--- type: Article title: "RebirthDay Attack: Reviving DNS Cache Poisoning with the Birthday Paradox" description: Studies how distinct EDNS Client Subnet values prevent DNS query aggregation while replies omitting that option create additional spoofed-response matches. Controlled resolver experiments and deployment measurements examine poisoning opportunities, configuration requirements and limits; Internet measurements identify prerequisites rather than confirmed poisoned services. resource: "https://doi.org/10.1145/3719027.3744832" tags: [article, webseclist-reference, acm, dns, cache-poisoning, measurement-study, parser-differential] generated: by: webseclist-refs/1 at: "2026-09-13T22:08:18+00:00" verified: - by: AI archive validation at: 2026-09-13 status: stable stale_after: 2027-09-13 sources: - id: original resource: "https://doi.org/10.1145/3719027.3744832" title: "RebirthDay Attack: Reviving DNS Cache Poisoning with the Birthday Paradox" author: Xiang Li, Mingming Zhang, Zuyao Xu, Fasheng Miao, Yuqi Qiu, Baojun Liu, Jia Zhang, Xiaofeng Zheng, Haixin Duan, Zheli Liu, Yunhai Zhang, Dunqiu Fan also_at: - "https://lixiang521.com/publication/ccs25/ccs2025-rebirthday-li.pdf" authors: - Xiang Li - Mingming Zhang - Zuyao Xu - Fasheng Miao - Yuqi Qiu - Baojun Liu - Jia Zhang - Xiaofeng Zheng - Haixin Duan - Zheli Liu - Yunhai Zhang - Dunqiu Fan canonical_url: "" cited_by: - "2025.md:116" commit: "" content_sha256: 97bdf7dec543f5c2baae030ae67cc7fb82392653cca433bcf35b7cb945b757f2 depth: full depth_reason: default kind: article language: "" licence: unknown original_url: "https://doi.org/10.1145/3719027.3744832" published: "" publisher: ACM publisher_english: "" raw_sha256: aa23ee8b6f118432652c574f5018f851da5b559720b573ad3c4bdaeddb59426d retrieved_from: "https://lixiang521.com/publication/ccs25/ccs2025-rebirthday-li.pdf" retrieved_kind: live retrieved_utc: "2026-09-13T22:08:18+00:00" slug: acm-rebirthday-attack-reviving-dns-cache-poisoning-birthday-paradox snapshot: "" title_english: "" translation_file: "" translation_of: "" --- # RebirthDay Attack: Reviving DNS Cache Poisoning with the Birthday Paradox **RebirthDay Attack: Reviving DNS Cache Poisoning with the Birthday Paradox** - Xiang Li, Mingming Zhang, Zuyao Xu, Fasheng Miao, Yuqi Qiu, Baojun Liu, Jia Zhang, Xiaofeng Zheng, Haixin Duan, Zheli Liu, Yunhai Zhang, Dunqiu Fan, ACM. - Published: date not stated - Original: - Also published at: - Preserved from: https://lixiang521.com/publication/ccs25/ccs2025-rebirthday-li.pdf (live) on 2026-09-13 - Licence: unknown Rights remain with the original author and publisher. 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RebirthDay Attack: Reviving DNS Cache Poisoning with the Birthday Paradox Xiang Li∗ Mingming Zhang Zuyao Xu Nankai University Zhongguancun Laboratory Nankai University Tianjin, China Beijing, China Tianjin, China lixiang@nankai.edu.cn zhangmm@mail.zgclab.edu.cn tochusc@gmail.com Fasheng Miao Yuqi Qiu Baojun Liu Tsinghua University Nankai Univeristy Tsinghua University Beijing, China Tianjin, China Beijing, China mfs24@mails.tsinghua.edu.cn qiuyuqi22@mails.ucas.ac.cn lbj@tsinghua.edu.cn Jia Zhang Xiaofeng Zheng† Haixin Duan‡ Tsinghua University Tsinghua University Tsinghua University Beijing, China Beijing, China Beijing, China zhangjia2017@tsinghua.edu.cn zxf19@mails.tsinghua.edu.cn duanhx@tsinghua.edu.cn Zheli Liu Yunhai Zhang Dunqiu Fan Nankai Univeristy NSFOCUS Technologies Group NSFOCUS Technologies Group Tianjin, China Beijing, China Beijing, China liuzheli@nankai.edu.cn zhangyunhai@nsfocus.com fandunqiu@nsfocus.com Abstract query aggregation mechanism and launch RebirthDay attacks. DNS cache poisoning is a persistent game of attack and defense, Through comprehensive evaluation, we showed that RebirthDay posing an enduring challenge for the DNS community. Significant attacks are highly practical and can have significant real-world im- efforts have been made to uncover, detect, and mitigate vulnerabili- pact, affecting 16 router vendors, 14 public DNS services, and 365K ties that increase the risk of cache poisoning. However, no work has (15%) open DNS resolvers. We have reported the identified vulnera- systematically revisited whether the original cache poisoning at- bilities to affected vendors and discussed mitigation solutions with tack based on the Birthday Paradox remains effective. In this work, them. To date, we have received acknowledgments from 8 vendors, we introduce RebirthDay, a novel DNS cache poisoning attack including BIND, Unbound, PowerDNS, and Quad9, and have been targeting recursive resolvers and forwarders, reviving the classic assigned 50 CVE-ids. Our study emphasizes the need for greater DNS Birthday attack that no longer works since 2002. RebirthDay attention to the importance of ECS verification and DNS extension exploits newly uncovered, protocol-compliant vulnerabilities in implementations, revealing new security risks introduced by them. DNS extension implementations to bypass the query aggregation mechanism intended to prevent DNS Birthday attacks that has not CCS Concepts been well understood. We uncovered that 18 out of 22 mainstream • Networks → Naming and addressing; • Security and privacy DNS software are vulnerable due to weaknesses in the processing → Network security; Authentication. of a DNS extension (i.e., ECS option), specifically lacking or incor- rectly implemented ECS coherence checks when handling DNS Keywords queries and responses, demonstrating the widespread susceptibility Domain name system; DNS security; Cache poisoning attack to RebirthDay. These flaws could be exploited to circumvent the ∗ Corresponding author. ACM Reference Format: † Also with QI-ANXIN Technology Research Institute. Xiang Li, Mingming Zhang, Zuyao Xu, Fasheng Miao, Yuqi Qiu, Baojun ‡ Also with Quan Cheng Laboratory. Liu, Jia Zhang, Xiaofeng Zheng, Haixin Duan, Zheli Liu, Yunhai Zhang, and Dunqiu Fan. 2025. RebirthDay Attack: Reviving DNS Cache Poison- Permission to make digital or hard copies of all or part of this work for personal or ing with the Birthday Paradox. In Proceedings of the 2025 ACM SIGSAC classroom use is granted without fee provided that copies are not made or distributed Conference on Computer and Communications Security (CCS ’25), October for profit or commercial advantage and that copies bear this notice and the full citation 13–17, 2025, Taipei, China. ACM, New York, NY, USA, 15 pages. https: on the first page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or //doi.org/10.1145/3719027.3744832 republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. CCS ’25, Taipei, China 1 Introduction © 2025 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 979-8-4007-1525-9/2025/10 The Domain Name System (DNS) provides translation between https://doi.org/10.1145/3719027.3744832 human-readable domain names and machine-readable IP addresses. CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. It serves as a fundamental infrastructure for locating Internet ser- vulnerable to RebirthDay. We also identified 16 router vendors vices through the domain name space and DNS resolution. The (e.g., ASUS, TP-Link, and ZTE), 14 DNS services (e.g., AdGuard, Ali, resolution process is simple and robust in design but vulnerable and Quad9), and 365K (15%) open DNS resolvers as vulnerable. to manipulation due to unencrypted communication. Extensive Disclosure and Mitigation. We claim the root cause of Rebirth- incidents have shown that DNS cache poisoning attacks remain a Day lies in the DNS protocol standards (RFC 7871), which specify significant threat to the Internet. These attacks exploit vulnerabili- that “a response that does not include the ECS option is still consid- ties in the resolution process to inject rogue resource records into ered valid”. We reported vulnerabilities to affected parties, includ- the DNS resolver’s cache and redirect user access to malicious tar- ing BIND, Unbound, PowerDNS, Google, Quad9, and Level3. We gets, leading to catastrophic security failures for national ISPs [62], have received acknowledgments from 8 vendors, including BIND, public key infrastructure [6], and other critical services [13]. Unbound, PowerDNS, Technitium, Dnsmasq, YogaDNS, Quad9, Ad- Over the past 30 years, DNS cache poisoning attacks have evolved guard, and been assigned 50 CVEs. We discussed the attack details from basic brute-force guessing of source ports and TxIDs to more and solutions with them. For mitigating RebirthDay, we suggest advanced techniques exploiting implementation vulnerabilities [15, the stakeholders to validate the response’s ECS option and aggre- 51, 59, 64], protocol flaws [20, 63, 73], and side channels [21, 44]. gate queries according to the ECS responding state of nameservers. This evolution has prompted DNS defenses such as source port Contributions. Our study makes the following contributions: and TxID randomization, Birthday protection, 0x20 encoding, and (1) We offered a comprehensive survey of previous DNS cache DNSSEC, which effectively mitigate off-path cache poisoning [15]. poisoning attacks and identified new vulnerabilities. However, researchers have also identified flaws in the implemen- (2) We proposed a novel threat model, RebirthDay, that re- tation of these defenses, creating new attack surfaces or allowing vives the classic DNS Birthday attacks, affecting 18 DNS bypass of the protections [18, 19, 37, 40, 42, 44, 45]. After systemat- implementations with newly uncovered vulnerabilities. ically reviewing the evolution of DNS cache poisoning threats, we (3) We demonstrated RebirthDay could cause prevalent real- noticed that DNS Birthday attacks [59, 64] have received limited world threats, affecting 16 famous router vendors, 14 DNS follow-up attention, resulting in an oversight of their mitigation service providers, and around 365K open DNS resolvers. effectiveness, which now needs to be studied in depth. (4) We introduced mitigation solutions, responsibly reported Our Study. We identify new protocol-compliant vulnerabilities findings to affected vendors, and assisted in resolving vul- in the DNS extension mechanisms as new attack surfaces, which nerabilities with an online DNS-OARC discussion group. enable a novel DNS cache poisoning attack, termed RebirthDay. Specifically, the attack exploits weaknesses in the EDNS Client 2 Background Subnet (ECS) mechanism’s option coherence checks to bypass DNS query aggregation policies adopted by resolvers to protect against In this section, we introduce the basic concepts of DNS, its resolu- DNS Birthday attacks. By bypassing query aggregation, attackers tion process, and packet formats [49, 50]. Additionally, we present can trigger numerous identical rogue DNS responses from different and analyze all types of DNS cache poisoning attacks to identify source ports. This increases the likelihood of colliding source ports potential areas that have not been thoroughly studied. and injecting malicious responses into the resolver cache. Via con- trolled experiments, we demonstrated that RebirthDay is totally 2.1 DNS Overview practical and can lead to prevalent real-world impacts. 2.1.1 DNS Concepts and Resolution. The Domain Name System Through code review and active testing, we analyzed ECS pro- (DNS) improves the usability of IP-based applications by mapping cessing implementations across 22 mainstream DNS software, in- domain names to IP addresses and vice versa. It consists of two cluding 9 recursive resolvers and 13 forwarders (Section 4). We iden- core components: the DNS namespace and the resolution process. tified a novel vulnerability in ECS processing, where DNS queries The DNS namespace is a hierarchical and distributed database with client subnets are cached without proper coherence checks that organizes domain names into zones, separated by periods (“.”). of subnet fields in responses. This flaw can bypass DNS query ag- Each zone is managed by an authoritative nameserver responsible gregation mechanisms and revive Birthday Paradox-based cache for maintaining authoritative data, referred to as resource records. poisoning. Affected software includes BIND, Unbound, PowerDNS, These include A (IPv4 address) and AAAA (IPv6 address) records Technitium, Dnsmasq, Pi-hole, and others. Additionally, we found for domain-to-IP mappings, and NS (nameserver) records, which that 11 DNS software lacks effective query aggregation mechanisms link parent zones to their child zones. As shown in Figure 1, the and one uses predictable TxID. These vulnerabilities collectively domain example.com consists of three zones: the Root zone (“.”), expose 18 software to exploitable DNS cache poisoning attacks. the Top-Level Domain (TLD) zone (“.com”), and the Second-Level To evaluate the capability of RebirthDay, we conducted cache Domain (SLD) zone (“example.com”). poisoning experiments across four representative vulnerable DNS The DNS resolution process translates domain names into IP ad- software implementations (Section 5), achieving a 20/20 success dresses iteratively. Figure 1 illustrates this process in detail. When rate by costing 358s on average. We then discovered the prevalence a user accesses a website like example.com, the client application of vulnerable DNS resolvers (Section 6) in real-world environments. interact with the stub resolver, such as systemd-resolved [65], With ethical considerations in mind, we analyzed 21 prominent Wi- within the TCP/IP stack. The stub resolver formulates a DNS query Fi routers, 6 router OSes, 45 public DNS services, and approximately and sends it to a pre-configured DNS forwarder, typically embed- 2.4M open DNS resolvers collected via Internet-wide scanning. Our ded in home or Wi-Fi routers [11, 36], to enhance performance by findings revealed that a significant portion of in-use resolvers are caching responses locally. The forwarder then relays the query to RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China Root (“.”) Query Response Cache Bogus Response Poisoned Cache Crafting a bogus response TLD (.com) TxID and Source Client DNS Forwarder Recursive Port Matched! Rogue Resolver SLD (example.com) Nameserver Query Response Cache Authoritative Nameserver Figure 1: General DNS Resolution Process. Malicious Client Recursive Resolver Authoritative Nameserver bit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Figure 3: General DNS Cache Poisoning Model. UDP Source Port (SrcPort) Destination Port (DstPort) Header Length Checksum Q A T R R DNS TxID R OpCode A C D A Z RCODE only if its source port and TxID match the query’s source port and Header QDCOUNT ANCOUNT TxID. According to Figure 3, a general DNS cache poisoning attack NSCOUNT ARCOUNT Question Section (QD) w/o Resource Records in Other Sections (AN/NS/AR) involves the following steps: Question (QD) www.example.com. A (1) Query Initiation or Interception: The attacker initiates or in- Answer (AN) www.example.com. A 1.2.3.4 Authority (NS) example.com. NS ns.example.com. tercepts a legitimate DNS query sent by the resolver to an Additional (AR) ns.example.com. A 5.6.7.8 authoritative nameserver. Figure 2: DNS Packet Format. (2) Crafting a Bogus Response: The attacker forges a DNS re- sponse packet, setting the TxID and source port to match those in the intercepted or guessed query. a recursive resolver, which handles the iterative resolution process. (3) Race Condition Exploitation: The attacker delivers spoofed re- The resolver starts at the Root nameserver, then queries the TLD and sponses to the resolver before the legitimate response arrives SLD nameservers sequentially. Each nameserver provides referral from the authoritative nameserver. information, guiding the resolver to the next “closer” authoritative (4) Cache Injection: Upon accepting the spoofed response, the nameserver. Finally, the authoritative nameserver for example.com resolver caches the falsified resource records, causing it to returns requested resource records, completing the resolution. If return incorrect data for future queries. caching is enabled, the recursive resolver stores the response in its local cache database for faster resolution of subsequent queries. On-Path-Based Attack Stage. Early DNS cache poisoning at- tacks were predominantly conducted by the on-path attacker who 2.1.2 DNS Packets. As shown in Figure 2, DNS typically uses UDP exploited their position within the communication path to inject to transmit query and response payloads. A DNS packet consists falsified resource records directly. of a header and a body. The header includes the transaction ID In 1990, Bellovin [5] identified a vulnerability in the file server (TxID), the query/response flag (QR), operation code (OpCode), re- that relied on hostname whitelists for user login verification. These turn code (RCODE), and the count of resource records. Specially, the servers performed a reverse DNS lookup using the gethostbyaddr TxID uniquely identifies each query-response pair, ensuring that function to obtain a hostname for the login host’s IP address and resolvers can correctly associate incoming responses with their cor- then verified the hostname against a whitelist to grant access. At- responding queries. The body contains four sections: the question tackers bypassed this verification by modifying the reverse lookup section (present in both queries and responses, specifying the query PTR record of their IP address to return a whitelisted hostname. In name and type), the answer section (providing resolution results 1993, Schuba [61] demonstrated how attackers could proactively from authoritative nameservers with the AA flag), and the authority inject falsified PTR records into the DNS cache using the additional and additional sections (providing referral information without the section of DNS packets. This allowed attackers to poison subsequent AA flag, from higher-level nameservers). reverse DNS lookups and bypass hostname-based authentication mechanisms. In 1995, Vixie [66] conducted a comprehensive anal- 2.2 DNS Cache Poisoning Attack ysis of DNS protocol design flaws, underscoring the insecurity of The DNS cache poisoning attack [12] aims to inject falsified resource hostname-based authentication [8]. To mitigate such vulnerabil- records into a resolver’s cache to manipulate subsequent queries. ities, Vixie introduced the Bailiwick Rules and Credibility Rules, By exploiting vulnerabilities in the resolution process, attackers which were incorporated into newer versions of BIND. These mea- can redirect users to malicious domains, disrupt network services, sures restricted the acceptance of invalid resource records from the or conduct man-in-the-middle attacks. These attacks have evolved additional section, significantly enhancing DNS security. over the years (1990 - 2025), transitioning from brute-force meth- The most infamous example of on-path cache poisoning occurred ods to sophisticated techniques that leverage protocol weaknesses in 1997 when Kashpureff [13] exploited recursive resolvers lack- or implementation flaws. This section provides a comprehensive ing Bailiwick Rules enforcement. By returning falsified NS records overview of DNS cache poisoning attacks, covering fundamental from the on-path-controlled AlterNIC authoritative nameserver, principles, methodologies, and the evolution of attack strategies. he redirected subsequent queries for InterNIC domains to a server Basic Attack. Cache poisoning attacks exploit the DNS resolution under his control, successfully poisoning the DNS cache. This in- process by injecting bogus response packets into a resolver’s cache. cident catalyzed the widespread adoption of Bailiwick Rules by As outlined in Section 2.1, DNS queries and responses depend on resolvers [64], which mitigated on-path cache poisoning attacks. matching the source port and TxID. A resolver accepts a response Since then, these attacks have become infeasible in modern DNS. CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. However, DNS remained vulnerable to prediction-based attacks. running Linux and Windows, leading to successful cache poisoning In 1997, researchers [9, 51] discovered that the TxID used by BIND attacks. By 2013, Herzberg et al. [22] highlighted that many DNS for external queries could be predicted due to its poor implemen- proxies, such as forwarding resolvers, had not fully adopted source tation. Coupled with the fixed source port for queries originating port randomization. These proxies often employed fixed or sequen- from the same client IP, the attacker were able to conduct off-path tially increasing source port allocation, exposing them to cache cache poisoning attacks by successfully guessing the TxID. poisoning risks. That same year, Herzberg et al. [21] introduced a Exploitation of Randomness Weakness Stage. Beginning in cache poisoning method leveraging Socket Overloading. By sending 2000, the security community identified multiple DNS vulnerabili- high volumes of traffic to the victim host, attackers could detect ties that could be exploited through weaknesses in TxID and source packet loss, indicating open source ports, and then inject falsified port verification mechanisms. replies by enumerating TxID values. In 2014, Shulman et al. [63] In 2000, researchers [52] discovered a flaw in the TxID generator utilized DNS fragmentation as a timing side channel to infer source within the GNU C Library, rendering TxID values predictable and ports. Attackers sent the first fragment of a DNS response con- enabling attackers to perform cache poisoning attacks. In 2001, Za- taining a source port to the target resolver. If the source port was lewski [70] conducted an extensive analysis of pseudorandom num- correct, the resolver accepted the response; otherwise, it rejected ber generator (PRNG) randomness across major operating systems, the response and initiated a new request. By measuring response visually illustrating the predictability of TxID values and TCP initial times, attackers could determine the correctness of the source port. sequence numbers [10]. In 2002, Sacramento [59] proposed leverag- In addition to guessing source ports, Herzberg et al. [20] in ing the “Birthday Paradox” to enhance brute-force attacks against 2013 proposed an alternative fragmentation-based cache poisoning TxID. Exploiting the behavior of BIND resolvers, which initiated attack. By predicting the IPID value and sending a false second multiple queries for repeated domain requests, attackers increased fragment to the target resolver, attackers bypassed the need to their success probability by issuing multiple resolution requests guess source ports and TxIDs, as the DNS verification fields were simultaneously. Theoretically, sending 425 forged packets achieved located in the first fragment. In 2018, Brandt et al. [6] extended a 50% success rate [23], effectively completing a cache poisoning fragmented cache poisoning to certificate application verification. attack [53]. In 2005, Kaminsky [28] highlighted the persistence of They demonstrated that off-path attackers could issue falsified cer- vulnerable configurations through network scans, discovering that tificates, compromising the security of the public key infrastructure. over 230K of 2.5 million tested resolvers were still using BIND8, Finally, in 2019, Alharbi et al. [1] introduced a cache poisoning which lacked sufficient defenses against cache poisoning. Between attack targeting DNS clients. By deploying malware to occupy local 2007 and 2008, Klein revealed that TxID generation remained pre- source ports, they forced DNS queries to use the only available dictable in several DNS implementations, including BIND8 [31], reserved port. Combined with TxID enumeration, this technique BIND9 [32], PowerDNS [35], Windows [34], and OpenBSD [33], enabled attackers to execute cache poisoning successfully. facilitating cache poisoning attacks. Revival Through Protocol Vulnerabilities Stage. Since 2020, The most prominent off-path attack during this stage was pro- the security community has leveraged various protocol vulnerabili- posed by Kaminsky in 2008 [29]. This off-path attack employed ties to propose new techniques for reviving DNS cache poisoning TxID brute-forcing alongside randomized domain names to bypass attacks rather than traditional guessing-style methods. cache limitations, enabling multiple rounds of attacks and the in- Following the development of fragmented cache poisoning at- jection of falsified NS records. The underlying vulnerability was tacks, academia and industry suggested minimizing fragmentation attributed to the lack of source port randomization, which affected as a mitigation measure. However, to bypass this limitation, Zheng nearly all domain resolver software at that time [54]. In 2009, Dagon et al. [73] in 2020 exploited the use of extended CNAME resolution et al. [15] developed a comprehensive cache poisoning attack model, chains to increase DNS packet sizes, forcing recursive resolvers to analyzing various defense techniques, including source port ran- fragment them. Using fragmented cache poisoning techniques, they domization, TxID randomization, and 0x20 case randomization [16]. successfully injected false resource records into DNS forwarders Their findings indicated that 57% of the 0.9 million tested resolvers that accepted fragments and lacked bailiwick checking. had not implemented source port randomization, and vulnerabili- In 2020 and 2021, Man et al. [44, 45] introduced two novel cache ties such as susceptibility to Birthday attacks persisted in systems poisoning attacks based on ICMP side channels. These attacks like djbDNS. Since then, widespread adoption of source port and allowed rapid source port guessing and false reply injection by enu- TxID randomization by resolvers [15] has substantially mitigated merating TxID values within a short time frame. The side channels the feasibility of off-path DNS cache poisoning attacks. exploited the global ICMP error message rate limiting counter, as Randomness Reduction Stage. Starting in 2010, the adoption well as fragmentation and redirection messages in the Linux oper- of source port and TxID randomization rendered brute-force tech- ating system. By observing the presence or absence of side channel niques for DNS cache poisoning increasingly infeasible. Conse- information, attackers could efficiently determine the source port quently, the security community began exploring new methods to used in DNS queries within tens to hundreds of millseconds. reduce the guessing space and achieve cache poisoning. In 2021, Klein [37] analyzed the pseudorandom number genera- In 2012, Herzberg et al. [19] identified flaws in the source port tor (PRNG) of the Linux operating system. By observing fields such allocation strategies for DNS queries and the authoritative name- as the IPv6 flow label and the IPv4 IPID in packets, Klein was able to server IP selection in NAT network environments. These vulner- reverse-engineer the initial state of the PRNG, calculate the source abilities allowed attackers to predict source ports and target au- port number, and successfully complete a cache poisoning attack. thoritative nameserver IPs for several NAT devices, including those Between 2021 and 2022, Jeitner et al. [26, 27] proposed two new RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China cache poisoning attacks based on ambiguities in special character ② N bogus responses. ... parsing. By leveraging inconsistencies in how resolvers processed special characters like “.” and “000”, they demonstrated how on- ① Trigger Q identical queries: Rogue path controlled authoritative nameservers could inject false replies. target.com. A? Nameserver These parsing ambiguities caused the same string to be decoded Query * Q Query * Q ... ... into different domain names, enabling cache poisoning. Response * Q In 2023, Heftrig et al. [18] discovered vulnerabilities in DNSSEC ... validation implemented by certain well-known public DNS reso- Malicious Client Victim DNS Resolver target.com lution services. These vulnerabilities exposed domains protected Nameserver by DNSSEC signatures to the risk of cache poisoning, highlighting (a) Threat Model of DNS Birthday Attack. new security risks even in environments utilizing DNSSEC. That same year, Li et al [40] introduced a new DNS cache poisoning Q identical queries: Queries with the same target.com. A? are aggregated attack, which exploits vulnerabilities in bailiwick checking algo- Query * Q rithms to perform DNS cache poisoning. This attack targets DNS ... Identical Queries Query * 1 servers acting as both recursive resolvers and forwarders, allowing ... Query Response * 1 attackers to take control of entire DNS zones. Response * Q Aggregator Most recently, in 2024, Li et al. [42] introduced a novel DNS cache target.com poisoning technique by exploiting logic vulnerabilities in DNS DNS Client DNS Resolver Nameserver response pre-processing. They demonstrated that malformed DNS (b) Mitigation: DNS Query Aggregation. packets, when processed incorrectly, could be leveraged as a side channel to inject malicious DNS records into the cache within 1s, Figure 4: DNS Birthday Attack and Mitigation Strategy. bypassing traditional defenses such as source port randomization. Insights from the Evolution. Existing DNS cache poisoning at- tacks, such as on-path and off-path methods leveraging source port malicious responses. This strategy effectively transforms the clas- and TxID guessing, have garnered significant attention and exten- sic brute-force attacks on TxIDs into a practical cache poisoning sive research, leading to numerous attack strategies and network technique by utilizing the Birthday Paradox. measurements. In contrast, DNS Birthday attacks [59], proposed 3.1.1 Birthday Paradox. The Birthday Paradox [46, 47] is a counter- and mitigated in 2002, lack systematic follow-up studies, leaving intuitive statistical phenomenon which states that the probability of critical questions about their impact on implementations unan- two people sharing the same birthday in a group grows rapidly with swered. Therefore, in this study, we systematically examine the the group size, even when the total possible outcomes (365 days) overlooked DNS Birthday attack vector, uncovering previously remain constant. In the context of DNS, this principle is applied to unidentified vulnerabilities that enable the revival of DNS cache increase the likelihood of TxID collisions. By generating multiple poisoning attacks and highlighting their extensive and widespread identical queries and spoofing a large number of forged responses, implications across modern DNS software, resolvers, and services. attackers can probabilistically achieve a collision between a legit- imate TxID and a spoofed one. For example, sending 425 forged 3 Overview of RebirthDay Attack packets theoretically yields a 50% success rate for collision [23], In this paper, we present a novel DNS cache poisoning attack, named which contributes to the DNS cache poisoning attack. RebirthDay attack, which leverages the DNS protocol extension as the new attack vector to revive the classic DNS Birthday attack. 3.1.2 Attack Steps. The threat model of DNS Birthday attacks is These protocol extensions, while compliant with DNS specifications, demonstrated in Figure 4(a). To initialize an attack, an attacker introduce new vulnerabilities, which bypass the mitigation strategy: needs to follow the following steps: Query Aggregation. With the new vulnerabilities reviving the DNS (1) The attacker identifies a vulnerable DNS resolver that issues Birthday attack that was no longer workable since 2002, we dubbed multiple queries for repeated domain name requests. them the RebirthDay attack (“Rebirth”). (2) The attacker floods the resolver with numerous identical This section begins by examining the fundamental of the DNS DNS queries for the target domain name, forcing it to initi- Birthday attack, followed by an introduction to the new threat ate multiple simultaneous resolution requests with different model and the basic attack workflow. Further attack details and ex- used TxIDs to an authoritative nameserver. periment results are provided in Section 4, Section 5, and Section 6. (3) The attacker simultaneously sends a large number of forged DNS responses with randomly generated TxIDs, hoping to 3.1 DNS Birthday Attack match one of the TxIDs generated by the resolver. (4) Upon achieving a TxID collision, the attacker could inject a The DNS Birthday attack leverages the statistical principle known malicious resource record into the resolver’s cache, indicat- as the “Birthday Paradox” [64] to significantly enhance the proba- ing a successful DNS cache poisoning attack. bility of brute-forcing DNS TxIDs. Initially proposed by Sacramento in 2002 [59], the attack exploits resolver behaviors such as issuing 3.1.3 Attack Metrics. To analyze the success probability of the multiple identical queries for repeated domain name requests, thus DNS Birthday attack, we introduce the following key metrics and allowing attackers to amplify the chances of successfully injecting equations, as well as their relationships. CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. ② Inject N bogus responses with matching and no ecs-option (subnet) in the packets. ... • 𝑄: Total number of DNS queries simultaneously triggered for the same domain (using different source ports or TxIDs). Rogue ① Trigger Q queries: ③ Arrive earlier Nameserver • 𝑇 : Total size of the randomness (source port or TxID) space, target.com. A? typically 𝑇 = 216 for the source port or TxID. Query * Q No ECS Checks!!! Query * Q • 𝑛: Number of unique source port or TxID collisions needed sport-1 for successful cache poisoning. ... sport-2 sport-q ... • 𝑃success : Probability of a successful DNS Birthday attack after DNS Victim Different Queries Response * Q target.com with Unique Source 𝑟 attack rounds. Client Resolver Ports!!! ... Nameserver Success Probability. The probability of success in a single attack Figure 5: Threat Model of the RebirthDay Attack. round is derived from the classic birthday problem: 𝑛−1 Ö 𝑇 − 𝑄 · 𝑖 𝑃single = 1 − (1) 3.2 Threat Model of RebirthDay Attack 𝑖=0 𝑇 Figure 5 illustrates the threat model of the RebirthDay attack. In where 𝑄 represents the number of triggered queries, and 𝑇 repre- this model, we assume that an attacker is a DNS client that can sents the size of the randomness space. trigger domain queries to target DNS resolvers and obtain their Cumulative Success Probability. To improve the overall suc- egress IP addresses. The attacker aims to inject malicious DNS re- cess rate, an attacker can repeat the attack multiple times. The sponses into the target resolvers’ DNS cache. Specifically, both DNS cumulative success probability after 𝑟 rounds is given by: forwarders and recursive resolvers are affected by RebirthDay. 𝑟 When sending DNS queries, attackers do not need to spoof the 𝑃success = 1 − 1 − 𝑃single (2) source IP address, as most resolvers do not verify the client’s IP or 3.1.4 Attack Effectiveness. The effectiveness of the DNS Birthday subnet information (see Section 6.2). For open resolvers distributed attack lies in its ability to amplify the collision probability using a across the Internet, attackers can directly send DNS queries to the high volume of simultaneous queries and forged responses. This resolver from any location [60]. For DNS resolvers serving more attack was demonstrated against BIND resolvers, where the attacker limited networks, such as those in home or enterprise networks, could achieve a 50% success probability with 425 forged packets to attackers can leverage large-scale measurement platforms [57] or collide TxIDs due to the lack of source port randomization [23, 59]. residential proxy networks [43, 48] to generate DNS queries. Before By exploiting resolver behaviors compliant with the DNS protocol, launching the RebirthDay attack, an attacker must collect the the attack significantly reduces the effort required for successful egress IP address of the target resolver to inject responses. This cache poisoning, posing a severe risk to DNS security. can be achieved by querying the target resolver, and then obtaining the egress IP address from the perspective of the authoritative 3.1.5 Mitigations. To address the vulnerabilities exploited by the nameserver controlled by the attacker. DNS Birthday attack, an effective mitigation strategy involves im- Additionally, we assume that the attacker is off-path and needs plementing the Query Aggregation mechanism [23, 53]. As shown to spoof the source IP address of a forged response. According to in Figure 4(b), the mechanism works by merging identical DNS re- recent data (Dec. 2024) from CAIDA [7], over 19% of IPv4 ASes are quests (identified if they share the same key: ) for classified as IP-spoofable, making it still feasible for an attacker the same domain name into a single query. Instead of sending out to use bulletproof hosting services [2] within these ASes to spoof separate queries for each identical request, the resolver combines the source address. Besides, to ensure that malformed packets are them and processes a single resolution workflow. Once the DNS re- delivered before legitimate responses, we assume the attacker can sponse is received, it is distributed to all pending queries, ensuring generate response packets from a neighboring host. Specially, we consistency and minimizing the attack surface. This mechanism exclude resolvers that enable DNSSEC validation [4] and 0x20 en- eliminates the creation of multiple DNS queries that an attacker coding [16] from our consideration. could exploit using the Birthday Paradox, effectively mitigating the probability of a successful cache poisoning attack. 3.3 Attack Workflow However, after discovery and mitigation, the DNS Birthday at- RebirthDay revitalizes the traditional DNS Birthday attack by tack was confirmed to affect a set of DNS software implementations, leveraging a novel attack vector stemming from protocol-compliant such as BIND and Microsoft DNS. Since then, despite significant DNS extensions. This newly identified vulnerability allows attack- advancements in DNS protocol standards and resolver implementa- ers to bypass the query aggregation mechanism of resolvers, forc- tions, there has been a noticeable lack of in-depth and systematic ing the target resolver to issue multiple DNS queries for the same analysis on the broader applicability and feasibility of the attack. domain by utilizing distinct source ports. This breakthrough signif- Specifically, questions such as whether this attack could still impact icantly increases the effectiveness of DNS Birthday attacks, even other DNS software or whether it remains a viable threat in today’s facing the widespread adoption of mitigation strategies. Internet have remained largely unexplored. As a result, the overall picture of the DNS Birthday attack and its potential impact across 3.3.1 Step-by-Step Workflow. The RebirthDay attack operates modern DNS systems remains unclear, leaving a significant gap in through three steps, as depicted in Figure 5, which exploit the understanding and an opportunity for further investigation. newly identified vulnerability to achieve a high success rate. RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China bit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Step ➀: Triggering Multiple Queries. The attacker begins by NAMEßà Root domain (Must be 0) initiating multiple carefully crafted DNS queries for a same do- TYPEßà OPT (41) main name to the victim resolver. By exploiting the identified logic CLASSßà Requestor's UDP payload size flaw in the DNS extension implementation, the resolver generates Extended RCODE Version TTLßà DO Z multiple identical queries for the same domain instead of aggre- RDLENßà Length of all RDATA gating them into a single query, using different source ports. This {Attribute, Value} behavior directly bypasses the query aggregation defense, laying RDATAßà {Attribute, Value} ... the groundwork for the subsequent steps of the attack. Step ➁: Injecting Malicious Responses. The attacker follows by Figure 6: DNS EDNS(0) Data Format. injecting a large number of bogus DNS responses. These responses exploit the resolver’s response handling vulnerabilities, increasing bit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 the collision probability between the attacker’s bogus responses Option Code and the resolver’s legitimate queries. By manipulating the source Option Length port (guessing a small range) and TxID (brute-forcing all 65,536), Address Family the attacker maximizes the effectiveness of the attack. Source Prefix-length Scope Prefix-length Step ➂: Returning before Legitimate Response. To succeed, Source Subnet ... the attacker ensures that the forged responses arrive before the legitimate responses. This is critical to poisoning the resolver’s Figure 7: ECS Option Format. cache with the malicious DNS record. If a single attack attempt is unsuccessful, the process is repeated starting from step ➀ with a bytes. By enabling larger DNS message sizes, EDNS(0) facilitates new domain name to bypass the impact of caching and scope-prefix. the transmission of additional information and supports advanced 3.3.2 Key Innovation. The critical innovation of the RebirthDay DNS features. EDNS(0) achieves this by adding optional fields in the attack lies in exploiting a protocol-compliant vulnerability to bypass DNS header, known as OPT pseudo-records, which allow extended query aggregation and trigger multiple DNS queries for a same functionality while maintaining backward compatibility. domain name. By systematically exploiting this novel vulnerability, The primary enhancement of EDNS(0) is its support for DNS mes- the RebirthDay attack reintroduces the DNS Birthday attack as a sages exceeding the original size limit, improving DNS efficiency significant threat to modern DNS infrastructures. and enabling extensions such as DNSSEC and ECS (EDNS Client Subnet) options. Furthermore, EDNS(0) permits the specification of 4 Bypassing DNS Query Aggregation extended flags and options in DNS queries and responses, offering DNS query aggregation has been recognized as an effective defense a flexible framework for future enhancements to the DNS protocol. mechanism against the DNS Birthday attack since its widespread Data Format. EDNS(0) extends the traditional DNS protocol by adoption by resolvers in 2002 [59, 64]. However, to date, no work introducing an optional OPT pseudo-record in the DNS additional has conducted a systematic and in-depth analysis of its software- section of DNS messages. The data format of such an OPT record is specific implementation and potential weaknesses. We find that the shown in Figure 6. It includes fields such as the Root name (set to introduction of DNS extensions, particularly EDNS(0) and Client 0), the fixed type (OPT, value 41), the UDP payload size, extended Subnet (ECS) mechanism, opens new attack surfaces for attackers, flags (e.g., DNSSEC signaling), and flexible option data structures enabling bypassing of DNS query aggregation and reigniting the for encoding new features. This backward-compatible extension potential of DNS Birthday attacks. enhances DNS capabilities while maintaining interoperability with In this section, we thoroughly analyze the EDNS(0) and ECS non-EDNS(0)-aware systems. mechanisms, including a summary of protocol specifications and an 4.1.2 ECS: EDNS Client Subnet. The EDNS Client Subnet (ECS) extensive examination of their implementation in 22 mainstream mechanism, standardized in RFC 7871 [14], is an extension option DNS software. These include 9 recursive resolvers and 13 DNS of EDNS(0) that aims to optimize DNS resolution for geograph- forwarders that are widely studied in prior works [38, 40–42, 69, ically distributed clients. ECS allows DNS resolvers to include a 72], as listed in Table 1. Our approach involves both source code portion of the client’s IP address (named as the client subnet) in inspection and local testing to elucidate the basic workflow of DNS queries sent to authoritative nameservers. This information ECS processing and to identify implementation-specific differences enables the authoritative nameserver to tailor DNS responses based across various software. Through detailed analysis and testing, we on the client’s geographical location, improving content delivery uncovered novel vulnerabilities that facilitate bypassing DNS query and reducing latency for end-users. aggregation, thereby enabling the reconstruction of DNS Birthday Option Format. The option format of ECS is shown in Figure 7. attacks (RebirthDay) to poison DNS caches effectively. This option is structured as follows: it begins with an option code identifying it as ECS, followed by a length field. It then specifies 4.1 EDNS(0) and ECS Mechanisms the address family (IPv4 or IPv6) and the source prefix length of the 4.1.1 EDNS(0): Extended DNS Mechanisms. EDNS(0), introduced client’s IP address, allowing the authoritative nameserver to use in RFC 2671 [67] and updated in RFC 6891 [17], is an extension the client’s subnet for more localized responses. The client’s subnet to the DNS protocol designed to overcome the limitations of the data (address) is included based on the specified scope prefix length original DNS specification, which restricted message sizes to 512 from the authoritative nameserver. CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. ECS Processing Steps. The following outlines how the ECS in- option, Unbound leverages this option to generate corre- teracts with resolvers, authoritative nameservers, and response sponding queries sent to the authoritative nameserver. Un- caching mechanisms during a DNS query-response cycle [14]. bound maintains distinct cache entries for different subnets. (I) Resolver Originating Query. When a resolver receives a query • PowerDNS supports ECS but disables it by default. It can from a client, it determines whether to include an ECS option based be enabled by configuring use-incoming-edns-subnet and on its configuration and policy. Typically, the client’s source IP edns-subnet-allow-list. Like Unbound, PowerDNS uti- address is truncated to a predefined prefix length (e.g., 24 bits for lizes ECS data from client queries for resolver queries, opti- IPv4 or 56 bits for IPv6), and this truncated subnet is included in the mizing responses based on the specified subnet. ECS option. If the client’s query already contains an ECS option, the • Knot, Microsoft, Simple DNS Plus, MaraDNS, and Hicko- resolver uses the subnet specified in the original ECS option. The ryDNS do not offer ECS functionality to handle ECS options. ECS option encodes this subnet with fields specifying the address • Technitium implements ECS, disabled by default. By en- family, source prefix length, and the truncated IP address. abling the ECS option, Technitium honors client ECS data (II) Authoritative Nameserver Generating Response. When an au- for resolution and maintains separate caches for each subnet. thoritative nameserver receives a query containing the ECS option, • Dnsmasq and Pi-hole, by default, only return ECS option to it determines whether to use the included subnet for its response. clients. When –add-subnet is enabled, they forward client’s If ECS processing is supported, the nameserver tailors its response ECS data to the upstream while ignoring the cache. based on the subnet, potentially providing more geographically • CoreDNS does not implement ECS functionality. or topologically relevant answers (e.g., selecting a closer content • DNSDist, Acrylic DNS, AdGuard, AdGuard Home, DNS delivery node). The nameserver includes an ECS option in its re- Safety, Dual DHCP DNS, and YogaDNS supports both sponse, reflecting the scope of the provided answer by specifying replying with ECS data to clients and forwarding the ECS a scope prefix length, which may differ from the original query. If option to upstream resolvers by default. the nameserver does not support ECS processing, it must omit the • SmartDNS enables ECS functionality by default and follows ECS option in the response, signaling to the client that ECS was the general ECS processing logic shown above for utilizing not used to generate the reply (Null ECS option). the ECS option in the client query. (III) Resolver Handling Response. When a resolver processes a • pdnsd only supports returning the ECS option to the client. response containing the ECS option, it checks the Family, Address, • NxFilter only supports forwarding ECS options to upstreams. Source Prefix-Length fields from the response against those in the original query. If there is any mismatch, the entire response is discarded to maintain consistency. If the ECS option matches, 4.2 Vulnerability in DNS Query Aggregation the resolver will use both the source and scope prefix length to We identified new DNS query aggregation vulnerabilities and other determine the subnet. If the response does not include an ECS poor source port or TxID randomization implementations. option, it is treated as applicable to all client addresses. Vulnerabilities in ECS Processing. As shown in Table 1 (Col- (IV) Caching and Subsequent Queries. When caching responses umn “No Query Aggregation” and “Vulnerable”), through analysis with ECS, the resolver associates the cached entry with the specific and testing, we identified a novel vulnerability affecting six DNS subnet used in the query. For subsequent queries from different sub- software that support ECS: BIND -S, Unbound, PowerDNS Recursor, nets, the resolver either uses an existing cache entry that matches Technitium DNS, Dnsmasq, and Pi-hole. This vulnerability arises the new subnet or issues a new query to the upstream. from two key issues in their ECS processing mechanisms. Firstly, when handling client queries containing ECS subnets, 4.1.3 Software Implementations. We summarize the implementa- these implementations verify the presence of a cache entry asso- tions related to ECS processing in Table 1 based on our analysis of ciated with the subnet. If such a cache exists, the query is directly 22 DNS software (9 resursives and 13 forwarders). resolved from it. If no cache entry exists, the software determines whether there is an ongoing query for the same subnet. If no such • BIND has two major versions with differing support for ECS query exists, the resolver initiates a new query using the speci- processing. The Open Source version does not support ECS fied subnet. Consequently, the identifier for determining identical functionality. When responding to client queries, it returns queries expands from the two-tuple to the three- the ECS option in the response exactly as it appeared in the tuple . This extension inadvertently per- client query, without further processing. The Subscription mits attackers to bypass query aggregation defenses by appending Edition (BIND -S), on the other hand, supports ECS. Upon re- different, spoofed subnets to otherwise identical queries, forcing ceiving a client query that includes an ECS option, it extracts the resolver to issue a large volume of queries for the same domain. the ECS information and uses it to construct the resolver Secondly, upon receiving responses from authoritative name- query sent to the authoritative nameserver, ensuring that servers, these implementations should validate that the subnet in the query is tailored based on the ECS subnet specified in the response aligns with the original query. However, as specified the original client request. by DNS protocol standards (RFC 7871), a response lacking the ECS • Unbound provides support for ECS (Disabled by default). option is still considered valid, indicating that the authoritative To activate ECS, the subnetcache module must be added, nameserver does not support ECS. Attackers can exploit this weak- and the client-subnet-always-forward option must be ness by injecting forged responses without the ECS option, relying enabled. Upon receiving a client query that includes an ECS solely on the two-tuple to bypass validation. RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China Table 1: ECS Processing Implementations of 22 DNS Software (18 Vulnerable). Resolver ECS No Query Aggregation # of Rate-limit Vulnerable Actor Index Software Version Reply Request Without ECS With ECS #1 BIND -S 9.18.37 ✓ ✓ ✗ ✓ 100 ✓ #2 Unbound 1.23.0 ✓ ✓ ✗ ✓ 20k ✓ #3 PowerDNS Recursor 5.2.2 ✓ ✓ ✗ ✓ 500 ✓ Recur- #4 Knot Resolver 5.7.5 ✗ ✗ ✗ ✗ 30k ✗ sive #5 Microsoft DNS 2025 ✗ ✗ ✗ ✗ 30k ✗ #6 Technitium DNS 13.3 ✓ ✓ ✗ ✓ 30k ✓ #7 Simple DNS Plus 9.1.116 ✗ ✗ ✗ ✗ 3k ✗ #8 MaraDNS 3.5.0036 ✗ ✗ ✗ ✗ 7 ✗ #9 HickoryDNS 0.26.0 ✗ ✗ ✓ ✓ 800 ✓ #10 Dnsmasq 2.91 ✓ ✓ ✗ ✓ 150 ✓ #11 CoreDNS 1.12.0 ✗ ✗ ✓ ✓ 3k ✓ #12 DNSDist 2.0.0 ✓ ✓ ✓ ✓ 450 ✓ #13 SmartDNS 46.1 ✓ ✓ ✗ ✗ 1k ✓* #14 Pi-hole 6.1.2 ✓ ✓ ✗ ✓ 150 ✓ Forw- #15 pdnsd 1.2.9a ✓ ✗ ✓ ✓ 40 ✓ sarder #16 Acrylic DNS 2.2.1 ✓ ✓ ✓ ✓ 14k ✓ #17 AdGuard 7.19 ✓ ✓ ✓ ✓ 9k ✓ #18 AdGuard Home 0.107.62 ✓ ✓ ✓ ✓ 20 ✓ #19 DNS Safety 2.1.0 ✓ ✓ ✓ ✓ 7k ✓ #20 Dual DHCP DNS 8.01 ✓ ✓ ✓ ✓ 1.5k ✓ #21 NxFilter 4.7.1.9 ✗ ✓ ✓ ✓ 3.4k ✓ #22 YogaDNS 1.47 ✓ ✓ ✓ ✓ 10k ✓ ✓: Yes. ✗: No. ✓: Vulnerable. ✗: Not vulnerable. * : Vulnerable due to poor source port or TxID randomization. # of Rate-limit is per IP. These flaws collectively introduce a novel attack vector that effec- 5 Evaluation of RebirthDay Attack tively circumvents query aggregation mechanisms and resurrects In this section, we develop end-to-end cache poisoning attack ex- the DNS Birthday attack, enabling DNS cache poisoning. periments exploiting the RebirthDay technique described above Flaws in Query Aggregation Implementations. We also find and evaluate them across four vulnerable DNS software imple- that several DNS software implementations remain vulnerable to mentations: Unbound, PowerDNS Recursor, Technitium DNS, and cache poisoning due to insufficient defenses against traditional CoreDNS. For ECS-enabled resolvers, we targeted three mainstream DNS Birthday attacks. Specifically, query aggregation mechanisms software: Unbound, PowerDNS Recursor, and Technitium DNS. Al- are absent or ineffective in HickoryDNS, CoreDNS, DNSDist, pdnsd, though the BIND Subscription Edition is not publicly available, Acrylic DNS, AdGuard, AdGuard Home, DNS Safety, Dual DHCP we verified its vulnerability through testing with industry part- DNS, NxFilter, and YogaDNS. ners (Quad9 DNS) utilizing this version. For non-ECS-supporting Flaws in Randomization Implementations. Furthermore, vul- resolvers, we selected CoreDNS due to its widespread use. While nerabilities in source port and TxID randomization still significantly Dnsmasq is also affected by RebirthDay, it does not enable caching exacerbate the risk of exploitation. For instance, SmartDNS exhibits for ECS, making it applicable only for attacks targeting unqueried or inadequate source port randomization over extended periods. DNS- nonexistent domains [71]. Other DNS software behave consistently Dist uses fixed source ports and sequential TxIDs for queries to with CoreDNS in our evaluations. the same upstream resolver. Acrylic DNS and YogaDNS employ in- crementing source ports, while AdGuard combines incrementing source ports with client query TxIDs for self-requests, and AdGuard 5.1 Attack Design Home exclusively relies on client query TxIDs too. Additionally, 5.1.1 Experiment Setup. For our evaluations, we installed Unbound, Dual DHCP DNS uses fixed source ports in conjunction with client PowerDNS, and CoreDNS on machines running Ubuntu 24.04 as the query TxIDs for resolution. These flaws collectively expose these host operating system, while Technitium was deployed on Windows implementations to easily exploitable cache poisoning attacks, un- Server 2022. Each DNS resolver was configured to enable ECS sup- dermining DNS security and corresponding Internet services. port. We utilized the domain victim.com for testing, configuring it In summary, we discovered new DNS query aggregation vulner- under a controlled authoritative nameserver. We deployed attackers abilities that could be exploited to conduct RebirthDay attacks, and clients on machines within the same local network as the DNS and classic flaws in source port or TxID randomization. Those vul- resolvers. Each experiment involved three to five clients issuing nerabilities could bypass the randomization defenses and enable queries to the resolvers, while the attacker machine sent multiple DNS cache poisoning attacks. queries with different ECS subnets to the target resolver and forged malicious responses targeting these queries. The query packets, CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. -- UDP -- UDP Table 2: RebirthDay Attack Results. Src Port, Dst Port: 53 Src Port, Dst Port: 53 -- DNS -- DNS Avg. Round Avg. Time Success TxID: {0-65k} TxID: {0-65k} Software Taken Taken Rate Flags: QR=0; Flags: QR=0; Question Section: Question Section: Unbound 263 593s 20/20 {sub}.victim.com A {sub}.victim.com A Answers Section: Answers Section: PowerDNS Recursor 328 237s 20/20 NULL NULL CoreDNS 20 245s 20/20 Authority Section: Authority Section: NULL NULL Additional Section: Additional Section: Subnet=x.x.{0-255}.0/24 Subnet=x.x.{0-255}.0/24 the legitimate response is received, as shown in Figure 8(b) and Fig- (a) Attacker Query. (c) Resolver Query. ure 8(d). The attack strategy involves guessing one or more source -- UDP -- UDP ports and brute-forcing 65,536 possible TxIDs. This is made feasible Src Port: 53, Dst Port Src Port: 53, Dst Port by the Birthday Paradox, where the greater the number of queries -- DNS -- DNS TxID: {0-65k} TxID: {0-65k} sent by the resolver (i.e., the more source ports used), the higher Flags: QR=1; AA=1; Flags: QR=1; AA=1; the probability of success. Lastly, after injecting the response, the Question Section: Question Section: {sub}.victim.com A {sub}.victim.com A attacker checks if the returned IP is a.t.k.r, indicating a successful Answers Section: Answers Section: {sub}.victim.com A a.t.k.r {sub}.victim.com A v.c.t.m attack. If the result is v.c.t.m, the attack has failed; the attacker Authority Section: Authority Section: then repeats the process by starting a new round of queries. victim.com NS ns.victim.com NULL Additional Section: Additional Section: Success Probability. According to the rate-limit testing results, ns.victim.com A a.t.k.r NULL we send 200 queries in each round (200 different source ports) and (b) Attacker Response. (d) Authoritative Response. guess only one random source port with brute-forcing 65,536 TxIDs. The success probability of the attack after 1,800 rounds, based on Figure 8: DNS Query and Response Packets. the Birthday Paradox, is calculated as follows (around 99.6%): 200 1,800 𝑃success = 1 − 1 − ≈ 0.99592 legitimate authoritative response packets, and forged attacker re- 65, 536 sponse packets are illustrated in Figure 8, respectively. Here, we Where 200 is the number of queries (or source ports) per round use victim.com for anonymity purposes. In actual experiments, and 65536 is the total number of possible source ports. real domain names are used. If the attack succeeds, victim.com will be hijacked to a.t.k.r controlled by attackers. The network 5.2 End-to-End Attacks was configured with a public IP assigned to the resolver, allowing We conducted the RebirthDay attack 20 times using programs access from both legitimate clients and potential external traffic. developed in Golang, which implement the techniques above. To in- 5.1.2 Rate-limit Testing. The feasibility of RebirthDay is signifi- troduce query latency, we delayed the response from each software cantly influenced by the number of queries a resolver accepts from by a specified duration before returning the legitimate response. The a single client. As discussed in Section 3.1, the probability of a experiment results are presented in Table 2. The attack success rate successful attack increases with the number of queries that can be is 20/20, except for Technitium. The maximum bandwidth required triggered. To investigate this aspect, we conducted a systematic to inject 65,536 packets with different TxIDs is about 119Mbps. evaluation of the rate-limiting configurations across 22 DNS soft- 5.2.1 Attacking Unbound. Due to Unbound’s retry algorithm that ware (Column “Rate-limit” in Table 1), measuring the maximum uses intervals starting from 0.1s, progressing to 0.5s, then 1.5s, number of queries permitted per client. Most impacted software and eventually increasing further, we introduce a 0.5s delay per allowed a rate limit exceeding 200 queries per client. For resolvers round. To enhance performance, we set the queries per thread to with lower thresholds, attackers could bypass these limitations in 200 (default is 30). On average, the attack takes only 263 rounds practical scenarios by employing multiple clients or spoofing source (593s). The minimum time is 23s while the maximum time is 2,426s. IP addresses to increase the number of identical queries generated. 5.2.2 Attacking PowerDNS Recursor. PowerDNS performs only 5.1.3 Attack Steps. The core of RebirthDay involves the follow- one retry with a timeout window of 1.5s. To account for this, we ing steps. In each round, the attacker first sends multiple queries introduce a 1s delay per round. Additionally, PowerDNS employs a containing the ECS option to the target resolver. In Figure 8(a), the spoof-nearmiss-max setting to detect spoofed responses (default attacker queries for the A record of {nonce}.victim.com, where 1). For testing, we modify the value to 0. On average, the attack nonce is a random string used to bypass the cache. The query also requires 328 rounds (237s). The attack took between 60s and 480s. contains a unique subnet value set to x.x.{0-255}.0/24. Sec- ondly, upon receiving the query, the target resolver will handle it 5.2.3 Attacking CoreDNS. Given CoreDNS’s 6s query timeout, we based on the three-tuple . The attacker introduce a 900ms delay per round. With a rate limit of 3k queries, uses this to bypass query aggregation and triggers a series of re- we increase the queries per round to 2,000 to optimize efficiency. solver queries to the authoritative nameserver, where each query Theoretically, with 2,000 queries, 300 rounds are required for a uses a different source port, as illustrated in Figure 8(c). Thirdly, the 99.9% success rate. In practice, the attack completes in an average attacker then injects fake responses to the target resolver before of 20 rounds (245s). The attack duration varied from 11s to 1,479s. RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China 5.2.4 Attacking Technitium DNS. Technitium performs a single Table 3: Results of 27 Wi-Fi Routers and OSes (16 Vulnerable). retry with a timeout of 1.5 seconds. We observed that Technitium No Q. deploys Delegation Revalidation [24] to validate NS records and Index Vendor Version Vul. Agg. only accepts NS records from referrals or authoritative answers. #1 360 WI-FI6 T7 4.2.4 ✗ ✗ During the experiment, when Technitium received a high volume #2 ASUS RT-AC66U 384.18 ✓ ✓ of random domain queries and fake responses, it initiated numerous #3 CISCO Router 1.2.1.7 ✓ ✓ revalidation queries but failed to respond to legitimate ones, making #4 D-Link 7001 17.01.11A1 ✓ ✓ it impossible to determine the success of the RebirthDay. However, #5 Fast FAC1200R 1.0.0 ✓ ✓ this resulted in a DoS effect. #6 Fiberhome SR4201SA RP0100 ✗ ✓* 5.2.5 Discussion. For practical attacks, we need to consider the #7 H3C Magic NX15 00R012 ✗ ✗ following factors that affect the attack result in the real world. #8 Honor X4 Pro 16.0.0.38 ✗ ✗ Multiple Nameservers. Many domains are configured with mul- #9 Huawei AX3 4.0.0.19 ✗ ✗ tiple nameserver IPs, which can potentially complicate the attack #10 iKuai IX-Q3600 3.7.15 ✗ ✗ surface. Our analysis of the latest zone files for .com and .net #11 Linksys 2.0.4.215745 ✓ ✓ domains [25], revealed the median number of nameserver IPs per #12 Mercury D191G 2.0.2 ✓ ✓ domain is 4. Given that our attack requires a maximum of 65,536 #13 Netgear AX5 1.0.8.82_1 ✗ ✓* packets to brute-force, attackers could exploit this by spoofing #14 PGY X4C-5131G 6.5.0 ✗ ✗ the IP addresses of all four nameservers simultaneously, sending #15 Redmi AX3000 1.0.68 ✓ ✓ 65,536×4 packets. #16 Skyworth WR9651X 1.1.0 ✓ ✓ Multiple Backend IPs. Public DNS services typically operate with #17 Tenda V1 16.03.29.50 ✓ ✓ multiple backend IPs. This increases the injection space for attack- #18 TP-Link XDR3230 1.0.22 ✓ ✓ ers. However, the selection of backend IPs is often influenced by #19 TP-Link XDR5430 1.0.14 ✓ ✓ factors such as geolocation and load balancing. In our experiments, #20 Xiaomi 4C 5.15.150 ✗ ✗ we found that affected public DNS services (tested in Section 6.2) #21 ZTE E2633 1.0.4 ✓ ✓ had limited backend IPs (e.g., 1-4) in use from a specific geolocation. #22 DD-WRT v3.0-r39296 ✗ ✗ This simplifies the attacker’s task, as they only need to spoof a few #23 Gargoyle 1.14.0 ✗ ✗ select IP addresses at once. #24 iKuai OS 3.7.17 ✗ ✓* #25 libreCMC 1.5.15 ✗ ✗ 6 Measurement of Vulnerable Resolvers #26 OpenWrt 23.05.3 ✗ ✗ In this section, we present a comprehensive measurement of the Re- #27 RouterOS 7.16.2 ✗ ✓* birthDay in real-world environments, analyzing 21 Wi-Fi routers, 6 router OSes, 45 public DNS services, and 2.4M open DNS resolvers. ✓: Yes. ✗: No. ✓: Vulnerable. ✗: Not vulnerable. No Q. Agg.: No Our extensive testing demonstrates that a significant portion of the query aggregation. * : Vulnerable due to poor randomization. resolver population remains vulnerable to RebirthDay, highlight- ing the widespread applicability and impact of RebirthDay. Testing Design. To evaluate vulnerable resolvers, we focus on as- to our authoritative nameserver. We then observed whether the sessing the key attack factors (ECS support and query aggregation) routers forward the ECS option and whether they aggregate queries. rather than conducting the attack itself. We perform two sets of tests to measure these factors. Firstly, we send DNS queries contain- 6.1.3 Results. We found that all tested routers successfully sup- ing the ECS option to the target resolver and observe whether the ported forwarding the ECS option. However, we identified key resolver correctly forwards the ECS information to the upstream differences regarding query aggregation. Specifically, 12 routers DNS servers and whether the ECS option is subsequently returned from ASUS, CISCO, D-Link, Fast, Linksys, Mercury, Redmi, Sky- to the client. Secondly, we send 200 DNS queries to each resolver worth, Tenda, TP-Link, and ZTE did not support query aggregation. to examine whether the resolver aggregates these queries, which Furthermore, we found that Fiberhome, Netgear, iKuai OS, and could potentially impact its vulnerability to attacks. On the name- RouterOS were vulnerable due to predictable source ports or TxIDs. server side, we could observe the number of DNS queries from each These vulnerabilities rendered 16 routers to be exploitable by the resolver. We can also identify how many backend IPs each resolver RebirthDay attack. uses. If no query aggregation is observed, we classify the target resolver as vulnerable to the attack. 6.2 Public DNS Services 6.2.1 Public DNS Service List. Based on statistics from APNIC [3], 6.1 Wi-Fi Routers and Router-OSes we selected 45 widely-used public DNS services and their corre- 6.1.1 Router and OS List. We collected 21 popular Wi-Fi routers sponding IP addresses for testing, as shown in Table 4. and 6 router OSes listed in [58, 68] to evaluate, shown in Table 3. 6.2.2 Testing. The testing was conducted using our own domain 6.1.2 Testing. We connected our client machine to these routers and involved multiple test rounds. Additionally, we measured their (OSes) and configured their upstream DNS servers to point directly rate-limiting behaviors that affected the attack success probability. CCS ’25, October 13–17, 2025, Taipei, China Xiang Li et al. Table 4: Results of 45 Public DNS Services (14 Vulnerable). Public DNS Service ECS No Query Aggregation # of Rate-limit Vulnerable Index Vendor IP Reply Request Without ECS With ECS #1 114DNS 114.114.114.114 ✗ ✗ ✗ ✗ 198 ✗ #2 360 Secure DNS 101.226.4.6 ✓ ✗ ✗ ✗ 150k+ ✓* #3 AdGuard DNS 94.140.14.14 ✓ ✓ ✗ ✓ 2.7k ✓ #4 AhaDNS 5.2.75.75 ✓ ✓ ✗ ✗ 11 ✗ #5 Akamai Vantio DNS 23.56.160.142 ✓ ✗ ✗ ✗ 10k+ ✗ #6 Ali DNS 223.5.5.5 ✓ ✓ ✗ ✓ 50k+ ✓ #7 Alternate DNS 76.76.19.19 ✓ ✗ ✗ ✗ 35 ✗ #8 Baidu DNS 180.76.76.76 ✗ ✗ ✗ ✗ 50k+ ✗ #9 ByteDance DNS 180.184.1.1 ✓ ✓ ✗ ✓ 1k+ ✓ #10 CenturyLink DNS 205.171.3.66 ✓ ✗ ✗ ✗ 2.5k ✗ #11 CIRA Shield DNS 149.112.121.10 ✓ ✓ ✗ ✓ 50k+ ✗ #12 Cisco OpenDNS 208.67.222.222 ✓ ✓ ✗ ✗ 25k ✓* #13 CleanBrowsing 185.228.168.10 ✗ ✓ ✗ ✗ 400 ✗ #14 CloudFlare DNS 1.1.1.1 ✗ ✓ ✗ ✗ 50k+ ✗ #15 CNNIC sDNS 1.2.4.8 ✗ ✗ ✗ ✗ 10k+ ✗ #16 Comodo Secure 8.26.56.10 ✓ ✓ ✗ ✗ 20k+ ✗ #17 Comss.one DNS 195.133.25.16 ✗ ✓ ✓ ✓ 1k+ ✓ #18 ControlD DNS 76.76.2.5 ✓ ✓ ✗ ✓ 15k+ ✓ #19 CZ.NIC ODVR 193.17.47.1 ✗ ✗ ✗ ✗ 120 ✗ #20 DNS for Family 94.130.180.225 ✓ ✓ ✓ ✓ 5k+ ✓ #21 DNS Forge 176.9.93.198 ✗ ✓ ✗ ✗ 1k ✗ #22 DNS.SB 185.222.222.222 ✗ ✗ ✗ ✗ 50k+ ✗ #23 DNS.WATCH 84.200.69.80 ✓ ✗ ✗ ✗ 5k+ ✗ #24 DNSPod Public DNS+ 119.28.28.28 ✓ ✓ ✗ ✓ 50k+ ✓ #25 Dyn DNS 216.146.35.35 ✗ ✓ ✗ ✗ 600 ✗ #26 FDN DNS 80.67.169.12 ✓ ✓ ✗ ✗ 10k+ ✗ #27 G-Core DNS 95.85.95.85 ✓ ✓ ✓ ✓ 10k+ ✓ #28 Google DNS 8.8.8.8 ✓ ✓ ✓ ✓ 500 ✓ #29 Hurricane DNS 74.82.42.42 ✗ ✗ ✗ ✗ 1k ✗ #30 Level3 DNS 4.2.2.1 ✗ ✗ ✗ ✗ 3.5k ✓* #31 LibreDNS 88.198.92.222 ✗ ✗ ✗ ✗ 10k+ ✗ #32 Neustar UltraDNS 156.154.70.1 ✓ ✓ ✗ ✗ 20k+ ✗ #33 NextDNS 45.90.30.118 ✗ ✗ ✗ ✗ 2.5k ✗ #34 Norton ConnectSafe 199.85.126.10 ✓ ✗ ✗ ✗ 20k+ ✗ #35 OneDNS 52.80.66.66 ✓ ✓ ✗ ✗ 540 ✗ #36 OpenNIC DNS 103.1.206.179 ✓ ✓ ✗ ✗ 15k+ ✗ #37 Quad101 DNS 101.101.101.101 ✓ ✗ ✗ ✗ 220 ✗ #38 Quad9 DNS 9.9.9.11 ✓ ✓ ✗ ✓ 5k+ ✓ #39 SafeDNS 195.46.39.39 ✗ ✗ ✗ ✗ 15k+ ✗ #40 SafeSurfer DNS 104.197.28.121 ✗ ✗ ✗ ✗ 3k+ ✗ #41 SkyDNS 193.58.251.251 ✗ ✗ ✗ ✗ 15k+ ✗ #42 Strongarm DNS 52.3.100.184 ✓ ✗ ✗ ✗ 150 ✗ #43 Tiarap Public DNS 174.138.21.128 ✗ ✗ ✗ ✗ 500 ✗ #44 Verisign Public DNS 64.6.65.6 ✓ ✗ ✗ ✗ 10k+ ✗ #45 Yandex DNS 77.88.8.1 ✓ ✗ ✓ ✓ 2.5k ✓ ✓: Yes. ✗: No. ✓: Vulnerable. ✗: Not vulnerable. * : Vulnerable due to poor query aggregation. # of Rate-limit is per IP. 6.2.3 Results. Our results showed that 32 services supported ECS. found that 11 services did not implement query aggregation that Specifically, 10 of these services only supported returning ECS were vulnerable to RebirthDay. Among these, 8 services, includ- to the client, 5 only supported querying upstream with ECS, and ing AdGuard DNS, Ali DNS, ByteDance DNS, CIRA Shield DNS, 17 supported both querying and responding with ECS. We also Comss.one DNS, ControlD DNS, DNSPod Public DNS+, and Quad9 RebirthDay Attack CCS ’25, October 13–17, 2025, Taipei, China DNS, were affected due to their vulnerable ECS implementations. Besides, 3 services, including 360 Secure DNS, Cisco OpenDNS, and Level3 DNS, were vulnerable because of poor query aggrega- tion (querying 25 - 100 times). The inconsistent multiple queries from resolvers are likely due to retry mechanisms for performance optimization or handling network issues. Specifically, only CIRA Shield DNS performs client IP and subnet verification; exploiting it would require attackers to spoof the client’s IP address. These vulnerabilities significantly increased the risk of the DNS Birthday attack. These findings demonstrate the critical impact of ECS in Figure 9: The Fraction of Query Aggregation Results. amplifying the risks of the RebirthDay attack. a lower thresholds (200). We configure the PTR record and a website to show our research objectives, and no opt-out requests have been 6.3 Open DNS Resolvers received. Finally, we report findings to all vendors. Notably, Quad9 6.3.1 Open DNS Resolver List. Since the open DNS resolver is actively provided services for us to test. highly volatile [60], we aimed to acquire the most recent snapshot Mitigation Solutions. RebirthDay exploits vulnerabilities in the of the Internet by scanning the IPv4 UDP port 53 on our controlled DNS Extension protocol, particularly with ECS processing. The root domain using XMap [39]. We discarded any results that were erro- cause lies in RFC 7871, which allows responses without ECS to be neous. Between October and December 2024, we discovered over considered valid. To mitigate this attack, we propose an enhanced 2.4 million open DNS resolvers, which are associated with 232 re- query aggregation strategy and urge additional defenses against gions and 25,711 autonomous systems (ASes). The top three regions cache poisoning. Firstly, we recommend that resolvers verify ECS with the highest number of resolvers are China, India, and Russia. consistency in responses, marking authoritative nameservers as in- valid if there is no ECS or the Scope Prefix-Length is 0. Resolvers 6.3.2 Testing. Instead of conducting actual attacks, we focused on should aggregate all subsequent queries containing ECS subnets testing whether the resolvers support ECS and query aggregation. for that server and domain, and also aggregate queries within the For ethical considerations and based on the rate-limiting results same ECS scope to ensure consistency. Secondly, to defend against from public DNS services, we sent 200 queries for the same domain cache poisoning, we urge enabling 0x20 encoding [16] to thwart in 1s. We then observed queries from our authoritative nameserver. case manipulation, and implementing DNSSEC [4] to cryptograph- 6.3.3 Results. Among the 2.4 million open DNS resolvers, approx- ically sign DNS responses. Finally, resolvers could block malicious imately 14.1% (343,133) supported querying the authoritative name- response injections through anomaly detection and rate limiting server with the ECS option, while about 29.0% (705,176) returned the like PowerDNS [56], further securing the DNS system. ECS option to the client. Around 5.2% (127,561) supported forward- Disclosure. We reported vulnerabilities to affected vendors and are ing ECS to both clients and upstream resolvers. The results of the discussing solutions with them. 8 vendors have confirmed Rebirth- query aggregation tests are shown in Figure 9. Upon receiving 200 Day, including BIND, Unbound, PowerDNS, Technitium, Dnsmasq, queries for the same domain, over 80% of resolvers exhibited no sig- YogaDNS, Quad9, and Adguard. BIND, Unbound, and PowerDNS nificant query aggregation (less than three queries). Specially, more have implemented a patched version based on our suggestions. than 15% of resolvers made 25 or more queries, and over 10% made They will resend the DNS query without ECS or switch to TCP 50 or more queries. According to the Birthday Paradox, resolvers when an ECS mismatch is detected. 50 CVE-ids were assigned. that queried 25 times would have a 50% success rate after 1,800 rounds of attacks, while those querying 50 times could reach a 50% 8 Conclusion success rate after only 900 rounds. For resolvers supporting ECS, In this study, we present a novel DNS poisoning attack model, the impact was even more significant. Over 30% of ECS-supporting RebirthDay, which exploits vulnerabilities in the poorly imple- resolvers made 25 or more queries, and approximately 20% made mented ECS mechanism. This flaw bypasses the query aggregation 50 or more queries. Consequently, the support for ECS signifi- policy designed to prevent DNS Birthday attacks, making DNS cantly increased the number of resolvers (by at least 100K) cache poisoning feasible once again. A solution involves enforcing vulnerable to Birthday attacks. We conclude that at least 365K strict ECS verification between DNS queries and responses. Besides, (15%) open DNS resolvers are vulnerable to Birthday attacks due to we found other extension-based risks and are conducting a no query aggregation. comprehensive analysis of DNS extensions to identify new weaknesses as our future work. 7 Discussion and Mitigation Ethical Considerations. We follow the ethical guidelines of the Acknowledgments Menlo Report [30] and best network measurement practices [55]. We thank all the anonymous reviewers for their valuable comments Firstly, we install all DNS software on our local machines. For the in helping us to improve this paper. 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