---
type: Article
title: "CDN Judo: Breaking the CDN DoS Protection with Itself"
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resource: "https://www.ndss-symposium.org/ndss-paper/cdn-judo-breaking-the-cdn-dos-protection-with-itself/"
title: "CDN Judo: Breaking the CDN DoS Protection with Itself"
author: Run Guo, Weizhong Li, Baojun Liu, Shuang Hao, Jia Zhang, Haixin Duan, Kaiwen Sheng, Jianjun Chen, Ying Liu
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authors:
- Run Guo
- Weizhong Li
- Baojun Liu
- Shuang Hao
- Jia Zhang
- Haixin Duan
- Kaiwen Sheng
- Jianjun Chen
- Ying Liu
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---
# CDN Judo: Breaking the CDN DoS Protection with Itself
**CDN Judo: Breaking the CDN DoS Protection with Itself** - Run Guo, Weizhong Li, Baojun Liu, Shuang Hao, Jia Zhang, Haixin Duan, Kaiwen Sheng, Jianjun Chen, Ying Liu, NDSS Symposium.
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CDN Judo: Breaking the CDN DoS
Protection with Itself
Run Guo∗§ , Weizhong Li∗§ , Baojun Liu∗ , Shuang Hao† , Jia Zhang∗¶ ,
Haixin Duan∗¶ , Kaiwen Shen∗ , Jianjun Chen‡ , Ying Liu∗¶
∗ Tsinghua University,
{gr15, lwz17, lbj15, skw17}@mails.tsinghua.edu.cn, {zhangjia, liuying}@cernet.edu.cn, duanhx@tsinghua.edu.cn
† University of Texas at Dallas, shao@utdallas.edu
‡ ICSI (International Computer Science Institute), jianjun@icsi.berkeley.edu
¶ BNRist (Beijing National Research Center for Information Science and Technology)
Abstract—A content delivery network (CDN) improves the having high capacities in terms of both computational re-
accessing performance and availability of websites via its globally sources and network bandwidth. Because of its traffic offload-
distributed network infrastructures, which contributes to the ing benefits and global accessibility, the CDN has become an
thriving of CDN-powered websites on the Internet. Because CDN- indispensable part of the Internet ecosystem. CDN vendors
powered websites normally operate important businesses or criti- have also been advertising their capability to protect against
cal services, attackers are mostly interested in taking down these
high-value websites, to achieve severe damage with maximum
DoS attacks, contributing to the successful expansion of CDNs
influence. Because the CDN absorbs distributed attacking traffic over the Internet, where increasingly more websites are being
with its massive bandwidth resources, it is commonly believed deployed behind CDNs. For example, more than 50% of the
that CDN vendors provide effective DoS protection for the CDN- Alexa 1K and more than 35% of the Alexa 10K websites are
powered websites. deployed behind CDNs [20].
However, we reveal that implementation or protocol weak- However, in this paper, by empirically exploring the for-
nesses in the forwarding mechanisms of the CDN can be exploited warding behaviors of six CDNs, we reveal that the CDN
to break this CDN protection. By sending crafted but legal itself can be abused to attack the origin (website server)
requests, an attacker can launch an efficient DoS attack against behind a CDN. By sending crafted but legal requests to a
the website origin behind it. In particular, we present three CDN, an attacker can initiate a DoS attack against the origin,
CDN threats in this study. By abusing the HTTP/2 request- breaking the CDN DoS protection. In short, our work reveals
converting behavior and HTTP pre-POST behavior of a CDN,
the following three threats:
an attacker can saturate the CDN–origin bandwidth and exhaust
the connection limits of the origin. What is more concerning is • HTTP/2 Bandwidth Amplification Attack. We find that
that some CDN vendors use only a small set of traffic forwarding
CDNs support only HTTP/2 in the client–CDN connection,
IPs with lower IP-churning rates to establish connections with
the origin. This characteristic provides a great opportunity for an and thus an attacker can abuse the HTTP/2–HTTP/1.1
attacker to effectively degrade the global availability of a website converting behavior of a CDN to launch a bandwidth
just by cutting off specific CDN–origin connections. amplification attack against the origin (e.g., reaching an
amplification factor of 132 for Cloudflare). We analyze
In this work, we examine the CDN request-forwarding behav- the HTTP/2-introduced HPACK compression mechanism,
iors across six well-known CDN vendors and perform real-world which contributes to the threat, and also reveal that the
experiments to evaluate the severity of the threats. Because the concurrent streams and Huffman encoding of HTTP/2 can
threats are caused by flawed trade-offs made by the CDN vendors
between usability and security, we discuss possible mitigation
be abused to further elevate the bandwidth amplification
and received positive feedback after responsible disclosure to the factor.
aforementioned CDN vendors. • Pre-POST Slow HTTP Attack. We find that three out of the
six CDNs we analyze in this study start forwarding HTTP
POST requests just upon receiving the POST header, without
I. I NTRODUCTION waiting for the whole POST message body. This pre-POST
behavior can be abused to exhaust the connection limit of the
Through the deployment of massive surrogate servers in origin and starve other legitimate user requests, resulting in
different geographical locations, often across multiple Internet a slow HTTP DoS attack against the origin. Even worse, the
backbones, a content delivery network (CDN) works as a HTTP/1.1 POST forwarding and HTTP/2 POST forwarding
geographically distributed network, supporting websites into behaviors of these CDNs are both susceptible to this threat.
§ Equal contribution joint first authors.
• Degradation-of-Global-Availability Attack. By sending
requests to the global surrogate IPs (ingress IPs) of each
Corresponding author.
CDN to simulate global client accessing, we perform a
Network and Distributed Systems Security (NDSS) Symposium 2020 large-scale measurement of the distribution of the traffic-
23-26 February 2020, San Diego, CA, USA
ISBN 1-891562-61-4 forwarding IPs (egress IPs) of each CDN. Results show that
https://dx.doi.org/10.14722/ndss.2020.24411 CDNs will assign a small set of egress IPs to access the
www.ndss-symposium.org origin, presenting a lower IP-churning rate. Therefore, this
Surrogate Egress
characteristic can be leveraged by an attacker to efficiently IP 1 IP 1
degrade the availability of a CDN-powered website just by Requests CDN Forwarded Requests
cutting off one or a small set of CDN–origin connections,
Replied Responses
thus preventing most global clients from accessing the Clients
Responses
Surrogate Egress Origin Server
services of the website. For example, with MaxCDN (which IP n IP m
has now been acquired by StackPath [53]), if just one
CDN–origin connection is cut off, more than 90% of global Fig. 1: CDN forwards requests and responses between client
accesses are stopped from fetching resources from the origin and origin.
behind the CDN.
already cached in the CDN, the surrogate serves the contents
In summary, we focus on how to break CDN security
directly to the client. Otherwise, the surrogate will forward the
protection, which is assumed to be trustworthy by many
requests to the origin via egress IPs, as shown in Fig. 1. As a
websites. By performing empirical security analysis on the
result, the CDN separates the traditional end-to-end connection
under-studied CDN back-to-origin connections, we explore
into two stages, i.e., client–CDN connection and CDN–origin
the feasibility of abusing the forwarding behaviors of a CDN
connection, working as a man-in-the-middle between the client
to launch DoS attacks against CDN-powered websites. The
and the origin.
HTTP/2 amplification attack is built on a previous study [7],
but whether it applies to CDN-protected websites has been un- Thus, a CDN, from its point of view, first has to work
explored, and thus we further present a real-world evaluation of as a protocol converter when the protocol of the client–
the HTTP/2 amplification attack through CDN platforms, with CDN connection differs from the protocol used in the CDN–
an in-depth analysis on the HPACK mechanism. Furthermore, origin connection, e.g., the CDN converts client–CDN HTTPS
we find vulnerable HTTP POST-forwarding strategies of CDN connections to CDN–origin HTTP connections, as in [41].
vendors, which can be exploited to launch pre-POST slow Second, the CDN aims to speed up the request delay of the
HTTP attacks. Lastly, based on our large-scale measurements end user, and thus the CDN has to optimize the back-to-
of CDN IP distribution, we exploit the low IP-churning rates of origin forwarding of the request to be as fast as possible.
CDNs, which can be used to launch a degradation-of-global- Lastly, to improve the CDN cache-hit ratio and reduce back-
availability attack. Our results show that these attacks pose a to-origin forwarding, the CDN may add an extra caching layer
severe threat to CDN-powered websites. to cache the resource contents of the websites for its global
surrogates [34]. In the following sections, we shall reveal how
DoS attacks are well known to cause severe damages
these three CDN features are able to lead to our attacks.
against websites, resulting in losses in terms of both money
and trust among the customers of these websites [22]. Because Request-Routing Mechanism. The request-routing mecha-
CDN-powered websites normally operate important business nism is critical for a CDN to provide the optimal CDN
services (e.g., banks, online shopping stores, news servers), a surrogates for processing requests. However, this mechanism
practical DoS attack against CDN-hidden origins can signif- can be bypassed if the surrogate IPs are pre-known; normal
icantly disrupt the businesses and reputations of these web- users can directly send requests to a chosen surrogate IP
sites [58]. without the request-routing stage, which otherwise maps the
Our work can help CDNs to raise security awareness and website domain name with the CDN surrogates. For example,
to enforce stricter secure validation that would result in the Holowczak et al. has shown that CDN-powered websites can
improved security of such critical Internet infrastructure. We be accessed from arbitrary CDN surrogates [30].
have responsibly disclosed our findings to all affected CDNs HTTP/2 Protocol in CDN. The HTTP/1.1 protocol builds the
and have received positive feedback for our work. foundation of the World Wide Web. However, the repeated
Roadmap. In Section II, we first present a background on redundant HTTP headers in each request and response wastes
CDNs and analyze the attack surface. We then sequentially network bandwidth and slows down connection performance.
expound in Sections III to V on the three threats that have Therefore, HTTP/2 was released to address those issues:
been introduced earlier. Possible mitigation are discussed in header compression reduces the unnecessary network traffic in
Section VI, related works are described in Section VII, and HTTP/1.1, and multiplexing streams allows multiple requests
our conclusion is presented in Section VIII. in a single TCP connection [8], [49]. Currently, almost all
CDNs claim that they support the HTTP/2 protocol [60].
II. BACKGROUND AND T HREAT M ODEL Brief Comparison of CDN Vendors. The global CDN service
market is worth billions of dollars and is growing at an
A. Background
increasingly fast rate, with several CDN vendors competing
Content Delivery Network. CDNs are widely used to improve in this booming market. According to CDN market share
the performance and security of websites. For a CDN-powered reports [18], [19], Akamai, CloudFront, Cloudflare, and Fastly
website, the CDN speeds up the connection performance by are the key players in this market [33], and thus these vendors
using request-routing mechanisms (e.g., Anycast or DNS- should naturally be in the scope of most research on CDN.
based) [55] that redirect the web requests of clients to geo- However, because Akamai provides CDN services to enterprise
graphically distributed CDN surrogates (CDN ingress IPs). customers only, it is not included in our study.
Upon receiving a web request, a CDN surrogate first Thus, for this study, we choose six CDN vendors (Cloud-
examines the HTTP header fields, especially the Host and Front, Cloudflare, and Fastly, which are three of the key
the URI header fields. If the requested web resources are players mentioned earlier, together with CDNSun, KeyCDN,
2
and MaxCDN) that provide free-trial account registrations to A. Attack Surface Analysis
individual users. Among these six CDN vendors, five require
email registration only, and only CloudFront requires an extra Half-Done HTTP/2 Support. Almost all CDN vendors claim
credit card verification. From the point of view of attackers, that they currently support HTTP/2 [60]. However, because a
these kinds of CDN vendors, that do not require stringent iden- CDN has to maintain both the client–CDN connection and
tity verification, enable attackers to reveal and exploit specific CDN–origin connection, the HTTP/2 forwarding behaviors
CDN forwarding behaviors, without exposing their sensitive have not yet been studied in detail. Here, we first explore the
personal information. Furthermore, these six CDN vendors HTTP/2 support behavior of a CDN by setting up the origin of
involve two primary request-routing mechanisms: Cloudflare our website as an HTTP/1.1-only server, as an HTTP/2-only
and MaxCDN use Anycast routing, whereas the other four server, and as an HTTP/1.1&HTTP/2 server. We then use the
CDNs use DNS mapping, which helps to increase our research tool curl as a client to access the CDN service in HTTP/2
coverage. In the following sections, although we explore only protocol.
the feasibility of our attacks against these six CDN vendors,
we believe that these attacks are also applicable to other CDN TABLE I: CDN–origin protocol. CDNs support HTTP/2 in
vendors not included in this study. client-facing connections but use only HTTP/1.1 to connect to
the origin.
B. Threat Model client–CDN CDN–origin Protocol CDN–origin Protocol CDN–origin Protocol
Protocol (HTTP/1.1 origin) (HTTP/2 origin) (HTTP/1.1-2 origin)
CloudFront HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
Crafted Legal Requests CDN-rendered Attacking Connections CloudFlare HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
CDNSun HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
Fastly HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
KeyCDN HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
Attacker CDN Origin Server
MaxCDN HTTP/2 HTTP/1.1 HTTP/1.1 HTTP/1.1
Fig. 2: Launching a DoS attack against a CDN-hidden origin.
Experiments have revealed that, as shown in Table I,
CDNs support HTTP/2 in client–CDN connection but use only
In general, websites employ CDNs to improve their se- HTTP/1.1 in the CDN–origin connection, even when the origin
curity and global availability. CDNs normally provide web supports HTTP/2. Consequently, these CDNs have to convert
application firewall (WAF) services to normalize requests to web requests between HTTP/2 and HTTP/1.1 protocols, which
the website origins. Furthermore, CDNs can absorb distributed may introduce new security threats. Even worse, as shown
denial-of-service (DDoS) attacks by leveraging a large number in Table II, these CDNs, except Fastly, turn on HTTP/2
of geo-distributed surrogates. Lastly, by hosting on CDNs, client–CDN connection support by default for their customer
websites can hide their “true” origin IP addresses away from websites, directly exposing their customer websites to possible
potential attackers. protocol conversion threats. Furthermore, the resulting severity
increases because three of the CDNs (Cloudflare, CDNSun,
In this study, we assume an attacker, as a normal client, and KeyCDN) do not even provide an option to turn off such
is able to craft malicious but legal requests to the CDN. We HTTP/2 support.
also assume that the victim website is being hosted on the
CDN (or being unwittingly hosted on the CDN by a mali-
cious CDN customer, further explained in Section VI). Here, TABLE II: HTTP/2 support statuses of the CDNs included in
through an empirical study, we aim to discover some specific this study. Five of the six CDNs enable HTTP/2 support by
but fundamental CDN characteristics that can be abused. In default for their customer websites.
particular, if the forwarding mechanisms of a CDN can be
CloudFront Cloudflare CDNSun Fastly KeyCDN MaxCDN
abused, an attacker may able to manipulate the CDN–origin
Default On Default Off Default On
connections. As a result, these malicious connections may HTTP/2 Support
Configurable
Default On Default On
Configurable
Default On
Configurable
exhaust the limited network resources of the origin, resulting
in a DoS attack against the origin, as shown in Fig. 2. Primer on HTTP/2. The primary goals of HTTP/2 are to
reduce latency and minimize protocol overhead. Primarily, the
HTTP/2 protocol supports multiple concurrent bidirectional
III. HTTP/2 BANDWIDTH A MPLIFICATION ATTACK streams within a single HTTP/2 connection, thus reducing
unnecessary TCP handshake processes and supporting full re-
Up until now, from our experiments, we find that in quest and response multiplexing [8]. For example, in a client–
their client–CDN and CDN–origin connections, CDNs support CDN connection, a client makes one HTTP/2 connection with
HTTP/2 only in client–CDN connection. Thus, when receiving the CDN, using two streams to request resources through
an HTTP/2 request, a CDN has to convert the HTTP/2 request “path1” and “path2,” as shown in Fig. 3.
into an HTTP/1.1 request, which could introduce new attacking
vectors during the protocol conversion process. In this section, In HTTP/1.1, header fields are not compressed. Because
by further exploring the protocol-converting behaviors across web pages have grown to require dozens to hundreds of
the six CDNs, we reveal that all six CDNs can be leveraged requests, the redundant header fields in these requests unnec-
to launch a bandwidth amplification attack against the origins essarily consume bandwidth. Therefore, in HTTP/2, HPACK
of the websites that they are servicing. header compression is introduced primarily to reduce unnec-
3
---------Stream1--------
Get /path1 HTTP/1.1 large-sized value, e.g., cookie with a large-sized value, given
:path: path1
:Authority: server.com Host: victim.com
Cookie: a=large-string
that it is widely used in HTTP requests. Besides the cookie
Cookie: a=large-string
Cookie: b=large-string Cookie: b=large-string field, the attacker can also use other header fields defined in
HTTP/1.1 the HTTP/2 protocol, such as user-agent and referer,
HTTP/2
One TCP Connection
which are also forwarded to the origin. The size of the header
Attacker CDN Get /path2 HTTP/1.1 Origin Server
2 Streams
Host: victim.com field value is limited by the size of the indexed dynamic table,
--------Stream2-------
:path: path2
Cookie: a=large-string
Cookie: b=large-string
which is also negotiated during the HTTP/2 connection. As
shown in Table III, the maximum table entry size across the
CDNs is 3072 B, and the table size is 4 kB. Thus, crafted
attacking HTTP/2 requests can use two header fields to fill
Fig. 3: HTTP/2-HTTP/1.1 conversion has to decompress and the indexed table, resulting in the converted HTTP/1.1 CDN–
expand HTTP/2 requests, resulting in bandwidth amplification. origin requests to have the maximum size.
essary network traffic caused by the repeated request and B. Real-World Attack Analysis
response headers in HTTP/1.1 [49].
Experiment Setup. Based on the previously explained anal-
According to the HPACK mechanism, within the client– ysis, we further evaluate the severity of such an amplification
CDN connection, both the client and CDN (as an HTTP/2 attack across the six CDNs. After deploying an Apache server
server) maintain an indexed dynamic table of previously behind each CDN, we initiate an HTTP/2 connection to each
seen header values, and subsequent repeated header fields of the six CDNs to send attacking requests which are crafted
are substituted as an index referencing a value in the table. as
Because many header fields, e.g., :authority, cookie,
and user-agent are repetitive, this mechanism has a very :path: /? (or /)
high table-hitting ratio. Thus, instead of full header fields, the :scheme: https
:authority: victim.com
substituted indexes are transmitted in the network, reducing :method: GET
the transferred bytes. cookie: A=X...X (a large-sized string)
cookie: B=X...X (a large-sized string)
Accordingly, when the client opens a second stream to send
another “path2” request, the repeated header fields, such as To achieve the maximum amplification ratio, we use two
cookie, are substituted as indexes (and thus these fields are cookie fields with large-sized strings to fill the 4 kB HTTP/2
not shown in “stream2” of Fig. 3). These mechanisms greatly dynamic table. Given that the maximum table entry size is
reduce the header overhead and improve transfer performance. 3072 B, the lengths of two cookie values are calculated
Attack Principle. When a CDN forwards these requests by subtracting additional overhead bytes from the total 4
to the origin, all header fields indexed in HTTP/2 must kB dynamic table size. The additional overhead bytes are
be expanded into HTTP/1.1 requests, leading to bandwidth determined by table entry overhead and other header field
amplification. As shown in Fig. 3, this mechanism results in values, e.g., :authority and user-agent. These two
two large-sized HTTP/1.1 requests with the same large-sized cookies stay the same in all concurrent streams, thus they will
cookie, which leads to a bandwidth amplification in the be transferred in the same way as indexes except for the first
CDN–origin connection, with an amplification ratio of almost stream. Note that we actually use two types of :path header
2. An unsymmetrical bandwidth-consuming attack that takes field values to evaluate the amplification ratio; the reason for
advantage of this mechanism was evaluated by Beckett et al. this will be discussed later in this section.
on an experimental testbed with proxy software Nginx and In our experiments, to explore the impact of concurrent
nghttp2 [7], but to our knowledge, no real-world experiments streams on the amplification ratio, we change the number of
on this kind of attack have been performed yet. concurrent streams within one HTTP/2 connection and use
As we can see, within one HTTP/2 connection, the ampli- tcpdump to capture the traffic in both the client–CDN connec-
fication ratio is linear with respect to the number of concurrent tion and CDN–origin connection to evaluate the amplification
streams. The maximum values for concurrent streams are nego- factor.
tiated when an HTTP/2 connection is established. We measure Experiment Results. According to Fig. 4, when the number
the stream limits of the CDNs and list them in Table III. Across of concurrent streams grows, the bandwidth amplification ratio
all six CDNs, the maximum allowed concurrent streams are all also grows. As shown in Fig. 5, when the number of concurrent
bigger than 100 (the recommended value in the RFC [8]). streams grows, the packet amplification ratio also grows. When
the concurrent streams reach the maximum allowed number
TABLE III: Limits set by CDNs on HTTP/2 streams. for one HTTP/2 connection, the amplification ratio reaches
the maximum. When the stream number grows beyond the
CloudFront Cloudflare CDNSun Fastly KeyCDN MaxCDN maximum allowed number for one HTTP/2 connection, our
Max Concurrent Streams 128 256 128 100 128 100 HTTP/2 client has to wait for the previous streams to close,
Dynamic Table Size
Max Entry Size
4KB
3072B
4KB
3072B
4KB
3072B
4KB
3072B
4KB
3072B
4KB
3072B
and the packets ratio drops, as shown in Fig. 5. Meanwhile,
the bandwidth amplification ratio fluctuates after the maximum
number of allowed concurrent streams is reached.
Therefore, for an attacker to achieve the maximum am-
plification ratio in CDN–origin connections, crafted attacking The bandwidth amplification ratios are summarized in
HTTP/2 requests can all use a header field with the same Table IV; we will illustrate the difference between the 2nd
4
TABLE IV: Maximum amplification ratios across the CDNs.
CloudFront Cloudflare CDNSun Fastly KeyCDN MaxCDN
Streams 128 256 128 100 128 100
Bandwidth Ratio
99.6 132.6 99.5 89.0 96.8 82.3
(:path: /?random string)
Bandwidth Ratio
116.9 166.1 118.7 97.9 105.5 94.7
(:path: /)
• :path header field: In our experiments, we also find
that the :path header field contributes to the amplification
ratio. If we use :path: / directly in all of our attacking
requests, given that it is a predefined index in the static
table1 , only 1 B needs to be transmitted in all of the HTTP/2
Fig. 4: Bandwidth amplification ratio when the number of con-
concurrent streams. However, in the World Wide Web, URLs
current streams increases (:path: /?random_string).
can be of varying lengths, and an attacker normally uses
random URLs to bypass CDN caching or WAF rules. There-
fore, we also append different random strings to the :path
header field in each HTTP/2 stream, in the form :path:
/?random_string.
TABLE V: Length of the :path header field during HTTP/2
and HTTP/1.1 conversion.
HTTP/2 client–CDN Connection HTTP/1.1 CDN–origin Connection
Header Field Transmission Length URL Transmission Length
:path: / 1 bytes / 1 bytes
:path: /?xxxyy 8 bytes /?xxxyy 7 bytes
As shown in Table V, the :path: / header field is also
Fig. 5: Packets amplification ratios when the number of con- converted into 1 B in each resulting HTTP/1.1 connection.
current streams increases (:path: /?random_string). When we use a random value /?random_string in the
:path header field in each HTTP/2 request, the random value
and 3rd rows later. We can see that this HTTP/2–HTTP/1.1 in that field is a non-repetitive value and is therefore not present
conversion threat is realistic; it can break the CDN protection in the dynamic table. According to the HPACK mechanism,
and cause a severe DoS attack against the origin. the value will be encoded in either its raw form or using the
Huffman encoding form (the shorter of the two). In Table V,
Analysis of Amplification Factors. From the given illus- we can see that :path: /?xxxyy consumes 8 B in HTTP/2
tration, we can see that the bandwidth amplification ratio is (1 B for the index of :path field, and 7 B for /?xxxyy 2 ),
determined primarily by the number of concurrent streams. and the converted URL in each HTTP/1.1 request will be 7 B.
From further analysis of the HTTP/2 specification, we also In our experiments, we send these two types of :path
find other influencing factors, such as the Huffman encoding headers to evaluate the bandwidth amplification ratio and
and the :path header field, that contribute to the bandwidth obtain different results, as listed in Table IV. The reason
amplification ratio. for these differences is that when the number of concurrent
HTTP/2 streams grows, the length of the :path header field
• Huffman encoding: In the HPACK compression mech- begins to influence the amplification ratio. In our experiments,
anism, Huffman encoding is applied to further compress the when the :path: / form is used, the network traffic of the
header values. This Huffman code is statistically generated for attacker–CDN connection front-end traffic (FB) is in the order
HTTP headers, wherein ASCII digits and lowercase letters are of thousands, e.g., 5,000 bytes per second, whereas the network
given shorter encoding [49]. The shortest encoding for one traffic of the CDN–origin connection back-end traffic (BB) is
byte is 5 bits long; therefore, the highest compression ratio in the order of millions, e.g., 600,000 bytes per second. The
achievable for one byte is 8 bits : 5 bits Huffman code (37.5% amplification ratio is BB /FB . On the other hand, when the
smaller). Thus, besides the concurrent streams, Huffman en- :path: /?xxxyy form is used, and we send n (e.g., 100,
coding can also be abused to maximize the amplification 128, or 256, i.e., the maximum values in Table III) concurrent
ratio. Because HTTP/2 headers are firstly compressed with HTTP/2 streams, the network traffic of the attacker–CDN
Huffman encoding and will be decompressed in the CDN– connection, compared with for the ‘:path: / form, will be
origin connections, the resulting HTTP/1.1 headers will be
nearly 8/5 = 160% larger in size. 1 According to the HTTP/2 specification, the HPACK mechanism uses
Therefore, in our experiments, the two cookie values are an additional static table to predefine common header fields associated with
composed of the characters 0, 1, 2, a, c, e, i, o, s, or t, frequently occurring values, e.g., :path: / is predefined in the indexed table
as index 4 [49].
which have the shortest Huffman encoding (5 bits) defined 2 We generate a different random string in each request. Here, we neglect
in the RFC [49]. With Huffman encoding, we achieve the the chances that the Huffman encoding may compress the random string to
amplification ratios listed in Table IV. shorter than 7 B.
5
(FB + 7n) (n HTTP/2 streams, 8 − 1 = 7 B larger in each allowing the attacker to exhaust the connection limits of the
stream), and the network traffic of the CDN–origin connections origin. In this section, we first review the traditional slow
will be (BB + 6n), (n HTTP/1.1 connections, 7 − 1 = 6 B HTTP attack, and then we further analyze how three out of
larger in each connection). Thus, for the :path: /?xxxyy the six CDNs are susceptible to this pre-POST threat.
form, the amplification ratio will be (BB + 6n)/(FB + 7n).
As we have illustrated, FB is in the order of thousands, Primer on Slow HTTP DoS Attack. According to the
whereas BB is in the order of millions. Therefore, we have the Kaspersky Q4 2018 Intelligence Report [47], the total duration
following mathematical inequality: of HTTP-related attacks has been growing, accounting for
about 80 percent of DDoS attack time for the whole year. This
BB BB + 6n report finding reveals that attackers are turning to sophisticated,
> . (1)
FB F B + 7n mixed HTTP attack techniques, such as slow HTTP DoS
attacks.
For example, assuming F B = 5, 000 and BB = 600, 000
for simplicity, when we use 128 or 256 for n (the number of Compared with brute-force flooding attacks, a slow HTTP
concurrent streams), the inequality becomes DoS attack is stealthier and more efficient. The slow HTTP
DoS attack takes advantages of the HTTP protocol having been
BB 600000 600000+128 ∗ 6 600000+256 ∗ 6 designed to keep the connection open until the receiving of
= > = 101.9 > = 88.6. (2)
FB 5000 5000+128 ∗ 7 5000+256 ∗ 7 data is finished [23], [55]. Therefore, different stages of the
request flow can be abused to launch slow HTTP DoS attacks.
Therefore, we can see that, to achieve the maximum A slow Header attack sends the partial header, a slow Read
amplification ratio, the HTTP/2 attacking requests should be attack intentionally receives response data slowly, and a slow
specially crafted to use the HPACK indexing mechanism as POST attack sends the posted data at an alarmingly slow rate.
much as possible. All these attacks aim to keep massive connections with the
target server for as long as possible, leading to an exhaustion
Summary. For the attack, we conducted a controlled ex- of the concurrent connections of the target and starving other
periment to obtain the network traffic amplification ratio by normal user requests [29], [48], [56].
establishing just one HTTP/2 connection with one CDN node.
However, from the perspective of an attacker as a client, he Attack Principle. Generally, to prevent unavailability due to
can initiate thousands of HTTP/2 connections with different DoS attacks, the CDN decouples attacker–CDN and CDN–
CDN nodes (e.g., we found 128,906 CloudFront IPs, which origin connections and absorbs any flooding traffic. However,
can be used for the attack; please refer to Table IX for the the applicability of slow HTTP DoS attack against CDN-
number of IPs of other CDNs). According to the amplification powered websites remains under-studied.
ratio we obtained, the network bandwidth of CDN–origin With our further analysis and real-world experiments, we
connection can be seriously consumed, adversely influencing find that each of the six CDNs forwards requests only until it
the performance of the origin. receives the full HTTP header, and is therefore able to defend
Given that HTTP/2 support is turned on by default across against slow Header attacks. Furthermore, when forwarding
five of these six CDNs, and cannot even be turned off across an HTTP GET request, the CDN–origin transmission is in-
three of the CDNs, we can see that this threat is severe and dependent of the attacker client–CDN transmission; therefore,
affects all websites hosted on these CDNs. the CDN is able to stop slow Read attacks.
However, we find that CDNs present two different POST-
IV. P RE -POST S LOW HTTP ATTACK forwarding behaviors. When a CDN receives a POST request
In this section, we introduce the pre-POST slow HTTP for the origin, the CDN faces the choice of when to forward
attack, which leverages CDN infrastructure to perform a DoS the POST request to the origin. For simplicity, the CDN
attack against the origin. Compared with traditional DoS can forward the POST request only after it finishes receiving
attacks that rely on massive bots [1], [43], [51], this attack is the whole POST message. However, the POST request may
stealthier and harder for the origin to defend against, because contain a large-sized message body, which would take a long
the crafted requests are legal and are initiated from the CDN. time to receive and therefore delay the request forwarding. The
CDN can also start forwarding the POST request just upon fin-
ishing receiving the POST request header and then sequentially
A. Attack Surface Analysis
forward the subsequently received POST message within the
The pre-POST slow HTTP attack aims to exhaust the same HTTP connection. This pre-POST-forwarding behavior
connection limits of the origin and starve other legitimate can certainly facilitate the origin into receiving the POST
user requests. To the origin, the attack acts the same as a request earlier; however, it also enables an attacker to keep
traditional slow POST attack [23], [55]. Normally, as the the CDN–origin connections open for as long as possible.
CDN decouples the client–CDN (including attacker–CDN) and
CDN–origin connections, the CDN naturally defends against B. Real-World Attack Analysis
traditional slow POST attacks. However, with experiments, we
find that three out of the six CDNs start forwarding HTTP Experiment Setup. In our experiment, we set up a self-built
POST requests just upon receiving the POST header, without Apache web-server and deploy it as a website origin behind
waiting for the whole POST message body. We reveal that this the six CDNs, one at a time. The concurrent connections limit
pre-POST behavior empowers an attacker to keep the CDN– of the Apache web server is configured with a default value
origin connections to remain open as long as possible, thus of 1000 [3].
6
From the view of an attacker, we craft POST requests
to explore the request-forwarding behaviors of the CDNs. In
particular, to POST a large message, the attacker can specify
the size directly in the Content-Length header field, or use
Chunked-Encoding to send dynamically generated data,
both aiming to send the POST message slowly. Here, for sim-
plicity, we specify the exact size of the HTTP message body
with the Content-Length field, and the POST message
body is sent quite slowly, taking 300 s to finish transmission.
POST /login.php? HTTP/1.1
Host: www.victim.com
Content-Length: 300
0101..... (300 bytes, 1 byte sent per second) Fig. 6: Establishing more than 1000 connections, from 100 s
to 400 s.
At the same time, at the website origin, we use the tool
tcpdump to capture the timestamp (relative to our request
sending time) upon receiving the CDN-forwarded HTTP POST
request, and how long the CDN–origin connection is kept
open. After sending 1000 concurrent POST requests, and
repeating this procedure for 30 times, we obtain the averaged
results shown in Table VI.
TABLE VI: Time data from sending slow POST requests
(lasting 300 s). Three CDNs start forwarding POST requests
as soon as they receive the POST header.
CloudFront Cloudflare CDNSun Fastly KeyCDN MaxCDN
Request
Receiving Time
0.87s 300.29s 299.92s 0.55s 299.79s 0.74s Fig. 7: Response time of a normal client during a slow HTTP
Connection
Keep-open Time
298.89s 0.12s 0.34s 299.32s 0.37s 15.01s POST attack.
connection resources are exhausted, and other requests are
We can see that CloudFront and Fastly start to forward starving. Thus, as shown in Fig. 7, the request delays of a
POST requests as soon as they receive the forwarding request normal client rise to 90 s for CloudFront (returns HTTP 504
header, whereas CDNSun, KeyCDN, and Cloudflare start to Gateway Time-out) and 15 s for Fastly (returns HTTP 503
forward a POST request only after receiving the whole mes- Service Unavailable), demonstrating the success of the DoS
sage. MaxCDN also starts to forward POST requests 0.74 attacks.
s later but aborts the connection when the kept-open time
exceeds 15 s. Because MaxCDN will abort the POST connection after 15
s, we periodically start 100 new concurrent connections every
Apparently, for CloudFront, Fastly, and MaxCDN, the second during the attack period. As shown in Fig. 6, the con-
kept-open time of the CDN–origin connection depends on nection number fluctuates at around 1500, as MaxCDN aborts
the kept-open time of the client–CDN connection, which is the previous 15-s-lasting connections sequentially. Meanwhile,
directly under the control of the client, and thus of a potential as shown in Fig. 7, the request delay of a normal client
attacker. Therefore, this pre-POST-forwarding behavior can be fluctuates below 15 s, as the normal client request competes
leveraged to launch a slow HTTP DoS attack: an attacker can with attacking requests for the released connection resources.
establish and maintain hundreds or even thousands of these This phenomenon of MaxCDN demonstrates a quality of
POST connections concurrently, leveraging the CDN (and thus service (QoS) attack, which aims to degrade performance
adversely affecting the origin). It will quickly exhaust all the rather than completely disable the service.
connection resources of the origin and starve other normal
requests, breaking the DoS protection given by the CDN. HTTP/2 Pre-POST Attack. Given that CDNs support HTTP/2
in client–CDN connections (as explained in Section III), we
Experiment Results. Further, we evaluate such pre-POST also further evaluate slow HTTP/2 POST attacks against the
attack against our self-built origin web server (with a connec- origin. To employ the multiplex stream feature of HTTP/2, we
tion limit of 1000), through CloudFront, Fastly, and MaxCDN establish 10 simultaneous HTTP/2 connections with the CDN
for 300 s. From another vantage point, as a normal client, and send 100 POST requests in each HTTP/2 connection. The
we periodically measure the client–CDN–origin request delay POST requests are crafted as follows:
every 5 s to probe whether the connection resources of the :method: POST
origin are exhausted or not. :scheme: https
:authority: www.victim.com
For CloudFront and Fastly, to exhaust the 1000-connection :path: /login.php?cdn=&a=