When malformed or abnormal HTTP requests are interpreted by one or more entities in the data flow between the user and the web server, such as a proxy or firewall, they can be interpreted inconsistently, allowing the attacker to "smuggle" a request to one device without the other device being aware of it. 1000 Weakness ChildOf 436 699 Category ChildOf 442 888 Category ChildOf 896 Request smuggling can be performed due to a multiple interpretation error, where the target is an intermediary or monitor, via a consistency manipulation (Transfer-Encoding and Content-Length headers). Resultant from CRLF injection. Architecture and Design Implementation Integrity Non-Repudiation Access_Control Unexpected state Hide activities Bypass protection mechanism An attacker could create a request to exploit a number of weaknesses including 1) the request can trick the web server to associate a URL with another URLs webpage and caching the contents of the webpage (web cache poisoning attack), 2) the request can be structured to bypass the firewall protection mechanisms and gain unauthorized access to a web application, and 3) the request can invoke a script or a page that returns client credentials (similar to a Cross Site Scripting attack). Implementation Use a web server that employs a strict HTTP parsing procedure, such as Apache (See paper in reference). Implementation Use only SSL communication. Implementation Terminate the client session after each request. System Configuration Turn all pages to non-cacheable. In the following example, a malformed HTTP request is sent to a website that includes a proxy server and a web server with the intent of poisoning the cache to associate one webpage with another malicious webpage. POST http://www.website.com/foobar.html HTTP/1.1 Host: www.website.com Connection: Keep-Alive Content-Type: application/x-www-form-urlencoded Content-Length: 0 Content-Length: 44 GET /poison.html HTTP/1.1 Host: www.website.com Bla: GET http://www.website.com/page_to_poison.html HTTP/1.1 Host: www.website.com Connection: Keep-Alive When this request is sent to the proxy server, the proxy server parses the POST request in the first seven lines, and encounters the two "Content-Length" headers. The proxy server ignores the first header, so it assumes the request has a body of length 44 bytes. Therefore, it treats the data in the next three lines that contain exactly 44 bytes as the first request's body. The proxy then parses the last three lines which it treats as the client's second request. The request is forwarded by the proxy server to the web server. Unlike the proxy, the web server uses the first "Content-Length" header and considers that the first POST request has no body, and the second request is the line with the first GET (note that the second GET is parsed by the web server as the value of the "Bla" header). The requests the web server sees are "POST /foobar.html" and "GET /poison.html", so it sends back two responses with the contents of the "foobar.html" page and the "poison.html" page, respectively. The proxy matches these responses to the two requests it thinks were sent by the client "POST /foobar.html" and "GET /page_to_poison.html". If the response is cacheable, the proxy caches the contents of "poison.html" under the URL "page_to_poison.html", and the cache is poisoned! Any client requesting "page_to_poison.html" from the proxy would receive the "poison.html" page. When a website includes both a proxy server and a web server some protection against this type of attack can be achieved by installing a web application firewall, or use a web server that includes a stricter HTTP parsing procedure or make all webpages non-cacheable. Additionally, if a web application includes a Java servlet for processing requests, the servlet can check for multiple "Content-Length" headers and if they are found the servlet can return an error response thereby preventing the poison page to be cached, as shown below. Java protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set up response writer object ... try { // check for multiple content length headers Enumeration contentLengthHeaders = request.getHeaders("Content-Length"); int count = 0; while (contentLengthHeaders.hasMoreElements()) { count++; } if (count > 1) { // output error response } else { // process request } } catch (Exception ex) {...} } In the following example, a malformed HTTP request is sent to a website that includes a web server with a firewall with the intent of bypassing the web server firewall to smuggle malicious code into the system.. POST /page.asp HTTP/1.1 Host: www.website.com Connection: Keep-Alive Content-Length: 49223 zzz...zzz ["z" x 49152] POST /page.asp HTTP/1.0 Connection: Keep-Alive Content-Length: 30 POST /page.asp HTTP/1.0 Bla: POST /page.asp?cmd.exe HTTP/1.0 Connection: Keep-Alive When this request is sent to the web server, the first POST request has a content-length of 49,223 bytes, and the firewall treats the line with 49,152 copies of "z" and the lines with an additional lines with 71 bytes as its body (49,152+71=49,223). The firewall then continues to parse what it thinks is the second request starting with the line with the third POST request. Note that there is no CRLF after the "Bla: " header so the POST in the line is parsed as the value of the "Bla:" header. Although the line contains the pattern identified with a worm ("cmd.exe"), it is not blocked, since it is considered part of a header value. Therefore, "cmd.exe" is smuggled through the firewall. When the request is passed through the firewall the web server the first request is ignored because the web server does not find an expected "Content-Type: application/x-www-form-urlencoded" header, and starts parsing the second request. This second request has a content-length of 30 bytes, which is exactly the length of the next two lines up to the space after the "Bla:" header. And unlike the firewall, the web server processes the final POST as a separate third request and the "cmd.exe" worm is smuggled through the firewall to the web server. To avoid this attack a Web server firewall product must be used that is designed to prevent this type of attack. CVE-2005-2088 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2089 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2090 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2091 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2092 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2093 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. CVE-2005-2094 Web servers allow request smuggling via inconsistent Transfer-Encoding and Content-Length headers. Chaim Linhart Amit Klein Ronen Heled Steve Orrin HTTP Request Smuggling http://www.cgisecurity.com/lib/HTTP-Request-Smuggling.pdf HTTP Request Smuggling HTTP Request Smuggling 26 105 33 PLOVER Eric Dalci Cigital 2008-07-01 updated Potential_Mitigations, Time_of_Introduction CWE Content Team MITRE 2008-09-08 updated Name, Relationships, Other_Notes, Taxonomy_Mappings CWE Content Team MITRE 2009-05-27 updated Name, Related_Attack_Patterns CWE Content Team MITRE 2010-02-16 updated Taxonomy_Mappings CWE Content Team MITRE 2011-06-01 updated Common_Consequences CWE Content Team MITRE 2012-05-11 updated Common_Consequences, Relationships CWE Content Team MITRE 2012-10-30 updated Demonstrative_Examples, Potential_Mitigations HTTP Request Smuggling Interpretation Conflict in Web Traffic (aka 'HTTP Request Smuggling') Inconsistent Interpretation of HTTP Requests (aka 'HTTP Request Smuggling')