Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling')
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.
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
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.
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.
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..
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.
Weaknesses in this category are related to the A04 "Insecure Design" category in the OWASP Top Ten 2021.
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