CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

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Description

The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

Extended Description

HTTP requests or responses ("messages") can be malformed or unexpected in ways that cause web servers or clients to interpret the messages in different ways than intermediary HTTP agents such as load balancers, reverse proxies, web caching proxies, application firewalls, etc. For example, an adversary may be able to add duplicate or different header fields that a client or server might interpret as one set of messages, whereas the intermediary might interpret the same sequence of bytes as a different set of messages. For example, discrepancies can arise in how to handle duplicate headers like two Transfer-encoding (TE) or two Content-length (CL), or the malicious HTTP message will have different headers for TE and CL. The inconsistent parsing and interpretation of messages can allow the adversary to "smuggle" a message to the client/server without the intermediary being aware of it. This weakness is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents.

ThreatScore

Threat Mapped score: 0.0

Industry: Finiancial

Threat priority: Unclassified

Observed Examples (CVEs)

Related Attack Patterns (CAPEC)

Attack TTPs

N/A

Modes of Introduction

Phase Note
Implementation N/A

Common Consequences

Potential Mitigations

Applicable Platforms

Demonstrative Examples

Intro: 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.

Body: When this request is sent to the proxy server, the proxy server parses the first four lines of the POST request and encounters the two "Content-Length" headers. The proxy server ignores the first header, so it assumes the request has a body of length 54 bytes. Therefore, it treats the data in the next three lines that contain exactly 54 bytes as the first request's body:

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: 54 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

Intro: 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.

Body: 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.

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

Intro: The interpretation of HTTP responses can be manipulated if response headers include a space between the header name and colon, or if HTTP 1.1 headers are sent through a proxy configured for HTTP 1.0, allowing for HTTP response smuggling. This can be exploited in web browsers and other applications when used in combination with various proxy servers. For instance, the HTTP response interpreted by the front-end/client HTTP agent/entity - in this case the web browser - can interpret a single response from an adversary-compromised web server as being two responses from two different web sites. In the Example below, notice the extra space after the Content-Length and Set-Cookie headers.

HTTP/1.1 200 OK Date: Fri, 08 Aug 2016 08:12:31 GMT Server: Apache (Unix) Connection: Keep-Alive Content-Encoding: gzip Content-Type: text/html Content-Length : 2345 Transfer-Encoding: chunked Set-Cookie : token="Malicious Code" <HTML> ... "Malicious Code"

Notes

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