HTTP Request Smuggling
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Today's web applications frequently employ chains of HTTP servers between users and the ultimate application logic. Users send requests to a front-end server (sometimes called a load balancer or reverse proxy) and this server forwards requests to one or more back-end servers. This type of architecture is increasingly common, and in some cases unavoidable, in modern cloud-based applications.
When the front-end server forwards HTTP requests to a back-end server, it typically sends several requests over the same back-end network connection, because this is much more efficient and performant. The protocol is very simple: HTTP requests are sent one after another, and the receiving server parses the HTTP request headers to determine where one request ends and the next one begins:
In this situation, it is crucial that the front-end and back-end systems agree about the boundaries between requests. Otherwise, an attacker might be able to send an ambiguous request that gets interpreted differently by the front-end and back-end systems:
Here, the attacker causes part of their front-end request to be interpreted by the back-end server as the start of the next request. It is effectively prepended to the next request, and so can interfere with the way the application processes that request. This is a request smuggling attack, and it can have devastating results.
Most HTTP request smuggling vulnerabilities arise because the HTTP specification provides two different ways to specify where a request ends: the Content-Length header and the Transfer-Encoding header.
The Content-Length header is straightforward: it specifies the length of the message body in bytes. For example:
The Transfer-Encoding header can be used to specify that the message body uses chunked encoding. This means that the message body contains one or more chunks of data. Each chunk consists of the chunk size in bytes (expressed in hexadecimal), followed by a newline, followed by the chunk contents. The message is terminated with a chunk of size zero. For example:
Since the HTTP specification provides two different methods for specifying the length of HTTP messages, it is possible for a single message to use both methods at once, such that they conflict with each other. The HTTP specification attempts to prevent this problem by stating that if both the Content-Length and Transfer-Encoding headers are present, then the Content-Length header should be ignored. This might be sufficient to avoid ambiguity when only a single server is in play, but not when two or more servers are chained together. In this situation, problems can arise for two reasons:
Some servers do not support the Transfer-Encoding header in requests.
Some servers that do support the Transfer-Encoding header can be induced not to process it if the header is obfuscated in some way.
If the front-end and back-end servers behave differently in relation to the (possibly obfuscated) Transfer-Encoding header, then they might disagree about the boundaries between successive requests, leading to request smuggling vulnerabilities.
Request smuggling attacks involve placing both the Content-Length header and the Transfer-Encoding header into a single HTTP request and manipulating these so that the front-end and back-end servers process the request differently. The exact way in which this is done depends on the behavior of the two servers:
CL.TE: the front-end server uses the Content-Length header and the back-end server uses the Transfer-Encoding header.
TE.CL: the front-end server uses the Transfer-Encoding header and the back-end server uses the Content-Length header.
TE.TE: the front-end and back-end servers both support the Transfer-Encoding header, but one of the servers can be induced not to process it by obfuscating the header in some way.
Here, the front-end server uses the Content-Length header and the back-end server uses the Transfer-Encoding header. We can perform a simple HTTP request smuggling attack as follows:
The front-end server processes the Content-Length header and determines that the request body is 13 bytes long, up to the end of SMUGGLED. This request is forwarded on to the back-end server.
The back-end server processes the Transfer-Encoding header, and so treats the message body as using chunked encoding. It processes the first chunk, which is stated to be zero length, and so is treated as terminating the request. The following bytes, SMUGGLED, are left unprocessed, and the back-end server will treat these as being the start of the next request in the sequence.
Here, the front-end server uses the Transfer-Encoding header and the back-end server uses the Content-Length header. We can perform a simple HTTP request smuggling attack as follows:
The front-end server processes the Transfer-Encoding header, and so treats the message body as using chunked encoding. It processes the first chunk, which is stated to be 8 bytes long, up to the start of the line following SMUGGLED. It processes the second chunk, which is stated to be zero length, and so is treated as terminating the request. This request is forwarded on to the back-end server.
The back-end server processes the Content-Length header and determines that the request body is 3 bytes long, up to the start of the line following 8. The following bytes, starting with SMUGGLED, are left unprocessed, and the back-end server will treat these as being the start of the next request in the sequence.
Here, the front-end and back-end servers both support the Transfer-Encoding header, but one of the servers can be induced not to process it by obfuscating the header in some way.
There are potentially endless ways to obfuscate the Transfer-Encoding header. For example:
Each of these techniques involves a subtle departure from the HTTP specification. Real-world code that implements a protocol specification rarely adheres to it with absolute precision, and it is common for different implementations to tolerate different variations from the specification. To uncover a TE.TE vulnerability, it is necessary to find some variation of the Transfer-Encoding header such that only one of the front-end or back-end servers processes it, while the other server ignores it.
Depending on whether it is the front-end or the back-end server that can be induced not to process the obfuscated Transfer-Encoding header, the remainder of the attack will take the same form as for the CL.TE or TE.CL vulnerabilities already described.
HTTP request smuggling vulnerabilities arise in situations where the front-end server and back-end server use different mechanisms for determining the boundaries between requests. This may be due to discrepancies between whether HTTP/1 servers use the Content-Length header or chunked transfer encoding to determine where each request ends. In HTTP/2 environments, the common practice of downgrading HTTP/2 requests for the back-end is also fraught with issues and enables or simplifies a number of additional attacks.
To prevent HTTP request smuggling vulnerabilities, we recommend taking the following general steps:
Use HTTP/2 end to end and disable HTTP downgrading if possible. HTTP/2 uses a robust mechanism for determining the length of requests and, when used end to end, is inherently protected against request smuggling. If you can't avoid HTTP downgrading, make sure you validate the rewritten request against the HTTP/1.1 specification. For example, reject requests that contain newlines in the headers, colons in header names, and spaces in the request method.
Make the front-end server normalize ambiguous requests and make the back-end server reject any that are still ambiguous, closing the TCP connection in the process.
As we've demonstrated in the learning materials, disabling reuse of back-end connections will help to mitigate certain kinds of attack, but this still doesn't protect you from attacks.
