CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies

Impact

A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.

Details

HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:

• chunk length
• \r\n
• length bytes of content
• \r\n

In versions of h11 up to 0.14.0, h11 instead parsed them as:

• chunk length
• \r\n
• length bytes of content
• any two bytes

i.e. it did not validate that the trailing \r\n bytes were correct, and if you put 2 bytes of garbage there it would be accepted, instead of correctly rejecting the body as malformed.

By itself this is harmless. However, suppose you have a proxy or reverse-proxy that tries to analyze HTTP requests, and your proxy has a _different_ bug in parsing Chunked-Encoding, acting as if the format is:

• chunk length
• \r\n
• length bytes of content
• more bytes of content, as many as it takes until you find a \r\n

For example, pound had this bug -- it can happen if an implementer uses a generic "read until end of line" helper to consumes the trailing \r\n.

In this case, h11 and your proxy may both accept the same stream of bytes, but interpret them differently. For example, consider the following HTTP request(s) (assume all line breaks are \r\n):

GET /one HTTP/1.1
Host: localhost
Transfer-Encoding: chunked

5
AAAAAXX2
45
0

GET /two HTTP/1.1
Host: localhost
Transfer-Encoding: chunked

0

Here h11 will interpret it as two requests, one with body AAAAA45 and one with an empty body, while our hypothetical buggy proxy will interpret it as a single request, with body AAAAXX20\r\n\r\nGET /two .... And any time two HTTP processors both accept the same string of bytes but interpret them differently, you have the conditions for a "request smuggling" attack. For example, if /two is a dangerous endpoint and the job of the reverse proxy is to stop requests from getting there, then an attacker could use a bytestream like the above to circumvent this protection.

Even worse, if our buggy reverse proxy receives two requests from different users:

GET /one HTTP/1.1
Host: localhost
Transfer-Encoding: chunked

5
AAAAAXX999
0

GET /two HTTP/1.1
Host: localhost
Cookie: SESSION_KEY=abcdef...

...it will consider the first request to be complete and valid, and send both on to the h11-based web server over the same socket. The server will then see the two concatenated requests, and interpret them as _one_ request to /one whose body includes /two's session key, potentially allowing one user to steal another's credentials.

Patches

Fixed in h11 0.15.0.

Workarounds

Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

Credits

Reported by Jeppe Bonde Weikop on 2025-01-09.