Know every vulnerabilitybefore it knows you.
DevGuard continuously monitors your dependencies and alerts you when CVEs like this one affect your stack — with real-time threat intelligence built for developers.
GHSA-vqfr-h8mv-ghfj
No affected components available
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\nlengthbytes of content\r\n
In versions of h11 up to 0.14.0, h11 instead parsed them as:
- chunk length
\r\nlengthbytes 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\nlengthbytes 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.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
Browse More
Continuously monitor your dependencies and get alerted when vulnerabilities like this one affect your stack.
Checkout DevGuard