Summary
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.
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.
Impact
A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.
CVE-2025-43859 has a CVSS score of 9.1 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.16.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Fixed in h11 0.15.0.
Frequently Asked Questions
- What is CVE-2025-43859? CVE-2025-43859 is a critical-severity security vulnerability in h11 (pip), affecting versions < 0.16.0. It is fixed in 0.16.0.
- How severe is CVE-2025-43859? CVE-2025-43859 has a CVSS score of 9.1 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of h11 are affected by CVE-2025-43859? h11 (pip) versions < 0.16.0 is affected.
- Is there a fix for CVE-2025-43859? Yes. CVE-2025-43859 is fixed in 0.16.0. Upgrade to this version or later.
- Is CVE-2025-43859 exploitable, and should I be worried? Whether CVE-2025-43859 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-43859 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-43859? Upgrade
h11to 0.16.0 or later.