CVE-2025-43859
ADVISORY - githubSummary
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.
Common Weakness Enumeration (CWE)
Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
GitHub
3.9
CVSS SCORE
9.1critical| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| h11 | pypi | - | - | <0.16.0 | 0.16.0 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any or all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
There is no impact to availability within the impacted component.
NIST
3.9
CVSS SCORE
9.1criticalDebian
-
CVSS SCORE
N/AlowUbuntu
-
CVSS SCORE
N/AmediumChainguard
CGA-4gc2-jvqg-rhhf
-
Chainguard
CGA-57x2-gghw-73hq
-
Chainguard
CGA-6568-886h-p4m7
-
Chainguard
CGA-65h3-fhpc-phj8
-
Chainguard
CGA-7qwr-ggrp-xcp2
-
Chainguard
CGA-899w-m26f-w6pw
-
Chainguard
CGA-fmxm-mhm4-g2g9
-
Chainguard
CGA-ggwp-jf43-x8c8
-
Chainguard
CGA-gv9p-2wx7-82mw
-
Chainguard
CGA-hjm6-8g6x-q4vc
-
minimos
MINI-qmq9-59w4-rgr3
-