CVE-2025-43859

h11 accepts some malformed Chunked-Encoding bodies

Description

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

Category

9.1
CVSS
Severity: Critical
CVSS 3.1 •
EPSS 0.04%
Affected: python-hyper h11
Published at:
Updated at:

References

Link Tags
https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj
https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed

Frequently Asked Questions

What is the severity of CVE-2025-43859?
CVE-2025-43859 has been scored as a critical severity vulnerability.
How to fix CVE-2025-43859?
To fix CVE-2025-43859, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2025-43859 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2025-43859 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2025-43859?
CVE-2025-43859 affects python-hyper h11.
This platform uses data from the NIST NVD, MITRE CVE, MITRE CWE, First.org and CISA KEV but is not endorsed or certified by these entities. CVE is a registred trademark of the MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. CWE is a registred trademark of the MITRE Corporation and the authoritative source of CWE content is MITRE's CWE web site.
© 2025 Under My Watch. All Rights Reserved.