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CVE-2025-43859

Critical

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.

SeverityCritical
CVSS9.1
Exploit probability—
ProductH11
PublishedApr 24, 2025
First seenApr 24, 2025
Weakness typeCWE-444CWE-444

How to fix

Update
  • h110.16.0 or later

Affected products

ProductAffected versionsFixed in
h11PyPIv < 0.16.00.16.0

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