The h11 library fixes a critical vulnerability

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[TLP:CLEAR] The h11 library fixes a critical vulnerability

the h11 Python library for implementing the HTTP/1.1 protocol fixes a critical vulnerability with version 0.16.0 [1].

h11 - request smuggling (CVE-2025-43859)

CVSS 9.1 (Critical)

Allows an unauthenticated remote attacker, due to insufficient validation of the termination of transferred data blocks (Chunked-Encoding bodies) combined with another parsing flaw in a (reverse) proxy server, to bypass server restrictions and gain unauthorized access to protected endpoints, or possibly compromise user sessions and potentially obtain user credentials [1].

The vulnerability is present in product:

*:h11;<=0.15.0;

CVSS: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

More information:

CVE-2025-43859 at cve.org, NVD
CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') at cwe.mitre.org

The vulnerability was publicly disclosed 24. 4. 2025.

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[TLP:CLEAR] The h11 library fixes a critical vulnerability

h11 - request smuggling (CVE-2025-43859)

CVSS 9.1 (Critical)

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