CVE-2025-43859

Name: CVE-2025-43859
Creator: NVD / NIST
Published: 2025-04-24T19:15:47.06+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.1/10EPSS 0.52%

Last modified Jun 17, 2026

CVE-2025-43859 is a critical-severity vulnerability rated 9.1/10 on the CVSS scale. 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. EPSS estimates a 0.52% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

9.1/10

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

EPSS Probability

0.52%

40.3th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

References

Timeline

Published: Apr 24, 2025
Last Modified: Jun 17, 2026
Status: Deferred

Frequently Asked Questions

What is CVE-2025-43859?

How severe is CVE-2025-43859?

CVE-2025-43859 has a CVSS score of 9.1/10 (CRITICAL severity). The EPSS model estimates a 0.52% probability of exploitation in the next 30 days.

How do I fix CVE-2025-43859?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2025-43859?

Run a free Strix scan to check your systems for this vulnerability.

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Source: NVD / NIST