Cyber Resilience

CVE-2025-43859

Critical

Published: 24 April 2025

Published
24 April 2025
Modified
15 April 2026
KEV Added
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Patch
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CVSS Score v3.1 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS Score 0.0024 47.7th percentile
Risk Priority 18 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2025-43859 is a critical-severity HTTP Request/Response Smuggling (CWE-444) vulnerability. Its CVSS base score is 9.1 (Critical).

Operationally, ranked at the 47.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

EU & UK References

Vulnerability details

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.…

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Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

CWE(s)
CWE-444

Related Threats

No named actor attribution yet. ATT&CK technique mapping in progress for this CVE.

Affected Assets

Mitigating Controls

No mitigating controls mapped yet. The per-CVE control annotator has not reached this CVE.

References