Is there a public exploit for CVE-2025-43859?

No public exploit for CVE-2025-43859 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2025-43859?

CVE-2025-43859 scores CVSS 9.1 (v3.1), rated critical. EPSS puts the chance of exploitation in the next 30 days at 0.59%.

What does CVE-2025-43859 affect?

CVE-2025-43859 affects python-hyper h11.

9.1 Critical

h11 is a Python implementation of HTTP/1.1.

CVE-2025-43859

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.

Assessment

CVSS: 9.1 (v3.1)
Severity: Critical
EPSS: 0.59% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 24 April 2025
Last updated: 17 June 2026
Affected: python-hyper · h11
Weakness: CWE-444
Assigner: security-advisories@github.com

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

At the source

References

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