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Vulnerabilities

Security fix · CVE-2025-43859

h11 is a Python implementation of HTTP/1.1.

Published Apr 24, 2025 · catalog updated Apr 24, 2025 · h11 is a Python

Check if you run it

No confirmed exploitation is on record for CVE-2025-43859. EPSS is 0.58%. Act only if a listed version of h11 is a Python is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (h11 is a Python).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Critical9.1
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.58%
Remediation
Version ranges on file
Affected products
vendor python-hyper h11 < 0.16.0

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • vendor python-hyper h11 < 0.16.0< 0.16.0

How to fix CVE-2025-43859

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2025-43859. EPSS is 0.58%. Act only if a listed version of h11 is a Python is installed.

Which versions are affected?

Compare what you have installed with python-hyper h11 < 0.16.0 < 0.16.0. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS 0.58%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Apr 24, 2025 · EUVD

    Detected

    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 mit

  2. Apr 24, 2025 · EUVD

    Updated

    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 mit

Sources

  • CVE.orgVulnerability recordCVE-2025-43859

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2025-43859

    30-day exploitation probability 0.6%

    first.org
  • EUVDVulnerability recordEUVD-2025-11981

    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 explo

    euvd.enisa.europa.eu

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.