mitmproxy binaries embed a vulnerable python-hyper/h2 dependency

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

mitmproxy 12.1.1 and below embed python-hyper/h2 ≤ v4.2.0, which has a gap in its HTTP/2 header validation. This enables request smuggling attacks when mitmproxy is in a configuration where it translates HTTP/2 to HTTP/1. For example, this affects reverse proxies to http:// backends. It does not affect mitmproxy's regular mode. All users are encouraged to upgrade to mitmproxy 12.1.2, which includes a fixed version of h2. More details about the vulnerability itself can be found at https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h.

Published by the source

25 Aug 2025, 21:01 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-63cx-g855-hvv4

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-63cx-g855-hvv4

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# Patch Next advisory brief: GHSA-63cx-g855-hvv4

Advisory: mitmproxy binaries embed a vulnerable python-hyper/h2 dependency
IDs: GHSA-63cx-g855-hvv4
Record: https://patchnext.com/advisories/ghsa-63cx-g855-hvv4 (revision 3, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mitmproxy 12.1.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mitmproxy: affected <= 12.1.1; fixed in 12.1.2

## Suggested inspection (read-only)
- `python -m pip show mitmproxy`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mitmproxy.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-63cx-g855-hvv4 (OSV record GHSA-63cx-g855-hvv4 lists affected ranges: PyPI mitmproxy >=0 <12.1.2 (fixed))
- Verified: https://github.com/advisories/GHSA-63cx-g855-hvv4 (GitHub-reviewed advisory GHSA-63cx-g855-hvv4 lists affected ranges: PyPI mitmproxy >=0 <=12.1.1)

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