---
id: GHSA-63cx-g855-hvv4
title: mitmproxy binaries embed a vulnerable python-hyper/h2 dependency
summary: mitmproxy binaries embed a vulnerable python-hyper/h2 dependency
severity: medium
vendor: mitmproxy
product: mitmproxy
ecosystem: pip
affected:
  - mitmproxy < 12.1.2
patched:
  - mitmproxy 12.1.2
published: '2025-08-25'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:26.707538775Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-63cx-g855-hvv4'
references:
  - url: >-
      https://github.com/mitmproxy/mitmproxy/security/advisories/GHSA-63cx-g855-hvv4
  - url: 'https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h'
  - url: 'https://github.com/mitmproxy/mitmproxy'
tags:
  - osv
  - pip
ingestedAt: '2026-09-12T03:13:01.663Z'
---

## Overview

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.

## Affected packages

- `mitmproxy < 12.1.2`

## Remediation

Upgrade to a patched release:

- `mitmproxy 12.1.2`