Summary
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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is GHSA-63CX-G855-HVV4? GHSA-63CX-G855-HVV4 is a medium-severity security vulnerability in mitmproxy (pip), affecting versions <= 12.1.1. It is fixed in 12.1.2.
- Which versions of mitmproxy are affected by GHSA-63CX-G855-HVV4? mitmproxy (pip) versions <= 12.1.1 is affected.
- Is there a fix for GHSA-63CX-G855-HVV4? Yes. GHSA-63CX-G855-HVV4 is fixed in 12.1.2. Upgrade to this version or later.
- Is GHSA-63CX-G855-HVV4 exploitable, and should I be worried? Whether GHSA-63CX-G855-HVV4 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-63CX-G855-HVV4 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-63CX-G855-HVV4? Upgrade
mitmproxyto 12.1.2 or later.