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GHSA-63cx-g855-hvv4

MediumCVSS 6.9 / 10
Published Aug 25, 2025·Last modified Mar 30, 2026
Affected Components(0)

No affected components available

Description

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.

Risk Scores
Base Score
6.9

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.

Threat Intelligence
2.7

Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.

EPSS
N/A

Probability that this vulnerability will be exploited in the wild within the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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