CVE-2021-39214 — mitmproxy
CRITICALCVE-2021-39214 is a critical-severity (CVSS 9.8) CWE-444 vulnerability in mitmproxy. A fix is available for mitmproxy — see the affected versions and patch details below.
Lacking Protection against HTTP Request Smuggling in mitmproxy
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-39214 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
mitmproxyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
In mitmproxy 7.0.2 and below, a malicious client or server is able to perform HTTP request smuggling attacks through mitmproxy. This means that a malicious client/server could smuggle a request/response through mitmproxy as part of another request/response's HTTP message body. While mitmproxy would only see one request, the target server would see multiple requests. A smuggled request is still captured as part of another request's body, but it does not appear in the request list and does not go through the usual mitmproxy event hooks, where users may have implemented custom access control checks or input sanitization.
Unless you use mitmproxy to protect an HTTP/1 service, no action is required.
Patches
The vulnerability has been fixed in mitmproxy 7.0.3 and above.
Acknowledgements
We thank João Sobral (@chinchila) for responsibly disclosing this vulnerability to the mitmproxy team.
Timeline
- 2021-09-08: Received initial report for mitmproxy <= 6.0.2.
- 2021-09-08: Requested clarification if 7.x is affected.
- 2021-09-10: Received additional details, 7.x situation still unclear.
- 2021-09-13: Internally determined that 7.x is also affected.
- 2021-09-13: Shared initial fix with researcher.
- 2021-09-14: Received confirmation that fix is working, but H2.TE/H2.CL should also be looked at.
- 2021-09-14: Shared revised fix that includes additional H2.TE mitigations.
- 2021-09-14: Received confirmation that revised fix is working.
- 2021-09-16: Completed internal patch review.
- 2021-09-16: Published patch release and advisory.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | mitmproxy | all versions | 7.0.3pip install --upgrade 'mitmproxy==7.0.3' |
Affected Products
mitmproxymitmproxyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mitmproxy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mitmproxy to 7.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39214 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is CVE-2021-39214 in your dependencies?
Find it across PyPI, including transitive dependencies.