Public proof of concept

CVE-2021-39214

A public proof of concept exists; no exploitation attested.

Published2021-09-16
WeaknessCWE-444

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

mitmproxy is an interactive, SSL/TLS-capable intercepting proxy. 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 a smuggled request is still captured as part of another request's body, 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 one uses mitmproxy to protect an HTTP/1 service, no action is required. The vulnerability has been fixed in mitmproxy 7.0.3 and above.

Timeline

dates in UTC

2021-09-1615:15CVE publishedNVD
2026-10-2319:23First public proof of conceptPoC-in-GitHub

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-39214?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-39214?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Mitmproxy

Branch	Affected	Fixed in	Source
7.0	≤ 7.0.2	no fix published	NVD

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

No fix published by the sources we read yet. NVD record

Detection

read 2026-10-23 19:23 UTC

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: 1 repository, unverifiedCyberCTF/secdevlabs-golden-hat

Sources

CNACVSS 3.18.1read 2026-10-23
NVDCVSS 3.19.8read 2026-10-23
NVDCVSS 2.07.5read 2026-10-23
CNAweakness (CWE)CWE-444read 2026-10-23
FIRST EPSSEPSS score1.1 %2026-10-22
PoC-in-GitHubproof of concept1read 2026-10-23

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Published: 2021-09-16
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2021-39214?

Is CVE-2021-39214 actively exploited?

Active exploitation of CVE-2021-39214 has not been confirmed. Its EPSS score was 1.1% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-39214?

CVE-2021-39214 has a CVSS v3 base score of 9.8 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

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