CVE-2021-39214Network attack vector

Access-control bypass via ambiguous HTTP framing

Published Sep 16, 2021 · Updated Aug 4, 2024

HTTP request smuggling in mitmproxy 7.0.2 and earlier allows remote attackers to bypass proxy event-hook controls via ambiguous HTTP messages. Its HTTP/1 parser accepts messages carrying both Content-Length and Transfer-Encoding headers, allowing downstream peers to split an embedded request that mitmproxy retains inside another message body. Exposure requires mitmproxy to protect an HTTP/1 service; the hidden request bypasses request-list visibility and event-hook access controls or sanitization.

CVSS severity8.1
High
EPSS probability1.05%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

HTTP request smuggling in mitmproxy 7.0.2 and earlier allows remote attackers to bypass proxy event-hook controls via ambiguous HTTP messages. Its HTTP/1 parser accepts messages carrying both Content-Length and Transfer-Encoding headers, allowing downstream peers to split an embedded request that mitmproxy retains inside another message body. Exposure requires mitmproxy to protect an HTTP/1 service; the hidden request bypasses request-list visibility and event-hook access controls or sanitization.

The record

CVE
CVE-2021-39214
Published
Sep 16, 2021
Updated
Aug 4, 2024
Vendor
mitmproxy
Product
mitmproxy
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 16, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 7.0.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

mitmproxy · mitmproxyversion=< 7.0.3

Published CVSS scores

8.1GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo