CVE-2021-41136: Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values w...

Description

Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. The only proxy which has this behavior, as far as the Puma team is aware of, is Apache Traffic Server. If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with `puma`.

Metrics

CVSS 3.1

3.7/10

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N

EPSS Probability

1.12%

62.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

Affected Software

Vendor	Product	Versions
Puma	Puma	<= 4.3.8
Puma	Puma	>= 5.0.0, <= 5.5.0
Debian	Debian Linux	10.0
Debian	Debian Linux	11.0

References

https://github.com/puma/puma/commit/436c71807f00e07070902a03f79fd3e130eb6b18
https://github.com/puma/puma/commit/acdc3ae571dfae0e045cf09a295280127db65c7fPatch, Third Party Advisory
https://github.com/puma/puma/commit/fb6ad8f8013ab5cdbb2f444cbfabd0b4fde71139
https://github.com/puma/puma/security/advisories/GHSA-48w2-rm65-62xxThird Party Advisory
https://github.com/puma/puma/commit/acdc3ae571dfae0e045cf09a295280127db65c7fPatch, Third Party Advisory
https://github.com/puma/puma/security/advisories/GHSA-48w2-rm65-62xxThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/08/msg00015.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202208-28Third Party Advisory
https://www.debian.org/security/2022/dsa-5146Third Party Advisory

Timeline

Published: Oct 12, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-41136?

Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. The only proxy which has this behavior, as far as the Puma team is aware of, is Apache Traffic Server. If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with `puma`.

How severe is CVE-2021-41136?

CVE-2021-41136 has a CVSS score of 3.7/10 (LOW severity). The EPSS model estimates a 1.12% probability of exploitation in the next 30 days.

How do I fix CVE-2021-41136?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.