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CVE-2021-41136: Puma with proxy which forwards LF characters as line endings could allow HTTP request smuggling

October 12, 2021 (updated May 27, 2025)

Prior to puma version 5.5.0, using puma with a proxy which forwards LF characters as line endings could allow HTTP request smuggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client.

This behavior (forwarding LF characters as line endings) is very uncommon amongst proxy servers, so we have graded the impact here as “low”. Puma is only aware of a single proxy server which has this behavior.

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.

References

  • github.com/advisories/GHSA-48w2-rm65-62xx
  • github.com/puma/puma
  • github.com/puma/puma/commit/436c71807f00e07070902a03f79fd3e130eb6b18
  • github.com/puma/puma/commit/acdc3ae571dfae0e045cf09a295280127db65c7f
  • github.com/puma/puma/commit/fb6ad8f8013ab5cdbb2f444cbfabd0b4fde71139
  • github.com/puma/puma/releases/tag/v4.3.9
  • github.com/puma/puma/releases/tag/v5.5.1
  • github.com/puma/puma/security/advisories/GHSA-48w2-rm65-62xx
  • github.com/rubysec/ruby-advisory-db/blob/master/gems/puma/CVE-2021-41136.yml
  • lists.debian.org/debian-lts-announce/2022/08/msg00015.html
  • nvd.nist.gov/vuln/detail/CVE-2021-41136
  • security.gentoo.org/glsa/202208-28
  • www.debian.org/security/2022/dsa-5146

Code Behaviors & Features

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Affected versions

All versions before 4.3.9, all versions starting from 5.0.0 before 5.5.1

Fixed versions

  • 5.5.1
  • 4.3.9

Solution

Upgrade to versions 4.3.9, 5.5.1 or above.

Impact 3.7 LOW

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

Learn more about CVSS

Weakness

  • CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Source file

gem/puma/CVE-2021-41136.yml

Spotted a mistake? Edit the file on GitLab.

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