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Puma with proxy which forwards LF characters as line endings could allow HTTP request smuggling

Low severity GitHub Reviewed Published Oct 12, 2021 in puma/puma • Updated May 4, 2023

Package

bundler puma (RubyGems)

Affected versions

>= 5.0.0, < 5.5.1
< 4.3.9

Patched versions

5.5.1
4.3.9

Description

Impact

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.

Patches

This vulnerability was patched in Puma 5.5.1 and 4.3.9.

Workarounds

This vulnerability only affects Puma installations without any proxy in front.

Use a proxy which does not forward LF characters as line endings.

Proxies which do not forward LF characters as line endings:

  • Nginx
  • Apache (>2.4.25)
  • Haproxy
  • Caddy
  • Traefik

Possible Breakage

If you are dealing with legacy clients that want to send LF as a line ending in an HTTP header, this will cause those clients to receive a 400 error.

References

For more information

If you have any questions or comments about this advisory:

References

@nateberkopec nateberkopec published to puma/puma Oct 12, 2021
Published by the National Vulnerability Database Oct 12, 2021
Reviewed Oct 12, 2021
Published to the GitHub Advisory Database Oct 12, 2021
Last updated May 4, 2023

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
High
Privileges required
Low
User interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N

EPSS score

0.131%
(49th percentile)

Weaknesses

CVE ID

CVE-2021-41136

GHSA ID

GHSA-48w2-rm65-62xx

Source code

Credits

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