Skip to content

Puma's Keepalive Connections Causing Denial Of Service

High severity GitHub Reviewed Published May 11, 2021 in puma/puma • Updated May 16, 2023

Package

bundler puma (RubyGems)

Affected versions

<= 4.3.7
>= 5.0.0, <= 5.3.0

Patched versions

4.3.8
5.3.1

Description

This vulnerability is related to CVE-2019-16770.

Impact

The fix for CVE-2019-16770 was incomplete. The original fix only protected existing connections that had already been accepted from having their requests starved by greedy persistent-connections saturating all threads in the same process. However, new connections may still be starved by greedy persistent-connections saturating all threads in all processes in the cluster.

A puma server which received more concurrent keep-alive connections than the server had threads in its threadpool would service only a subset of connections, denying service to the unserved connections.

Patches

This problem has been fixed in puma 4.3.8 and 5.3.1.

Workarounds

Setting queue_requests false also fixes the issue. This is not advised when using puma without a reverse proxy, such as nginx or apache, because you will open yourself to slow client attacks (e.g. slowloris).

The fix is very small. A git patch is available here for those using unsupported versions of Puma.

For more information

If you have any questions or comments about this advisory:

Acknowledgements

Thank you to @MSP-Greg, @wjordan and @evanphx for their review on this issue.

Thank you to @ioquatix for providing a modified fork of wrk which made debugging this issue much easier.

References

@nateberkopec nateberkopec published to puma/puma May 11, 2021
Published by the National Vulnerability Database May 11, 2021
Reviewed May 18, 2021
Published to the GitHub Advisory Database May 18, 2021
Last updated May 16, 2023

Severity

High

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
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

2.089%
(89th percentile)

Weaknesses

CVE ID

CVE-2021-29509

GHSA ID

GHSA-q28m-8xjw-8vr5

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.