CVE-2023-40175: Inconsistent Interpretation of HTTP Requests in puma

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-08-18 · updated 2026-06-17
Weakness: CWE-444

Description

Puma is a Ruby/Rack web server built for parallelism. Prior to versions 6.3.1 and 5.6.7, puma exhibited incorrect behavior when parsing chunked transfer encoding bodies and zero-length Content-Length headers in a way that allowed HTTP request smuggling. Severity of this issue is highly dependent on the nature of the web site using puma is. This could be caused by either incorrect parsing of trailing fields in chunked transfer encoding bodies or by parsing of blank/zero-length Content-Length headers. Both issues have been addressed and this vulnerability has been fixed in versions 6.3.1 and 5.6.7. Users are advised to upgrade. There are no known workarounds for this vulnerability.

Affected products

puma: before 5.6.7; from 6.0.0 before 6.3.1 — fixed in 5.6.7, 6.3.1
Rubygems Puma: before 5.6.7; from 6.0.0 before 6.3.1; 0.8.0, 0.8.1, 0.8.2, 0.9.0, 0.9.1, 0.9.2 (+155) — fixed in 5.6.7, 6.3.1

How to fix

Upgrade to a fixed version: puma 5.6.7, 6.3.1; Rubygems Puma 5.6.7, 6.3.1. Check the vendor advisory for mitigations.

References

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