CVE-2020-5247: HTTP Response Splitting in Puma

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 2.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2020-02-28 · updated 2026-06-17
Weakness: CWE-113

Description

In Puma (RubyGem) before 4.3.2 and before 3.12.3, if an application using Puma allows untrusted input in a response header, an attacker can use newline characters (i.e. `CR`, `LF` or`/r`, `/n`) to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting. While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS). This is related to CVE-2019-16254, which fixed this vulnerability for the WEBrick Ruby web server. This has been fixed in versions 4.3.2 and 3.12.3 by checking all headers for line endings and rejecting headers with those characters.

Affected products

Debian Linux: 9.0
Fedoraproject Fedora: 30, 31, 32
puma: through 3.12.3; from 4.0.0 through 4.3.2
Ruby: through 2.3.0; from 2.4.0 through 2.4.7; from 2.5.0 through 2.5.6; from 2.6.0 through 2.6.4; 2.7.0-preview1
Rubygems Puma: before 3.12.4; from 4.0.0 before 4.3.3; 0.8.0, 0.8.1, 0.8.2, 0.9.0, 0.9.1, 0.9.2 (+108) — fixed in 3.12.4, 4.3.3

How to fix

Upgrade to a fixed version: Rubygems Puma 3.12.4, 4.3.3. Check the vendor advisory for mitigations.

References

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