CVE-2020-5247: Debian Linux

High severity, CVSS 7.5. EPSS: 2.5% chance of exploitation in the next 30 days.

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 Debian Linux: version 9.0 only
• Fedoraproject Fedora: version 30 only; version 31 only; version 32 only
• Puma Puma: up to and including 3.12.3; from 4.0.0, up to and including 4.3.2
• Ruby-Lang Ruby: up to and including 2.3.0; from 2.4.0, up to and including 2.4.7; from 2.5.0, up to and including 2.5.6; from 2.6.0, up to and including 2.6.4; version 2.7.0 only

Published 2020-02-28. Last modified 2026-06-17.