HTTP Response Splitting (Early Hints) in Puma

Impact

If an application using Puma allows untrusted input in an early-hints header,
an attacker can use a carriage return character 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-2020-5247,
which fixed this vulnerability but only for regular responses.

Patches

This has been fixed in 4.3.3 and 3.12.4.

Workarounds

Users can not allow untrusted/user input in the Early Hints response header.