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.
This has been fixed in 4.3.3 and 3.12.4.
Users can not allow untrusted/user input in the Early Hints response header.
If you have any questions or comments about this advisory: * Open an issue in puma * Email us a project maintainer. Email addresses are listed in our Code of Conduct.