Directive injection when using dynamic overrides with user input

Impact

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

This could be used to e.g. override a script-src directive. Duplicate directives are ignored and the first one wins. The directives in secure_headers are sorted alphabetically so they pretty much all come before script-src. A previously undefined directive would receive a value even if SecureHeaders::OPT_OUT was supplied.

The fixed versions will silently convert the semicolons to spaces and emit a deprecation warning when this happens. This will result in innocuous browser console messages if being exploited/accidentally used. In future releases, we will raise application errors resulting in 500s.

> Duplicate script-src directives detected. All but the first instance will be ignored.

See https://www.w3.org/TR/CSP3/#parse-serialized-policy

> Note: In this case, the user agent SHOULD notify developers that a duplicate directive was ignored. A console warning might be appropriate, for example.

Patches

Depending on what major version you are using, the fixed versions are 6.2.0, 5.1.0, 3.8.0.

Workarounds

If you are passing user input into the above methods, you could filter out the input:

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

References

Reported in https://github.com/twitter/secure_headers/issues/418
https://www.w3.org/TR/CSP3/#parse-serialized-policy

For more information

If you have any questions or comments about this advisory:

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