WordPress should return nocache headers for requests with comment cookies

[barry]

Most themes, when displaying the comment form, change the HTML to pre-fill username, email address, and website when comment cookies are received in the HTTP request. Since the response does not have explicit nocache headers, per RFC2616 (​http://www.ietf.org/rfc/rfc2616.txt) intermediate caches can use heuristics to determine the cache TTL for the response. Since there is 0 freshness data in the response, it is not really possible to perform good heuristics, but in practice, caches will assign a default TTL to this type of response. The result is that private information input by user A when submitting a comment can be returned to user B when making a request for the same URL.

To protect ourselves against this, we should call nocache_headers() when comment cookies are sent and the comment form is being displayed. Alternatively, we can send nocache headers for all requests with comment cookies regardless of the comment form being displayed or not (probably easier and maybe safer).

​http://humboldtherald.wordpress.com/2011/01/27/gremlins/ is a story likely caused by an aggressive cache and the lack of nocache headers.

[westi]

The fix :)

[nacin]

Or, per a discussion, "Vary: Cookie" could be sent.

[thomaswm]

Refreshed patch

[thomaswm]

Added refreshed version of the patch as 16612.diff​.

[DrewAPicture]

#35266 was marked as a duplicate.

[helen]

Was Vary: Cookie ever discussed? If going with the existing patch, it needs a different function name.

@rachelbaker: Still want to do this 4.7-early?

[rachelbaker]

Renames the function to wp_check_comment_cookies

[rachelbaker]

@helen I am not aware of any discussion or plans to implement Vary: Cookie headers. I defer to @wonderboymusic or @markjaquith for a more informed response.

[rmccue]

Related: #38060.