authentication: use hidden form instead of referer

This also gives us some CSRF protection. Note that we make use of the
hmac to protect the redirect value.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/cgitrc.5.txt b/cgitrc.5.txt
index c45dbd3..682d8bb 100644
--- a/cgitrc.5.txt
+++ b/cgitrc.5.txt
@@ -662,7 +662,8 @@ auth filter::
         the http cookie and return a 0 if it is invalid or 1 if it is invalid,
         in the exit code / close function. If the filter action is
         "authenticate-post", this filter receives POST'd parameters on
-        standard input, and should write to output one or more "Set-Cookie"
+        standard input, and should write a complete CGI request, preferably
+        with a 302 redirect, and write to output one or more "Set-Cookie"
         HTTP headers, each followed by a newline.
 
         Please see `filters/simple-authentication.lua` for a clear example