Fix redirect URL validation bypass

It turns out that browsers silently convert backslash characters into
forward slashes, while apr_uri_parse() does not.

This mismatch allows an attacker to bypass the redirect URL validation
by using an URL like:

  https://sp.example.org/mellon/logout?ReturnTo=https:%5c%5cmalicious.example.org/

mod_auth_mellon will assume that it is a relative URL and allow the
request to pass through, while the browsers will use it as an absolute
url and redirect to https://malicious.example.org/ .

This patch fixes this issue by rejecting all redirect URLs with
backslashes.

auth_mellon_util.c

@@ -927,6 +927,13 @@ int am_check_url(request_rec *r, const char *url)
                           "Control character detected in URL.");
             return HTTP_BAD_REQUEST;
         }
+        if (*i == '\\') {
+            /* Reject backslash character, as it can be used to bypass
+             * redirect URL validation. */
+            AM_LOG_RERROR(APLOG_MARK, APLOG_ERR, HTTP_BAD_REQUEST, r,
+                          "Backslash character detected in URL.");
+            return HTTP_BAD_REQUEST;
+        }
     }
 
     return OK;