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=============================================================================
FreeBSD-SA-96:13              Security Advisory
                      FreeBSD, Inc.

Topic:    unauthorized mail reading via comsat

Category:  core
Module:    comsat
Announced:  1996-06-05
Affects:  All FreeBSD versions
Corrected:  2.1-stable and 2.2-current as of 1996-06-04
FreeBSD only:  yes

Patches:  ftp://freebsd.org/pub/CERT/patches/SA-96:13/

=============================================================================

I.   Background    

     FreeBSD made an extension to the comsat daemon to support
     e-mail in alternate locations.  A bug was found in this
     extension which allows an unprivileged local user to read
     portions of other users' mail.

     This problem is present in all source code and binary
     distributions of FreeBSD released before 1996-06-03.


II.  Problem Description

     The comsat daemon does not properly set privileges before
     attempting to read mail files for display on a user terminal.


III. Impact

     The comsat daemon may be used to read portions of files
     in /var/mail that would not normally be accessible to a
     particular user.  This ability is limited to the /var/mail
     directory hierarchy.

     This vulnerability can only be exploited by users with a valid
     account on the local system.


IV. Workaround

     One may simply disable the comsat daemon in /etc/inetd.conf.
     This will disable asynchronous notification of received mail
     (biff messages).  This workaround will suffice for
     all versions of FreeBSD affected by this problem.
     
     As root, edit the file /etc/inetd.conf and change the line

     comsat  dgram  udp  wait  root  /usr/libexec/comsat  comsat

     to read

     #comsat  dgram  udp  wait  root  /usr/libexec/comsat  comsat

     and then reboot the system or restart the inetd daemon and kill
     off any active comsat daemons.

V. Solution

     The following patch fixes the permissions problem.  It should
     apply cleanly to all FreeBSD 2.x systems.  It has not been tested
     with FreeBSD 1.x but this change, if applied by hand, should work.

    --- comsat.c  Mon Jun  3 09:07:49 1996
    +++ comsat.c  Mon Jun  3 09:17:11 1996
    @@ -73,7 +73,7 @@
     time_t  lastmsgtime;
     int  nutmp, uf;
     
    -void jkfprintf __P((FILE *, char[], off_t));
    +void jkfprintf __P((FILE *, char[], char[], off_t));
     void mailfor __P((char *));
     void notify __P((struct utmp *, char[], off_t, int));
     void onalrm __P((int));
    @@ -238,15 +238,16 @@
    cr, name, (int)sizeof(hostname), hostname,
    folder ? cr : "", folder ? "to " : "", folder ? file : "",
    cr, cr);
    -  jkfprintf(tp, file, offset);
    +  jkfprintf(tp, name, file, offset);
      (void)fclose(tp);
      _exit(0);
     }
     
     void
    -jkfprintf(tp, name, offset)
    +jkfprintf(tp, user, file, offset)
      register FILE *tp;
    -  char name[];
    +  char user[];
    +  char file[];
      off_t offset;
     {
      register char *cp, ch;
    @@ -256,10 +257,10 @@
      char line[BUFSIZ];
     
      /* Set effective uid to user in case mail drop is on nfs */
    -  if ((p = getpwnam(name)) != NULL)
    +  if ((p = getpwnam(user)) != NULL)
        (void) setuid(p->pw_uid);
     
    -  if ((fi = fopen(name, "r")) == NULL)
    +  if ((fi = fopen(file, "r")) == NULL)
        return;
     
      (void)fseek(fi, offset, L_SET);


=============================================================================
FreeBSD, Inc.

Web Site:      http://www.freebsd.org/
Confidential contacts:    security-officer@freebsd.org
PGP Key:      ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications:    security-notifications@freebsd.org
Security public discussion:  security@freebsd.org

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================

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