RAZOR Advisory: Multiple Local Sendmail Vulnerabilities
=======================================================

Author: Michal Zalewski <lcamtuf@razor.bindview.com>
Release Date: 10/01/2001

Assigned CVE numbers: CAN-2001-0713, CAN-2001-0714, CAN-2001-0715

Topic:
------

   The Sendmail mail delivery subsystem is vulnerable to multiple local
   attacks that lead to information loss, information leaks and mail
system
   compromise.

Affected Systems:
-----------------

   The mail system privileges compromise affects Sendmail 8.12.0. Other
   problems affect all versions up to 8.12.0.

Vulnerability 1: Mail System Compromise -- CAN-2001-0713
--------------------------------------------------------

  Sendmail 8.12.0, in its default installation, is no longer using a
setuid
  root binary to manipulate the mail queue and submit mail. This
security
  enhancement is supposed to minimize the eventual impact of local
Sendmail
  vulnerabilities. The new Sendmail binary is setgid smmsp, where smmsp
is a
  special group with read-write queue access permissions.

  From previous versions, Sendmail 8.12 inherits a functionality that
allows
  users to specify custom configuration files or configuration
parameters. In
  this case of processing of untrusted configurations, Sendmail was
supposed
  to drop all extra privileges and continue to run at user level,
causing no
  security risk. This mechanism worked fine in Sendmail versions prior
to
  8.12.0. Because of a programming error, this inherited code fails to
drop
  extra group privileges completely in new setgid conditions, leaving
the
  saved gid value untouched. By calling the setregid() function, the
attacker
  will be able to regain dropped privileges. Extra privileges expose a
  security risk to the mail subsystem and, in specific conditions, might
lead
  to further privilege elevation (see discussion below).

  In order to issue setregid(), the attacker has to take over control of
the
  program itself. A possible attack vector is to exploit configuration
file
  parsing code. This code is not supposed to be bullet-proof - it is
intended
  to parse trusted content or untrusted content, but run with no extra
  privileges. We have determined that there are multiple potentially
  exploitable conditions that can be triggered by custom configuration
files 
  (-C parameter):

  * Parsing one-letter macro names in rewrite rules that have the high
bit set
    cause a delayed crash in miscellaneous locations, e.g. the queueup()
    function during sendall(). This indicates memory corruption problem:

    R$-           $ó: $(dequote $1 $) < @ *LOCAL* > dequote "foo"

  * The function setoption() in readcf.c file, invocation of bitnset()
can
    result in a memory overwrite if the following line is found in a
    configuration file:

      Oó some=thing

    A crash occurs immediately.

  * The function getmodifiers() in daemon.c, invocation of setbitn() in
the
    following case:
 
      O DaemonPortOptions=Port=587, Name=MSA, M=ó

  In all examples, 'ó' can be replaced with any ASCII character with the
high
  bit set (this causes the signed char value to be negative and the
table to
  be accessed outside the assigned area).

  Note that our .cf parser code audit is not supposed to be complete or
  accurate, and results are only provided to demonstrate the possibility
of
  attack. The proper solution to this problem is to fix the
privilege-dropping
  code.

  Once the Sendmail process is controlled by the attacker, s/he can
issue the
  setregid() system call and gain smmsp group access level. This group
is
  allowed to manipulate the user-level queue (/var/spool/clientmqueue).
This 
  allows the attacker to read and/or modify other users' mail, and to
enable
  further privilege elevation due to a practice suggested by Sendmail
8.12
  documentation (sendmail/SECURITY):

    "In general it is necessary to clean the queue either
    via a cronjob or by running a daemon, e.g.,

    /PATH/TO/sendmail -L sm-msp-queue -Ac -q30m"

  While Sendmail, in this particular case, is supposed to run with smmsp
user
  and group privileges, it fails to drop group privileges and the gid
remains
  set to 0. Again, the queue file processing algorithm is not
bullet-proof. 
  This is acceptable, because ordinary users should be not able to
modify
  queue files. Unfortunately, due to the vulnerability described above,
it
  suddenly becomes a problem.

  One of the potential problems in the qf parser: expired T tag in the
queue
  file, in conjunction with no valid Return-Path header, can cause a
crash.
  The failure occours when Sendmail tries to generate return envelope
for
  this mail and is not able to connect master SMTP daemon (which can be
  DoSed or rendered unusable in other way). Because of heap corruption,
  depending on run parameters and queue contents, this vulnerability can

  cause a crash by following invalid pointer, freeing non-allocated
memory, 
  etc, and, due to user-dependent queue contents, is likely to be
  exploitable.

  Note:
  -----

  We have noticed that there are other alarming symptoms related to
recent
  changes in Sendmail code. This condition shall not be considered a
  vulnerability, but might lead to serious problems. The "harmless"
practice of
  giving a+x permissions to user queue (/var/spool/clientmqueue), which,
at
  first sight, does not cause a major security problem and might be a
result
  of overlooking (especially if Sendmail 8.12.x is shipped in packages),
  actually allows mail system compromise. The attacker can read and/or
modify 
  other users' mail by initiating bogus queue runs (sendmail -q -Ac
-h1000 or
  similar) and "taking over" the ownership of queue entries: new queue
files
  for existing mail that belong to other users will be owned by the
attacker.
  Additionally, attacker-owned queue files can be altered to cause
privilege 
  elevation, as described above.

  To prevent this condition, it should be at least documented; and at
best,
  Sendmail should refuse to run in an insecure environment or generate
an
  appropriate warning.

Vulnerability 2: Queue Manipulation and Destruction -- CAN-2001-0714
--------------------------------------------------------------------

  All versions of Sendmail allow any user to process the whole mail
queue,
  unless this feature is administratively disabled. This feature itself
is not
  dangerous. Due to a programming bug, specific attacker-specified mail
  delivery options will be honored. It is possible to, for example,
force 
  Sendmail to drop queue contents by setting initial message hop count
above 
  the limit:

    sendmail -q -h1000

  Specific queue entries can be targeted using parameters like -qR, -qS,
etc.

  This can be considered a DoS / data loss attack. Systems that do not
allow 
  users to run the queue (RestrictQRun option) are not vulnerable to
this.
  RestrictQRun is not set by default.
 
Vulnerability 3: Debug Mode Leaks Information -- CAN-2001-0715
--------------------------------------------------------------

  This is a fairly low-risk vulnerability related to user-driven queue
  processing abilities. Debugging flags can be used to obtain the
complete mail
  system configuration, gather potentially interesting information about
the
  mail queue (full message path, subject, mail software, etc.) even if
local 
  users (and the attacker) are not allowed to read the configuration or
mail
  queue directly. This can be achieved by issuing the following command:

    sendmail -q -d0-nnnn.xxx

  where nnnn and xxx specify debugging levels.

  Systems that do not allow users to run the queue (RestrictQRun option)

  are not vulnerable to this. RestrictQRun is not set by default.

Vendor Response and Fix Information:
------------------------------------

  The vendor has been contacted, and released the following statement:

  We would like to thank Michal Zalewski for bringing these problems
  to our attention. We are sorry that these issues haven't been found
  earlier despite extensive beta testing. In response to his advisory
  we will release sendmail 8.12.1 that addresses the issues as follows:

  - For operating systems where the system call setgid() does not
    irrevocably give up privileges obtained via the set-group-ID binary,
    three other options are available: setegid(), setregid(), and
    setresgid(). Additionally sendmail checks whether the privileges
    can be regained when they should have been dropped in which case
    sendmail refuses to run.

  - The PrivacyOptions RestrictQRun is set by default for mail
submission
    (submit.cf) to avoid privacy leaks or mail loss due to mailicous
    local users.

  - 8.12.1 prevents information leakage when sendmail has extra
    privileges by disabling debugging (command line -d flag) during
queue
    runs and disabling ETRN when sendmail -bs is used.

  - 8.12.1 limits mail submission command line flags (i.e., -G, -h,
    -F, etc.) to mail submission operating modes (i.e., -bm, -bs, -bv,
    etc.) when sendmail is running with extra privileges.

  - Additonal test programs are distributed with sendmail that
    can check whether the required functionality of system calls is
    available. See test/README in 8.12.1 for further information.

  - The potential memory corruption problems have been fixed.