Unsafe Signal Handling in Sendmail

RAZOR advisory: Unsafe Signal Handling in Sendmail

Issue Date: May 28, 2001
Contact: Michal Zalewski <[email protected]>

Topic:

Sendmail signal handlers used for dealing with specific signals are
vulnerable to numerous race conditions.

Affected Systems:

Any systems running sendmail (tested on sendmail 8.11.0, 8.12.0-Beta5)

Details:

Sendmail signal handlers used for dealing with specific signals
(SIGINT, SIGTERM, etc) are vulnerable to numerous race conditions,
including handler re-entry, interrupting non-reentrant libc functions
and entering them again from the handler (see "References" for more
details on this family of vulnerabilities). This set of
vulnerabilities exist because of unsafe library function calls from
signal handlers (malloc, free, syslog, operations on global buffers,
etc).

As sendmail is setuid root and can be invoked by user, and - moreover

• keeps running with root privileges almost all the time, there is no
  problem with delivering signals at a specific moment.

It is worth mentioning that not only sendmail is suspectible to have
this kind of problems. Moreover, in some situations, unsafe signal
handlers can be even exploited remotely, by delivering SIGURG over TCP
stream (OOB message). Whenever SIGURG is handled in remote daemons in
verbose way using unsafe functions, this is an exploitable condition.
Note, sendmail is not vulnerable to this.

Impact:

One of the attack paths we can see is delivering SIGTERM while
sendmail is working in 'verbose debugging' mode (-d switch). SIGTERM
handler works less or more this way:

 - ...
 - syslog(...) call with user-dependent information
 - ...
 - fclose(...)
 - free(...)
 - free(...)
 - ...
 - exit(...)

This is important that syslog() function effectively calls malloc()
code to allocate a temporary buffer. As exactly the same handler is
used for SIGINT, and there is no re-entry protection in this handler,
we can reach appropriate (usually the second) free() call, and deliver
SIGTERM. Then, already free()d memory will be overwritten with
user-dependent data from syslog() buffer, as new memory chunk would
fit in the place of free()d buffers. Then, duplicate free() attempt on
the memory region containing user-dependent data will be performed,
which would lead to program execution path compromise. This is a
difficult race, but can be attempted numerous times.

Note that avoiding re-entry into signal handler is not the only thing
that has to be done. Other possibilities include e.g. re-entering
functions like malloc() - in this case, signal has to be delivered
only once, in the middle of malloc() call. That would lead to heap
corruption. Any functions that are not reentrant should be protected
in a special way or not used at all in signal handlers.

Vendor response / fix info:

From [email protected]:

We agree with Michal Zalewski's comments regarding the possibility of
heap corruption due to signal delivery. We do not believe the heap
corruption to be easily exploitable due to the complexity involved
with timing and the little control the user has over the contents of
memory in the signal handler. This is different than buffer overflows
attacks which occur on the stack and allow users to insert specific
instructions at a known location. At the present time, there is no
proof that this is exploitable as there are no known exploits.

However, the corruption could crash the process and we have taken
measures to reduce this possibility in 8.11.4. We have eliminated the
ability to reenter a signal handler making the attack discussed above
impossible. Additionally, sendmail 8.12 will no longer require a
set-user-id root binary.

Note that this attack can only be used by a process started by the
user and therefore can not be used as a denial of service attack and
also is not remotely exploitable. The information regarding remote
attacks and SIGURG does not apply to sendmail as SMTP does not use out
of band messages.

References:

For more information on signal delivery race conditions, please
refer to RAZOR whitepaper at:

 http://razor.bindview.com/publish/papers/signals.txt