It is possible for an attacker to bypass the restrictions imposed by The Sendmail Consortium's Restricted Shell (SMRSH) and execute a binary of his choosing by inserting a special character sequence into his .forward file. SMRSH is an application intended as a replacement for sh for use in Sendmail. There are two attack methods both of which are detailed below.
Method One was exclusively disclosed to iDEFENSE by zen-parse.
Method Two was discovered during the verification process by Pedram Amini.
/tmp/unauth is executed despite the fact that it is not located in the SMRSH restricted directory /etc/smrsh. This happens because SMRSH first checks for '.', which exists, and does no further verification on the files listed after '||'. The same attack would look like the following in the attackers .forward file:
"| . \|| . /tmp/unauth \|| ."
This method takes advantage of the following routine from smrsh.c:
/* search backwards for last / (allow for 0200 bit) */
while (cmd > q)
if ((*--cmd & 0177) == '/')
It is possible to feed SMRSH a command line that will be internally converted to a space thereby bypassing all filters, yet will still execute. Examples of these include:
Notice that despite the double space 'command' will still execute. The .forward variation of this attack works the same way.
The following are required conditions for successful and meaningful exploitation of this vulnerability:
* The target system must be utilizing SMRSH.
* The attacker must have a valid local account on the system.
* In method one the attacker must be able to create files.
While this exploit obviously removes the restrictions imposed by SMRSH, it also allows users to execute programs on systems that they do not have shell access to. Utilizing either of the above-described methods, an attacker who can modify his own .forward file can execute arbitrary commands on the target system with the privileges of his own account. Systems that forbid shell access generally do not have tightened local security the ability to execute arbitrary commands through the SMRSH vulnerability opens the target system to local privilege escalation attacks that otherwise would not be possible.
The latest versions of SMRSH are vulnerable. Including the version packaged with Sendmail 8.12.6 and Sendmail 8.11.6-15 (default install of RedHat 7.3). Older versions of SMRSH do not appear to be vulnerable (8.11 5/19/1998). The version of SMRSH available at ftp://ftp.uu.net/pub/security/smrsh is also not vulnerable.
Sendmail.org's official comment:
"We would like to thank iDEFENSE, zen-parse, and Pedram Amini for bringing these problems to our attention.
If you actually use a vulnerable smrsh version (which can be tested according to the descriptions given before), please apply the patch that has been made available. To figure out whether your configuration uses smrsh, check your sendmail.mc file, i.e., look for
And check your sendmail.cf file (usually located in /etc/mail or /etc): grep '^Mprog.*smrsh' sendmail.cf
Also consider whether you actually need this feature, e.g., if you make procmail available to your users then smrsh is basically useless."
9/1/2002 Disclosed to iDEFENSE
9/24/2002 Disclosed to email@example.com
9/24/2002 Disclosed to iDEFENSE clients
9/24/2002 Response from Greg Shapiro firstname.lastname@example.org
9/25/2002 Coordination from Claus Assmann email@example.com
10/1/2002 Public Coordinated Disclosure