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Sudo Password Prompt Vulnerability
26 April 2002
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Global InterSec LLC

http://www.globalintersec.com

GIS Advisory ID: 2002041701
Changed: 25/04/2002
Author: research@globalintersec.com
Reference: http://www.globalintersec.com/adv/sudo-2002041701.txt

Summary:

Sudo - A popular utility for allowing users to execute commands as other users contains a vulnerability which may be exploited to execute arbitrary commands.

Impact:

A local user may gain root access through corruption of the heap (Off-By-Five).

Versions Tested To Be Vulnerable:

1.6.5p2
1.6.4
1.6.3p7 (SuSE 7.1 Update, RedHat 7.2)
1.6.3 (SuSE 7.1)
1.6.2

Description:

Sudo, an open source utility shipped with many linux distributions enables the super user to grant non root users permission to execute commands as other users (including root). Access to sudo is normally regulated via /etc/sudoers.

One of the functions of sudo enables users to specify the password prompt given when challenged for their password to 'sudo'. On parsing this parameter (-p) to sudo, a user may also specify characters which expand to either the hostname (%h) or the username (%u). On the reception of these expansion characters, sudo will malloc() memory for the string the argument to -p will form.

Unfortunately it is possible to trick sudo into allocating less memory than it should for the prompt. Under certain conditions it is possible to exploit this bug to corrupt the heap in a way in which could be used to execute arbitrary commands.

Because of the nature of sudo, it is installed by default as suid() root and therefore could be used to escalate normal user privileges to that of the root user, taking into account the restrictions noted below.

Scope for attack:

As noted above, this vulnerability could be used to escalate user privileges to that of the super user. However, for this to happen several factors must be considered.

- The options which sudo were compiled with may determine if it is exploitable or not. In our tests we used binaries compiled with PAM support, which we found to be exploitable. Many distributions of linux ship with PAM enabled builds of sudo.

Distributions shipping PAM enabled sudo binaries include those of SuSE and RedHat.

- The length of the hostname on the system a vulnerable binary is on is a critical factor in the way in which the bug is exploited.

Work around:

Remove the suid bit from the sudo binary and remove any entries in /etc/sudoers.

Credit:

This vulnerability was originally discovered by fc, a GIS affiliate. Research and vendor coordination were carried out by the Global InterSec research division, under Tom Parker.

Please email research@globalintersec.com for more information.

Thanks also to Todd Miller, the sudo maintainer for responding so quickly to our email.

Vendor Solutions:

A patch for version 1.6.5p2 is available at:
http://www.globalintersec.com/adv/files/sudo-1.6.5p3.patch

Both the Sudo maintainer and Vendors shipping vulnerable versions of sudo have been notified well in advance of the release date. A list of advisories by individual vendors will be appended to this advisory as they become available.

The most recent copy of this advisory is available at the "Reference" URL noted in the header of this advisory.

Proof of Concept, Semi-Technical Details:

When sudo is called with the -p parameter, expand_prompt() is called to check for and expand any special characters parsed as arguments to -p (%h or %u).

expand_prompt will then calculate space for the expanded prompt and malloc() the calculated amount. On miscalculation of the required space, the place in which sudo break will depend on:

- The string used to cause sudo to miscalculate the required space and the length which any expansion character(s) expand to.
- The compilation options sudo was built with.

These factors therefore have a direct influence on how the bug is to be exploited, if at all.

In the case of a string 'h%h%' being parsed to the -p option, miscalculation of the prompt length occurs due to the first h in our string being treated as an %h and the last character still having the value of % where it should of been given the value '\0' if *lastchar had been re-initialised correctly.

In the example below we used a system who's hostname was 7 bytes long. Because of the length of the hostname, we were able to trigger the vulnerability, but without causing a SEGV, before we were able to write additional data into memory for sudo to read into.

In the case of a system with a hostname over 8 bytes, you may find that the expansion of the hostname has written so far into memory that sudo segfaults before additional memory can be written via the password prompt.

In this case an alternative method would be needed to write into memory so that relevant registers are corrupted. This could possibly be in parameters to -p or in the environment variable 'SUDO_PROMPT' (which -p overrides).

user@defiant:~/research/sudo/dist/sudo-1.6.5p2 > gdb sudo

GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.

(gdb) r -p h%h% -s
Starting program: /research/sudo/dist/sudo-1.6.5p2/sudo -p h%h% -s

efiantdefian! <4 Bytes>\xef\xbe\xad\xde\<84 Bytes> # Password Challenge Sorry, try again.

Program received signal SIGSEGV, Segmentation fault.
0x400d49c1 in chunk_alloc () from /lib/libc.so.6
(gdb) i r $edi
edi 0xdeadbeef -559038737
(gdb)

Note that %ecx and %edx were also within our reach.

Our example used a sudo 1.6.5p2 binary with --with-pam enabled at build time.

The off-by-five condition still occurs when sudo is compiled without PAM as we can see from the following example, using a slightly modified version of sudo.

user@defiant:~/research/sudo/dist/sudo-1.6.5p2 > ./sudo -p h%h% -s
Allocating 9 bytes for prompt: efiantdefiant% (14 bytes long)
efiantdefiant%
Sorry, try again.
efiantdefiant%
^C
./sudo: 1 incorrect password attempt
user@defiant:~/research/sudo/dist/sudo-1.6.5p2 >

To this end - sudo without pam support (or any other configuration) must be considered vulnerable as alternative ways to cause functions in sudo to read into corrupted areas of memory and gain flow control of sudo (other than the pam functions) may exist.

Please see: http://www.phrack.org/show.php?p=57&a=8 for more information on exploiting this type of vulnerability,.

Legal:

This advisory is the intellectual property of Global InterSec LLC but may be freely distributed with the conditions that:

a) No fee is charged
b) Appropriate credit is given.

(c) Global InterSec LLC 2002




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