Global InterSec Advisory: sudo
Apr 25, 2002, 15:33 (0 Talkback[s])
Global InterSec LLC
GIS Advisory ID: 2002041701
Sudo - A popular utility for allowing users to execute
commands as other users contains a vulnerability which
may be exploited to execute arbitrary commands.
A local user may gain root access through corruption of
the heap (Off-By-Five).
Versions Tested To Be Vulnerable:
1.6.3p7 (SuSE 7.1 Update, RedHat 7.2)
1.6.3 (SuSE 7.1)
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
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
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.
Remove the suid bit from the sudo binary and remove any
entries in /etc/sudoers.
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 firstname.lastname@example.org for more
Thanks also to Todd Miller, the sudo maintainer for
responding so quickly to our email.
A patch for version 1.6.5p2 is available at:
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
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)
Sorry, try again.
./sudo: 1 incorrect password attempt
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,.
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