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Date: Wed, 27 Feb 2013 11:23:36 -0500
From: "Todd C. Miller" <Todd.Miller@...rtesan.com>
To: oss security list <oss-security@...ts.openwall.com>
Subject: CVE request: potential bypass of sudo tty_tickets constraints
Sudo 1.8.6p7 and 1.7.10p6 are now available which include a fix for
the following bug:
Potential bypass of sudo tty_tickets constraints
Summary:
When a user successfully authenticates with sudo, a time stamp
file is updated to allow that user to continue running sudo
without requiring a password for a preset time period (five
minutes by default).
This time stamp file can either be common to all of a user's
terminals, or it can be specific to the particular terminal the
user authenticated themselves on. The terminal-specific time
stamp file behavior can be controlled using the "tty_tickets"
option in the sudoers file. This option has been enabled by
default since sudo 1.7.4. Prior to sudo 1.7.4, the default was
to use a single time stamp for all the user's sessions.
A vulnerability exists because the user can control which
terminal the standard input, output and error file descriptors
(0-2) refer to. A malicious user could use this to run commands
via sudo without authenticating, so long as there exists a
terminal the user has access to where a sudo command was
successfully run by that same user within the password timeout
period (usually five minutes).
The vulnerability does not permit a user to run commands other
than those allowed by the sudoers policy.
Sudo versions affected:
Sudo 1.3.5 through 1.7.10p6 and sudo 1.8.0 through 1.8.6p7 when
the "tty_tickets" option is enabled. This option is enabled
by default in sudo 1.7.4 and above.
Details:
The vulnerability can be triggered when the standard input,
output and error file descriptors (0-2) of a process are closed
and a different terminal device is opened and connected to those
descriptors. When sudo tries to determine the terminal device
via the ttyname() function, it will get the name of the other
terminal instead. The core problem is that while ttyname() can
be used to determine the name of the terminal device connected
to a specific file descriptor, there is no portable way to
determine the name of the terminal associated with the session
the process belongs to. However, on many systems it is possible
to determine this by using the /proc file system or the sysctl()
function.
Most operating systems that have the /proc file system provide
a way to determine the controlling terminal device number for
a process; this information is used by the ps command for
example. On Linux, this is the tty_nr field in /proc/self/stat
(the seventh entry). On systems with an SVR4-style /proc, this
is the pr_ttydev member of struct psinfo, which comes from
/proc/self/psinfo. Most BSD systems that support the sysctl()
function also provide a way to get the terminal device number
via the KERN_PROC_PID sysctl. By mapping this device number
to a file name, it is possible to get the name of the terminal
file without resorting to ttyname(). Sudo began using this
method to determine the process's terminal starting with version
1.8.5 and 1.7.10.
However, sudo still used the ttyname() function as a fall back
when no controlling terminal was found via /proc or sysctl().
This allowed a malicious process to cause sudo to use ttyname()
simply by creating a new session without a controlling tty
before executing sudo. In sudo 1.8.6p6 and 1.7.10p5, this fall
back behavior was removed. This fixed the vulnerability for
systems where the process's controlling terminal could be
determined via /proc or sysctl().
Sudo 1.8.6p7 and 1.7.10p6 contain an additional fix for systems
without /proc or sysctl() that stores the POSIX session ID in
the time stamp file itself. The controlling terminal is specific
to the POSIX session it is associated with. It is not possible
for two processes in different sessions to have the same
controlling terminal. Sudo will now compare the current session
ID with the one in the time stamp file and ignore the time stamp
file if the session ID does not match. This has the additional
benefit of making it much less likely that a user will be able
to reuse the time stamp file after logging out and back in again
on the same terminal.
Impact:
A (potentially malicious) program run by a user with sudo access
may be able to bypass the "tty_ticket" constraints. In order
for this to succeed there must exist on the machine a terminal
device that the user has previously authenticated themselves
on via sudo within the last time stamp timeout (5 minutes by
default).
This program may use sudo's -n flag to "probe" the terminals
in question to see if there is an active time stamp file for
the user. Prior to sudo 1.8.6 and 1.7.10, if a password was
required when the -n flag was specified the failure would not
be logged, allowing the program to perform such probes without
being detected. The successful command (if any), would still
be logged.
Fix:
The bug is fixed in sudo 1.8.6p7 and 1.7.10p6.
Credit:
Ryan Castellucci brought the initial ttyname() issue to my
attention. Subsequently, James Ogden discovered that using
setsid() to create a new session would cause sudo to fall back
to using ttyname().
Other shortcomings in sudo's "tty_tickets" functionality have
been known and discussed openly for some time. There is a long
discussion about them at:
https://bugs.launchpad.net/ubuntu/+source/sudo/+bug/87023
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