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Date: Wed, 05 Mar 2014 20:29:10 -0700
From: "Todd C. Miller" <Todd.Miller@...rtesan.com>
To: oss-security@...ts.openwall.com
Subject: sudo: security policy bypass when env_reset is disabled
Summary:
If the env_reset option is disabled in the sudoers file, a
malicious user with sudo permissions may be able to run arbitrary
commands with elevated privileges by manipulating the environment
of a command the user is legitimately allowed to run.
Sudo versions affected:
Sudo 1.6.9 through 1.8.4p5 inclusive. Sudo 1.8.5 and higher
are not affected.
CVE ID:
This vulnerability has been assigned CVE-2014-0106 in the Common
Vulnerabilities and Exposures database.
Details:
Sudo has two methods of constructing the environment that the
command run by it will use. The default method (since sudo
1.6.9) is to execute the command with a new, minimal environment.
The new environment contains the TERM, PATH, HOME, MAIL, SHELL,
LOGNAME, USER, USERNAME, SUDO_COMMAND, SUDO_USER, SUDO_UID and
SUDO_GID variables in addition to variables from the invoking
process permitted by the env_check and env_keep options. This
is effectively a whitelist for environment variables.
If, however, the env_reset option is disabled, any variables
not explicitly denied by the env_check and env_delete options
are inherited from the invoking process. In this case, env_check
and env_delete behave like a blacklist. Since it is not possible
to blacklist all potentially dangerous environment variables,
use of the default env_reset behavior is encouraged.
Beginning with sudo 1.6.9, it is also possible to specify extra
environment variables on the command line. These variables are
supposed to be subject to the same restrictions as the invoking
user's environment, unless the user is allowed to set arbitrary
variables either via the SETENV attribute or by virtue of having
sudo "ALL".
Due to a logic bug in the validate_env_vars() function, if the
env_reset option is disabled, environment variables specified
on the command line are permitted when they should not be (and
vice versa). This can be used by a malicious user to run
arbitrary programs by manipulating the environment of a command
the user is legitimately allowed to run. For example, on many
systems the LD_PRELOAD environment variable is used to load a
dynamic shared object before any shared libraries are loaded.
By either replacing a library function called by the program,
or by including an _init() function in the shared object, the
user can execute arbitrary commands with elevated privileges.
The code that contains the bug was rewritten for sudo 1.8.5,
which does not suffer from the same security issue.
Impact:
For sudo versions prior to 1.8.5, if the env_reset option is
explicitly disabled in the sudoers file, a malicious user with
sudo permissions may be able to run arbitrary commands with
elevated privileges. There is no impact for sudo 1.8.5 and
higher, or when the sudoers file does not disable env_reset.
Fix:
A fix for the sudo 1.7.x branch is included in sudo 1.7.10p8.
The actual fix is a single line change to env.c:
http://www.sudo.ws/repos/sudo/rev/748cefb49422
Sudo versions 1.8.5 and higher are not vulnerable.
Workaround:
Only systems with sudoers files that explicitly disable env_reset
are affected. As such, a simple workaround is to simply not
disable env_reset, which is the default behavior.
Credit:
I'd like to thank Sebastien Macke for reporting this bug and
providing a fix.
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