Security policy bypass when env_reset is disabled

Release Date:

March 5, 2014

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. 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.