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Vigil@nce: Linux, privilege elevation via login

December 2008 by Vigil@nce

A local attacker member of the utmp group can obtain the root
privilege.

 Gravity: 1/4
 Consequences: administrator access/rights
 Provenance: user shell
 Means of attack: 1 attack
 Ability of attacker: technician (2/4)
 Confidence: confirmed by the editor (5/5)
 Diffusion of the vulnerable configuration: low (1/3)
 Creation date: 02/12/2008

IMPACTED PRODUCTS

 Unix - plateform

DESCRIPTION

The /var/(log|run)/utmp files contain information about sessions
opened by users. These files belong to the utmp group, so
authentication programs are only sgid utmp instead of sgid root.

An attacker member of the utmp group can add an entry in the utmp
file, indicating for example the /tmp/x TTY. When the "login"
program is run, it does a chown() on the TTY with root privileges.
However, if /tmp/x is a symbolic link pointing to the /etc/shadow
file, the attackers becomes the owner of /etc/shadow.

A local attacker member of the utmp group can therefore obtain the
root privilege. It can be noted that another vulnerability has to
be used to become a member of the utmp group.

CHARACTERISTICS

 Identifiers: 505271, VIGILANCE-VUL-8278
 Url: http://vigilance.fr/vulnerability/8278


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