A weakness has been reported in Windows XP, which can be exploited to bypass certain rules in the Internet Connection Firewall (ICF).

The problem is caused due to the firewall by default accepting incoming connections to ports listened on by the "sessmgr.exe" process.

This can e.g. be exploited by malicious, unprivileged users to host an unauthorised service or by a trojan to accept incoming connections by starting "sessmgr.exe" and then inject malicious code into the running process.

Successful exploitation does not require administrative privileges on an affected system.

The weakness has been reported in Windows XP SP2. Other versions may also be affected.

NOTE: This is a general problem with personal firewalls and can be exploited via any program granted access through the firewall without user interaction. It is a known issue and have been discussed in the security community about 2 years ago. PoC exploit code has also priorly been released by Oliver Lavery.

Solution:
Uncheck "Remote Assistance" under the "Exceptions" tab for the ICF configuration. Don't rely solely on personal firewalls to prevent unauthorised Internet access.

Provided and/or discovered by:
Reported in ICF via "sessmgr.exe" by: americanidiot[at]hushmail.com