Exploit Out for Critical Microsoft Agent Flaw

An exploit attacks a critical Microsoft Agent vulnerability less than 24 hours after the patch came out.

An exploit that attacks a critical Microsoft Agent vulnerability was published less than 24 hours after Microsoft released a relevant security advisory in its Sept. 11 Patch Tuesday set of releases.

The security advisory for Microsoft Agent, MS07-051, was the only critical release out of four security advisories. It addresses a vulnerability whereby the Microsoft Agent—a set of software services for developers to enhance the user interface of Web-based applications—can get hoodwinked by a malicious URL and can then be used to take over a targeted system without ever appearing to the user.

Microsoft Agent (agentsvr.exe) is prone to the stack-based buffer-overflow vulnerability because it fails to adequately bound check user-supplied data. The issue occurs when the "agentdpv.dll" ActiveX control processes maliciously craft URLs, resulting in memory corruption.

If the exploit succeeds, the attacker gains system control. If it fails, a denial-of-service occurs.

Symantec has exploit code available to members of its Immunity Partners Program.

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Proof-of-concept code is also available here.

For users who cant apply the patch straight away, Symantec and Microsoft recommend the typical mitigation strategies when dealing with an exploit that requires user interaction:

Run software as a non-privileged user with minimal access rights, and always run non-administrative software as an unprivileged user with minimal access rights. Also, stay away from links coming from strangers or untrustworthy sources.

Keep out of sites that have questionable integrity. Set browser security to disable execution of script code or active content. Dont open e-mail from strangers or those whom you dont trust, given that attackers could exploit the vulnerability through HTML e-mail.

As a workaround, Symantec suggests setting the kill bit for the following CLSIDs to prevent instantiation of malicious Microsoft Agent ActiveX controls:

D45FD31B-5C6E-11D1-9EC1-00C04FD7081F
F5BE8BD2-7DE6-11D0-91FE-00C04FD701A5
4BAC124B-78C8-11D1-B9A8-00C04FD97575
D45FD31D-5C6E-11D1-9EC1-00C04FD7081F
D45FD31E-5C6E-11D1-9EC1-00C04FD7081F

Microsoft describes how to disable ActiveX controls here. Microsoft also suggests unregistering "AgentSvr.exe" as a workaround.

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