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Microsoft has issued a 'critical' security update after the discovery of a buffer overflow vulnerability which may allow an attacker to execute malicious code against the majority of MSN Messenger users.

The problem stems from a flaw in an ActiveX control, called MSN Chat, which is included with MSN Messenger since version 4.5 and Exchange Instant Messenger. MSN Chat allows groups of users to gather in a single, virtual location online to engage in text messaging.

Researchers at eEye Digital Security have discovered that an unchecked buffer exists in one of the functions that handles input parameters in the MSN Chat control. Because of this users enticed to open a maliciously crafted HTML mail or visit a maliciously constructed Web site might potentially fall victim to attack.

In mitigation, Microsoft states that Outlook Express 6.0 and the Outlook Email Security Update and can thwart such attacks through their default security settings. It also points out that the version of Windows Messenger which ships with Windows XP does not include the MSN Chat control.

This still leaves a vast number of people vulnerable (Outlook Email Security Update take-up is worryingly low, witness the continuing spread of email worms it was designed to stop) so it is not without good reason that Microsoft defines the update as 'critical'.

The vulnerability is ripe for exploitation and of a type that means it is likely to hand around for some time before people wake up to the problem.

Buffer overflows are a common class of security vulnerability, associated with sloppy programming, which allow arbitrary and potentially malicious code to be injected into a system through a carefully crafted, malformed data entry.

Generally, this spurious input is much longer than a program expects, causing code to overflow the buffer, crash a process and enter parts of a system where it may be subsequently executed. ®

External Links

Microsoft's security advisory

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