Microsoft has released its security updates for the month of September, but a couple of unpatched flaws have some security experts wondering if the software company will be forced to release an emergency patch sometime in the month ahead.

Security researchers believe that an unpatched flaw in the SMB (Server Message Block) 2 software that ships with Windows Vista and Windows Server 2008 could turn into a major headache.

Proof-of-concept code showing how the bug could be used to crash a Windows machine was posted on Monday to the Full Disclosure mailing list by Laurent Gaffie. But security experts believe that more serious attacks are possible.

Kostya Korchinsky, a senior security researcher with security-assessment software vendor Immunity, said the flaw could be exploited in a privilege-escalation attack. This type of attack is used once the attacker has already found a way to run software on the victim's machine. It gives the hacker a way of accessing system resources that would otherwise be prohibited.

A more dangerous 'remote-code execution' attack "might be possible, but it would be a lot more difficult", Korchinsky said. With remote-code execution, the attacker is able to run unauthorised software on the victim's machine.

Security vendor SourceFire is examining the bug too. "We're unwilling to call it a DoS-only, but we're not willing to call it a remote-code-execution [flaw] either," said Matt Watchinski, the company's senior director of vulnerability research, referring to a denial-of-service attack.

SMB 2 is typically blocked at the firewall, so even if these attacks could be written, they would have a hard time spreading from company to company.

Gaffie said the flaw most likely works on Windows 7, Windows Vista and Windows Server 2008. Earlier versions of Windows do not use SMB 2 and are thought to be immune.

Meanwhile, Microsoft has yet to patch a flaw in its Internet Information Services (IIS) software that was disclosed last week. That bug could let an attacker crash an IIS server, or even install unauthorised software in certain configurations.

The flaw could be used in a remote-code execution attack, but only in very specific circumstances. For the attack to work, the victim must run the older IIS 5 software on Windows 2000 and allow the attacker to create an ftp directory on the server.

Although Microsoft says it's seen a "limited number" of attacks that leverage this bug, Watchinski said it's unlikely to affect most IIS users.

Microsoft issued five security patches Tuesday, fixing eight vulnerabilities in Windows.

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Windows security advice