Cybercriminals are already exploiting an unpatched hole in IIS server software, made public last week, according to Microsoft.
The flaw can be exploited to let an attacker take control of an older IIS 5.0 server running on Windows 2000, provided the hacker has some way of creating an FTP directory on the server. Attack code that exploits the bug was posted last week.
Other IIS users could also be hit with a denial of service (DoS) attack, thanks to a second attack, posted to the Milw0rm website last Thursday.
This new code could be used to launch a DoS attack against IIS 5.0, 5.1, 6.0 and 7.0, and could affect users running IIS on Windows XP and Windows Server 2003, Microsoft said. For the attack to work, however, the server needs to be running the FTP service, and the attacker must be able to read files on the system.
Microsoft has updated its security advisory on the issue, saying it was starting to see "limited attacks that use this exploit code."
That generally means that only a handful of attacks have been spotted. Other security vendors contacted said they had not seen the IIS bug being used in attacks.
Microsoft will release its scheduled September security updates on Tuesday, but it is not expected to fix this bug until it has had more time to test and develop a patch.
"The initial vulnerability was not responsibly disclosed to Microsoft, which has led to limited, active attacks putting customers at risk," Microsoft said in a blog posting last Thursday.
Microsoft didn't say whether the attacks it had seen involved installing malicious software on an IIS server or simply making it crash. The company did not respond to a request for more information on the subject.