Security researchers have detected exploit code for three vulnerabilities that Microsoft plugged earlier this month, the SANS Internet Storm Centre has warned. 

Microsoft had issued a severity rating of 'critical' for two of the exploited vulnerabilities.

The first concerns a flaw in the Server component of Windows that could allow an attacker to take control of a system. McAfee warned that hackers could exploit the hole to launch a worm outbreak. 

Security researchers had warned that the vulnerability was the most likely candidate of the July patches for attackers to target.

A second exploit targets a vulnerability in the DHCP Client Service that could lead to a buffer overflow, allowing an attacker to take control of a system. The flaw affects several versions of Windows including XP and Server 2003. 

The final exploit uses a flaw in Microsoft Internet Information Services for some Windows versions. Although XP Pro and Windows Server 2003 are vulnerable, users of the consumer version of XP are not affected. 

The flaw could allow attackers to take control of a system through a specially crafted Active Server Pages file.

Microsoft rated the vulnerability as 'important' because most systems are configured in such a way that they do not allow the code to be executed.

The software giant released updates for the three holes as part of July's security patch, at which time the company fixed a total of 18 vulnerabilities.