Symantec issued an alert Monday that it had detected unusual amounts of scanning of a port normally associated with Microsoft SQL Server, a possible precursor to an attack.

The Cupertino, Calif.-based company's DeepSight Threat network, a global collection of sensors that tracks developing attacks, reported an increase in the number of scanned sensors and the number of attacking IP addresses scanning TCP port 1433, which is commonly used by Microsoft's server software.

Although there are no known vulnerabilities in SQL Server, Symantec's alert said it was still a likely preface to an attack. "This activity likely represents active scanning by a malicious code network in an attempt to locate machines susceptible to password-based brute force attacks," the DeepSight team wrote in the warning. "Code components designed to accomplish the above task are publicly available, and have been included in a number of active worms, including Spybot and Gaobot."

Symantec's analysts recommended that administrators block access to port 1433 at the perimeter, and within the network, limit access to the port to trusted hosts.