Some security patches take time.

Seven-and-a-half years, in fact, if you count the time it's taken Microsoft to patch a security issue in its SMB (Server Message Block) service, fixed Tuesday. This software is used by Windows to share files and print documents over a network.

In a blog posting, Microsoft acknowledged that "Public tools, including a Metasploit module, are available to perform this attack." Metasploit is an open-source toolkit used by hackers and security professionals to build attack code.

According to Metasploit, the flaw goes back to March 2001, when a hacker named Josh Buchbinder (a.k.a Sir Dystic) published code showing how the attack worked.

Symantec Research Manager Ben Greenbaum said the flaw may have first been disclosed at Defcon 2000, by Veracode Chief Scientist Christien Rioux (a.k.a. Dildog)

Whomever discovered the flaw, Microsoft seems to have taken an unusually long time to fix the bug.

"This is definitely out of the ordinary," Greenbaum said. He said he did not know why Microsoft had waited so long to fix the issue.

"I've been holding my breath since 2001 for this patch," said Shavlik Technologies Chief Technology Officer Eric Schultze, in an e-mailed statement. Buchbinder's attack, called a SMB relay attack, "showed how easy it was to take control of a remote machine without knowing the password," he said.

For the attack to work, a victim could be sent a malicious e-mail message that, when opened, would try to connect to a server run by the attacker. That machine would then steal network authentication credentials from the victim, which could then be used to gain access to the victim's machine.

This type of attack would be blocked by a firewall, so a hacker would have to already be on a computer within the network in order to launch the SMB relay. Microsoft rates the flaw as "important" for Windows XP, 2000 and Server 2003 users, and as "moderate" for Vista and Server 2008.

Nevertheless Schultze said he considered the patch "critical" for machines on a corporate network.

He said the attack is "pretty easy" to pull off today. "It's a great vector of attack on a corporate network where file and print sharing ports and services may be unprotected," he said.

Microsoft representatives were unable to comment for this story Tuesday afternoon.