Bit9, a software security company that says three of the top 10 Fortune 500 banks are among its clients, has been hacked and was temporarily being used by criminals to help spread malicious software, the company said in a blog post.

The Waltham, Mass., firm said the attack was short-lived, and only affected three of its customers.

The news was first reported, ahead of the Bit9's announcement, by technology and security blog KrebsOnSecurity. Reuters later followed up on the story.

Bit9 provides security services that whitelist applications and control what software and devices are allowed to operate on a customer's network.

"Due to an operational oversight within Bit9, we failed to install our own product on a handful of computers within our network," the Bit9 blog explained. "As a result, a malicious third party was able to illegally gain temporary access to one of our digital code-signing certificates, which they then used to illegitimately sign malware."

At one time, Bit9's clients included Omgeo, Putnam Investments and Thomson Financial. Bit9 was highlighted by Bank Technology News' magazine in 2008 as one of the top 10 companies to watch that year. The company was founded in 2002.