Security software maker Bit9 said that computer hackers have breached its network, then launched a second round of attacks against some of its customers.

The hackers accessed a system that Bit9 said it uses to digitally sign its software to let customers know it is safe to run on their computers, said Reuters. The hackers then forged Bit9's digital signature on malicious software, which they used to attack some of its customers, according to the Reuters report.

The report said that Bit9 believed the hackers were able to access one of its internal systems because the company had failed to properly install its own software throughout its network.

Bit9, which has about 1,000 customers including U.S. government agencies and major defense, energy and financial companies, is one of the leading providers of security technology known as "white listing," Reuters said.

Unlike traditional anti-virus software, which seeks to block malicious programs, white listing looks to protect systems from attack by only allowing computers to run programs from trusted vendors.

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