The moderator for the company's U.K. customer list on Feb. 26 forwarded an e-mail generated by the virus NetSky.B. As a result, the company will no longer accept outside e-mail messages to its list, and attachments will be blocked, said Mikko Hypponen, director of antivirus research for the company.
"We have taken several steps after (the incident) to make sure it will never happen again," he said.
The virus e-mail had been sent to the antivirus list's moderator after a PC in the United States--with no association to F-Secure--became infected. The address for the list happened to be on the computer. When the moderator received the e-mail, he accidentally gave permission for the software to forward the message to subscribers.
"We immediately understood what happened," said Hypponen, who called the incident a "human error."
No customers were infected as a result of the mistake, he added.
"We were lucky that this was a virus that we already knew about," he said.
Between last Friday and this Wednesday, more than 10 variants of the NetSky, Bagle and MyDoom worms have been discovered by antivirus firms.
Variants spreading among computers on the Internet in the past three days have contained messages, complete with vulgar taunts, that indicate that the authors of MyDoom and Bagle have.