A computer virus has stolen 45,000 Facebook passwords from users, according to security experts.
The virus, known as Ramnit, was first identified in April 2010 and has previously stolen banking information.
The Security firm Seculert says 800,000 computers were infected with the virus from September to the end of December 2011.
Seculert warns that the theft of private data, which seems to have mainly affected users in France and the U.K., stands to compromise other Internet services.
"Cybercriminals are taking advantage of the fact that users tend to use the same password in various web-based services to gain remote access to corporate networks," the firm said Friday on its blog.
"We suspect that the attackers behind Ramnit are using the stolen credentials to login to victims' Facebook accounts and to transmit malicious links to their friends, thereby magnifying the malware's spread even further," the company added.
Facebook said in a statement that it has taken steps to restore the security of any users affected by the virus.
"Thus far, we have not seen the virus propagating on Facebook itself," said the social network, adding that users can help protect themselves by not clicking on strange links.
Users infected with Ramnit are advised to run anti-virus software.
Securlert notes that hackers are moving beyond writing email-based malware to more sophisticated code that preys on information-rich social networking sites.
