Care2.com, a social networking website for activists, has decided to reset the account passwords of almost 18 million registered members after hackers gained access to its servers.

The compromise was discovered last Tuesday and, according to the company operating the website, it only affected a limited number of accounts. However, because Care2 was not able to determine the full extent of the breach it has opted to revoke all login credentials.

"To protect Care2 members we are resetting access to all Care2 accounts. The next time you login to Care2, you will be automatically emailed a new password, which will enable you to access your Care2 account as usual," the company said.

The identities of the hackers or their reasons for targeting the social networking website are not clear. The only information available at this time is that Care2's servers were attacked from an IP address in Russia.

"Hackers are most likely looking for login information they can exploit on financial websites," the company said. "Individuals often use the same login information on multiple sites, so if a hacker can get your login credentials on one site, they can then try using those same details to login to a financial site."

Password reuse is a common practice and security experts have long advised users to generate unique access codes for every website they use. Despite this, many database breaches have resulted in compromised accounts on multiple websites in the past.

Care2 did not specify how user credentials are being stored in its database, but its password recovery process sends the user's password in plain text to their corresponding email address. This behavior suggests that the method used is not safe.

The industry standard is to never store passwords in recoverable form, regardless of whether they are in plain text or encrypted with a key accessible to the server. Instead, websites should store unique cryptographic hashes generated with secure algorithms.

Authenticating users in this way involves generating hashes from supplied passwords on the fly and comparing them to the ones stored in the database. When users can't remember their passwords, the server shouldn't be able to recover them and should generate new ones.

Core2 did not immediately return a request for comment, but an automated email reply from its customer support department states that "for security reasons we cannot provide details on the system, as to do so would give other would-be hackers clues to exploit."