It's not a good day for LinkedIn. After reports that its iOS app potentially violates user privacy by sending detailed calendar entries to its servers, comes a report that 6.46 million encrypted LinkedIn passwords have leaked online.

A Russian forum user claims he has hacked LinkedIn, uploading 6,458,020 encrypted passwords (without usernames) as proof.

The passwords are encrypted with the SHA-1 cryptographic hash function, used in SSL and TLS and generally considered to be relatively secure, but not foolproof. Unfortunately, it also seems that passwords are stored as unsalted hashes, which it makes it much easier to decipher them using pre-computed rainbow tables.

In simple terms, this means an attacker might be able to crack many of the passwords using very cheap resources in a relatively short amount of time.

SEE ALSO: How to Change Your LinkedIn Password

While there's a possibility that the password collection is not genuine, some reports on Twitter add credibility to the story. LinkedIn said on Twitter it's looking into the issue.

Our team is currently looking into reports of stolen passwords. Stay tuned for more. — LinkedIn (@LinkedIn) June 6, 2012

Finnish security company Cert-Fi has posted a warning about the incident, saying it is "likely" that whoever hacked LinkedIn possesses the accompanying user names as well.

If you're a LinkedIn user, we recommend you change your password right now. Furthermore, if you used that password on any other online service, we recommend you change those passwords as well.