I’ve championed the use of LastPass as a master password system for the Internet for years now, and of course as recently as this past week it was again a software pick on the Windows Weekly podcast. With that in mind, it’s important that all LastPass users read the following warning and, when possible, proactively change their master password with the service accordingly.
We noticed an issue yesterday and wanted to alert you to it. As a precaution, we're also forcing you to change your master password.
We take a close look at our logs and try to explain every anomaly we see. Tuesday morning we saw a network traffic anomaly for a few minutes from one of our non-critical machines. These happen occasionally, and we typically identify them as an employee or an automated script.
In this case, we couldn't find that root cause. After delving into the anomaly we found a similar but smaller matching traffic anomaly from one of our databases in the opposite direction (more traffic was sent from the database compared to what was received on the server). Because we can't account for this anomaly either, we're going to be paranoid and assume the worst: that the data we stored in the database was somehow accessed. We know roughly the amount of data transfered and that it's big enough to have transfered people's email addresses, the server salt and their salted password hashes from the database. We also know that the amount of data taken isn't remotely enough to have pulled many users encrypted data blobs.
If you have a strong, non-dictionary based password or pass phrase, this shouldn't impact you - the potential threat here is brute forcing your master password using dictionary words, then going to LastPass with that password to get your data. Unfortunately not everyone picks a master password that's immune to brute forcing.
To counter that potential threat, we're going to force everyone to change their master passwords. Additionally, we're going to want an indication that you're you, by either ensuring that you're coming from an IP block you've used before or by validating your email address. The reason is that if an attacker had your master password through a brute force method, LastPass still wouldn't give access to this theoretical attacker because they wouldn't have access to your email account or your IP.
We realize this may be an overreaction and we apologize for the disruption this will cause, but we'd rather be paranoid and slightly inconvenience you than to be even more sorry later.
Please read the entire post, only part of which is duplicated here.
Thanks to Deren S. for the tip.