LastPass officials warned Monday that attackers have compromised servers that run the company's password management service and made off with cryptographically protected passwords and other sensitive user data. It was the second breach notification regarding the service in the past four years.

In all, the unknown attackers obtained hashed user passwords, cryptographic salts, password reminders, and e-mail addresses, LastPass CEO Joe Siegrist wrote in a blog post . It emphasized that there was no evidence the attackers were able to open cryptographically locked user vaults where plain-text passwords are stored. That's because the master passwords that unlock those vaults were protected using an extremely slow hashing mechanism that requires large amounts of computing power to work.

"We are confident that our encryption measures are sufficient to protect the vast majority of users," Siegrist wrote. "LastPass strengthens the authentication hash with a random salt and 100,000 rounds of server-side PBKDF2-SHA256, in addition to the rounds performed client-side. This additional strengthening makes it difficult to attack the stolen hashes with any significant speed."

By contrast, many sites have used extremely fast hashing algorithms that provide minimal protection. Despite the rigor of the LastPass hashing regimen, the job of cracking a single hash belonging to a specific, targeted individual would be considerably less difficult and potentially within the ability of determined attackers, especially if the underlying password is weak. To prevent such attacks, LastPass officials are requiring all users who log in from new devices or IP addresses to first verify their account by e-mail unless they have multifactor authentication enabled. As an added precaution, LastPass is also prompting users to update their master passwords. LastPass users who haven't already done so should strongly consider enabling multifactor authentication.

The breach comes four years after LastPass officials detected anomalies in their server logs that were consistent with a network compromise. The data that may have been accessed in that 2011 event included hashed passwords, the underlying cryptographic salts, and user e-mail addresses. That same year, a security researcher reported finding a cross-site scripting (XSS) bug on the LastPass website that he said made it possible for attackers to steal sensitive user data. Information at risk, the researcher said, included e-mail addresses, password reminders, the list of sites users logged into, and the time, dates, and IP addresses of those logins. LastPass promptly fixed the bug.

The hack is sure to reopen the age-old debate about the wisdom of storing dozens or possibly hundreds of passwords in the cloud. Even when those passwords are robustly protected, as they appear to have been by LastPass, many experts say the cloud remains an unsuitable storage environment given the vulnerability of Internet-facing servers.

Then again, end-user computers are also notoriously easy to compromise, making it hard to argue there's any safe haven for such sensitive data. As Ars has long documented, the risks of using no password manager at all are also significant. Further compounding the password predicament are studies showing vulnerabilities in specific password managers that make it possible for attackers to obtain vault contents.

Update: In an e-mail to reporters, Ars resident password expert Jeremi Gosney said the real-world risks the breach posed to end users was minimal. He based his assessment on the LastPass response to the breach and the system that was in place when it happened. He paid particular attention to the 100,000-round hashing routine, which he said was among the strongest he has ever seen. Gosney, a password security expert at Stricture Group, wrote:

On an NVIDIA GTX Titan X, which is currently the fastest GPU for password cracking, an attacker would only be able to make fewer than 10,000 guesses per second for a single password hash. That is proper slow! Even weak passwords are fairly secure with that level of protection (unless you’re using an absurdly weak password.) And this doesn’t even account for the number of client-side iterations, which is user-configurable. The default is 5,000 iterations, so at a minimum we’re looking at 105,000 iterations. I actually have mine set to 65,000 iterations, so that’s a total of 165,000 iterations protecting my Diceware passphrase. So no, I’m definitely not sweating this breach. I don’t even feel compelled to change my master password.

Post updated to correct Jeremi Gosney's math in the last paragraph. Gosney previously said an attacker with an Nvidia GTX Titan X would be able to make only 10 guesses per second. He later discovered an error in his calculations and concluded that the correct number of guesses per second was 10,000. He said 10,000 guesses per second remains extremely slow, and he stands by the rest of what he had to say.