Updated Password vault LastPass is scrambling to patch critical security flaws that malicious websites can exploit to steal millions of victims' passphrases.

The programming cockups were spotted by Tavis Ormandy, a white-hat hacker on Google's crack Project Zero security team. He found that the LastPass Chrome extension has an exploitable content script that evil webpages can attack to extract usernames and passwords.

LastPass works by storing your passwords in the cloud. It provides browser extensions that connect to your LastPass account and automatically fill out your saved login details when you surf to your favorite sites.

However, due to the discovered vulnerabilities, simply browsing a malicious website is enough to hand over all your LastPass passphrases to strangers. The weak LastPass script uncovered by Ormandy can be tricked into granting access to the manager's internal mechanisms, which is rather bad news.

The script can also be abused to execute commands on the victim's computer – Ormandy demonstrated this by running calc.exe simply by opening a webpage. A malicious website could exploit this hole to drop malware on a visiting machine. A victim must have the binary component of LastPass installed to be vulnerable to this attack.

"This script will proxy unauthenticated window messages to the extension. This is clearly a mistake," Ormandy explained in a bug report today.

"This allows complete access to internal privileged LastPass RPC [remote procedure call] commands. There are hundreds of internal LastPass RPCs, but the obviously bad ones are things copying and filling in passwords (copypass, fillform, etc)."

All that's needed to exploit the vulnerability are two simple lines of JavaScript code, which Ormandy supplied:

win = window.open("https://1min-ui-prod.service.lastpass.com/"); win.postMessage({}, "*");

The password manager developer has experience with Ormandy after he found another flaw in its code last year that could compromise a punter's passwords just by visiting the wrong website.

"We greatly appreciate the work of the security community to challenge our product and uncover areas that need improvement," Joe Siegrist, cofounder and VP of LastPass, told The Register.

"We have made our LastPass community aware of the report made by Tavis Ormandy and have confirmed that the vulnerabilities have been fixed. We were notified early on – our team worked directly with Tavis to verify the report made, and worked quickly to issue the fix. As always, we recommend that users keep their software updated to the latest versions."

It appears LastPass's fix for the Chrome extension issue was to quickly disable 1min-ui-prod.service.lastpass.com – although some say the server is still working for them, so they are still vulnerable. That LastPass backend system resolves to 23.72.215.179 for us right now, and is still up.

It's probably best to disable the Chrome extension until a version newer than 4.1.42, dated March 14, is sent out with an actual working fix.

And now its Firefox add-on

It has been a busy weekend for LastPass software engineers. Late last week, Ormandy found another LastPass vulnerability, this time in its Firefox extension. Again, the vulnerability can be exploited by malicious webpages to extract passwords from the manager.

Wrote a quick exploit for another LastPass vulnerability. Only affects version on https://t.co/lGcefN9YXM (3.3.2), report on way. ¯\_(ツ)_/¯ pic.twitter.com/AgjASiQMfJ — Tavis Ormandy (@taviso) March 16, 2017

That extension bug has been addressed, we're told, but the security patch won't be pushed out to people until the update is approved by Firefox-maker Mozilla. "The team has already issued a patch to fix [version] 3.3.2 and that updated version is currently in the Mozilla review process," a LastPass spokeswoman told us. She also said the 3.x branch of the add-on is being retired, and people should move onto the version 4.x family.

As we've said in the past, keep your password managers up to date. They're like any other software, and all software is exploitable. If you're a LastPass user, disable your Chrome and Firefox extensions until a fix is definitely available. ®

Stop press: Ormandy has found another password-leaking bug in LastPass for Firefox 4.1.35. That'll need patching, too. Or just dump LostPass and find other manager.

Updated to add on March 22

LastPass has put out an "incident report" that insists its browser extensions have been patched to squash the above reported bugs, and these builds are being pushed to users: check to see if you're running the latest version of LastPass on your computer, and update your extension if the software hasn't automatically fixed itself. LastPass has also made server-side changes to close the security holes.

You should be using LastPass version 4.1.36 with Firefox, 4.1.43.82 with Chrome, 4.1.30 with Edge, and 4.1.28 with Opera. Note that the add-on maker is waiting for Microsoft and Opera to approve the updates from their browser extension stores, so the new builds may or may not be there right this second for those particular platforms. Check back later if they are not.

"Our investigation to date has not indicated that any sensitive user data was lost or compromised," LastPass added. "Our mobile apps for Android and iOS were not affected. No master password change is required. No site credential passwords need to be changed."