Anthropic warns Claude accounts are being hijacked by password-stealing malware

Key points

• Anthropic is emailing Claude users whose login sessions were stolen by infostealer malware, small programs that quietly copy passwords and browser data from an infected computer.
• The company has linked the theft to well-known Windows infostealers Vidar, LummaC2, StealC, RedLine and Acreed, plus Atomic Stealer (AMOS) on a small number of Macs.
• Because the attackers stole live session cookies, they could open Claude accounts without needing the password or a two-factor code.
• Anthropic is signing affected users out, wiping saved payment methods, and refunding charges it identifies as unauthorised.
• At least one victim, first reported by BleepingComputer, traced the infection to a pirated game download.

Anthropic has told a group of Claude users that criminals have been signing into their accounts and running up usage, after malware on their own computers quietly copied their login sessions.

The company is emailing affected customers, signing them out of Claude, removing saved cards, and refunding charges it can identify as fraudulent.

"If your usage limits looked like they refilled and then drained while you weren't using Claude, this was likely the cause," Anthropic wrote in the notice, which one recipient posted to Reddit.

This is not a hack of Anthropic. The break-in happened on the customer's own PC.

How did the attackers get in without a password?

They didn't need one. The malware stole something called a session cookie: a small file your browser holds after you log in, which tells a website "this person is already signed in." Copy that file to another computer and the site treats the attacker as you, no password or two-factor code required.

That is why Anthropic's first move is to sign affected users out. Killing the session invalidates the stolen cookie.

What malware is behind it?

Anthropic named a familiar lineup of commodity infostealers, meaning cheap, off-the-shelf malware sold in criminal forums that scoops up whatever it can find on an infected machine. The company listed Vidar, LummaC2, StealC, RedLine and Acreed on Windows, along with Atomic Stealer (AMOS) on a small number of Macs.

These families are not targeted spy tools. They are broad nets. They grab saved browser passwords, cookies, cryptocurrency wallet files, and credentials from other installed apps, then send the lot back to whoever deployed them.

With medium confidence, the pattern here looks like a criminal buying stolen logs in bulk and then sifting them for Claude sessions to resell or use, rather than a nation-state operation. Anthropic has not publicly attributed the activity to a tracked cluster.

How do people catch this malware in the first place?

Usually by installing something they shouldn't. Pirated software, cracked games, fake browser extensions and dodgy "free" tools are the classic delivery routes. The Reddit user whose email kicked off the story said they had downloaded a pirated game.

Anthropic was clear that the malware has nothing to do with Claude itself: "We have no reason to believe that this malware is related to Claude, installed through Claude, or related to anything you did with Claude."

What should affected users do?

Signing out of Claude stops the current theft but does not clean the PC. If the infostealer is still running, the next login can be stolen the same way.

Anthropic's advice, in plain terms:

The wider lesson for anyone using AI services with a paid plan: a saved session in your browser is worth money to someone. Treat it like cash.