Anthropic emailed Claude users about stolen sessions. What was taken was not a password.

On 30 August 2026 BleepingComputer published the text of an email Anthropic has been sending to Claude users whose accounts somebody else was using. The notice, as quoted there:

We have recently become aware of a bad actor that is using common infostealer malware to steal Claude login sessions from people's computers, then using those login sessions to access Claude accounts and consume their usage.

Login sessions. Not passwords, and the difference decides what a recipient should actually do.

A session cookie is the receipt for an authentication that already succeeded. Replaying it presents no password and raises no second-factor prompt, because as far as the server is concerned the login happened days ago and this is the same browser coming back. CyberPress, on 31 August 2026, states the consequence: stealing "already-authenticated session cookies rather than passwords" means "the theft bypasses two-factor authentication and single sign-on entirely, letting attackers replay a victim's session." Switching on 2FA afterwards guards a door the intruder is not using.

Anthropic's remedy is the right one and it is not a password reset. From the email as reproduced by Search Engine Journal, retrieved 31 August 2026: "We recently signed you out of Claude and removed the payment method saved on your account, so you'll need to log back in and re-add your card." On why the sign-out is the operative act rather than a precaution: "Signing you out cancels that session everywhere, so the stolen copy stops working." A third action shows up only in BleepingComputer's reporting, nowhere in the email text Search Engine Journal reproduces: the company is "refunding charges it identifies as unauthorized."

Then the sentence worth forwarding, which BleepingComputer carries:

Signing you out of Claude stops the stolen sessions, but it doesn't remove the malware. If it's still on your computer, your next login session could be stolen the same way.

Those two instructions have an order, and it is not the order they arrive in. The revocation already happened, so the account is not where the remaining exposure sits. Logging back in on a machine you have not cleaned issues a fresh cookie into the same collection, and re-adding the card gives the next session something to spend. Clean the machine first.

If you are wondering whether this was you, the notice offers a symptom: "If your usage limits looked like they refilled and then drained while you weren't using Claude, this was likely the cause."

The malware is unremarkable, which is the interesting part. Anthropic named Vidar, LummaC2, StealC, RedLine and Acreed on Windows, plus Atomic Stealer (AMOS) on a small number of Macs. Not one of them was built for this. They are commodity credential thieves that arrive, in BleepingComputer's description, through downloads or malicious apps, and sweep up "browser passwords, login cookies, and credentials belonging to other apps." A Claude cookie is a line item in that haul, not the objective. What is new sits on the other side: a subscription that converts into inference is now worth stealing on the same terms as a bank login.

Two limits on all of the above. Anthropic has published nothing of its own. status.claude.com listed no incident of this kind for August 2026 when checked on 31 August 2026, and none of the outlets carrying the story links an Anthropic page, so the chain runs entirely through one user's email, screenshotted on Reddit and reproduced by reporters. And there are no figures: not how many accounts, not how much usage, not who the bad actor is. The notice does not say, and nobody outside Anthropic is positioned to.

All four accounts below were retrieved on 31 August 2026. BleepingComputer published on 30 August 2026. Cybersecurity News and CyberPress both published on 31 August 2026. Search Engine Journal carries more of the email text than the others and shows a relative timestamp rather than a date.