Anthropic logs out Claude users hit by infostealer malware

Anthropic has started notifying Claude users whose accounts were accessed by attackers using stolen login sessions. It has signed the affected users out, stripped saved payment methods from their accounts, and refunded charges it flagged as unauthorized.

According to the notice Anthropic sent affected users, commodity infostealer malware running on victims' own computers collected their active Claude session cookies along with other locally stored browser data. Whoever obtained that stolen data then pulled the Claude sessions out and used them to sign in, running up paid usage on Pro and Max plans without ever needing the password or a second factor. The tell, per the notice, is a usage limit that appears to refill and then drain while the account owner is not using Claude.

The malware families named in the campaign are Vidar, LummaC2, StealC, RedLine, and Acreed on Windows, plus Atomic Stealer on a smaller number of Macs. None are new or specific to Claude. They are the same credential stealers that have been harvesting browser passwords and session tokens for years, typically arriving through pirated software, fake installers, or malicious browser extensions. Anthropic said it has no reason to believe the malware is connected to Claude or was installed through it. One affected user said publicly they were infected after downloading cracked software.

Session-cookie theft matters here because it sidesteps both the password and multi-factor authentication. A valid session is enough. For anyone running a paid Claude or Claude Code subscription with a card on file, the practical exposure is a drained quota and a stored payment method on an account someone else controls.

If you get the notice, or see usage you did not generate, change the password, revoke every active session, and remove any saved payment method. Treat any machine that has run unofficial or cracked software as compromised until it is cleaned.