Infostealers replayed stolen Claude session cookies into paid accounts without ever touching the login page two-factor authentication guards.

The accounts Anthropic flagged were card-billed, self-serve accounts, which is the population no corporate identity provider governs, and no admin console can sign out. Session-cookie replay bypasses SSO as thoroughly as it bypasses 2FA. What SSO provides here is revocation and visibility, not prevention. The company disclosed the campaign in notification emails to affected users, named six stealer families, signed the accounts out, stripped the saved payment methods, and refunded the charges it found.

The burned usage is the small loss. What those sessions could reach is the exposure, and none of it sat behind an identity controlled by an enterprise.

Anthropic told affected users that a bad actor was using common infostealer malware to lift Claude login sessions off their computers and then replaying them to burn the accounts' usage, according to the notification an affected user posted to Reddit and BleepingComputer reported on August 30.

It named Vidar, LummaC2, StealC, RedLine and Acreed on Windows and Atomic Stealer on a small number of Macs, and it described general-purpose malware that copies browser login cookies along with saved passwords. "Your Claude session was likely one of the many things it collected," the email said.

A session cookie is the proof that a login already happened

The attack chain runs in one direction, from an infected machine through a stolen cookie past a checkpoint that never fires, and into everything the account can reach.