Anthropic has warned Claude users that hackers are using infostealer malware to steal login sessions and consume their token allowances. The company said it became aware of a bad actor using common infostealer malware to steal Claude login sessions from people's computers, then using those sessions to access accounts and consume usage.
One affected user, Grant de Swardt, an independent AI consultant in East Sussex, UK, noticed his Claude Max 20x account was consuming tokens on August 4 even though he was not working. After he disabled everything attached to Claude, token consumption still increased. Anthropic suspended his account, invalidated his sessions and tokens, and issued a partial refund of £44.49 for the remaining time on his $200-per-month subscription.
Anthropic told de Swardt that a compromised Claude session key was used to mint unauthorized Claude Code OAuth tokens. The company said the account appeared to have been used by an unauthorized-looking third-party service, but it could not determine how access was obtained. De Swardt said he found no evidence his computer was compromised.
Other users reported similar issues on Reddit and GitHub, including accounts being auto-upgraded without consent and usage rising sharply without activity. Anthropic said the malware did not come from using Claude itself, and it has signed out affected users, invalidated authorizations, and issued some refunds.