Anthropic's Claude users are facing token theft through compromised session keys. In one case, a user with a $200 monthly subscription noticed token consumption increase from 45% to 55% while inactive. Anthropic responded by suspending the account, invalidating sessions, and issuing a partial refund of £44.49.
The company confirmed that a compromised session key was used to mint unauthorized Claude Code OAuth tokens. Anthropic stated that the account was used by an unauthorized third-party service to handle activity for other people, though the exact method of access was not determined.
Anthropic later warned some users via email that 'bad actors' are using infostealer malware to steal login sessions from computers. This malware, which steals passwords and session data, does not originate from Claude itself but from external sources like infected software or ads.
Several users on Reddit and GitHub reported similar issues, where token usage jumped from 0% to 100% or 0% to 49% in 12 minutes. Some users experienced maximum token burn through every day for three days. Anthropic has signed out affected users and invalidated authorizations to mitigate the risk.
A user who experienced the theft reported that the lack of itemized usage tracking tools for users prevents them from the ability to protect themselves. The user eventually cancelled their subscription in favor of of Cursor, which supports multiple models including open-source options.