Anthropic Warns of Malware Stealing Claude Sessions to Exhaust User Credits

How Attackers Exploit Stolen Sessions to Abuse AI Access

Anthropic has alerted certain Claude users that infostealer malware present on their personal computers is actively hijacking authenticated sessions. This allows attackers to gain unauthorized access to accounts and consume allocated usage credits without the user’s knowledge. The warning emerged in late August 2026, targeting individuals whose devices may be compromised by credential-stealing software.

The malware operates by extracting active login tokens or session cookies from browsers or applications where Claude is logged in. Once obtained, these tokens enable threat actors to impersonate legitimate users and make API calls or interact with the Claude interface directly. Anthropic confirmed it is responding by signing affected users out of all active sessions as a precautionary measure to halt unauthorized access.

What Steps Is Anthropic Taking to Protect Users?

By stealing session data rather than passwords, attackers bypass traditional login protections and appear as legitimate traffic to Anthropic’s systems. This method allows them to avoid triggering login alerts while still draining computational resources tied to a user’s subscription or free tier limits. The company emphasized that no breach occurred on its servers; the compromise originates solely from infected user devices.

Anthropic is notifying impacted users directly and advising them to run antivirus scans and update their security software. The firm also recommends re-authenticating only after confirming devices are clean. While no data theft beyond usage abuse has been reported, Anthropic is monitoring for patterns of anomalous activity linked to known malware families.

How can users tell if their Claude session has been compromised? Users may notice unexpected usage spikes or find themselves logged out without action. Anthropic advises checking account activity logs and signing out of all sessions if suspicious behavior is detected.

Frequently Asked Questions

Is personal data at risk if a Claude session is stolen? Anthropic states that session hijacking primarily allows misuse of computational credits, not direct access to stored conversations or personal information, though users should still treat any compromise seriously.

Should users change their passwords after such an incident? While password changes are good practice, the immediate priority is removing malware from the device, as stolen sessions can be reused until invalidated by logout or expiration.