Anthropic warns infostealer malware is hijacking Claude sessions to drain usage

Infostealer malware infecting user PCs is stealing active Claude login sessions by capturing authenticated browser sessions, enabling attackers to access Claude accounts without needing passwords or 2FA. This results in unauthorized usage consumption and potential charges. Anthropic's investigation links the malware to known infostealers like Vidar, LummaC2, StealC, RedLine, and others on Windows, and Atomic Stealer on Macs. The malware is delivered through common infection vectors such as pirated software downloads and malicious apps. Anthropic is mitigating impact by signing out affected users, removing saved payment methods, and issuing refunds for unauthorized usage. The company emphasizes that the malware is not related to Claude's software or infrastructure and that users must remove the malware from their devices to prevent further session theft.

Attackers gain unauthorized access to Claude user accounts by hijacking active login sessions stolen from infected PCs. This allows attackers to consume usage quotas and potentially incur charges on victims' accounts. The compromise bypasses normal authentication mechanisms including passwords and two-factor authentication since the attackers use stolen authenticated sessions. Anthropic has taken steps to limit financial impact by revoking sessions, removing payment methods, and refunding unauthorized charges. However, if the malware remains on the user's device, subsequent sessions remain vulnerable to theft.

Anthropic has implemented mitigation by signing out affected users, removing saved payment methods, and refunding unauthorized charges. Users should remove the infostealer malware from their devices using appropriate anti-malware tools. They should also change their Claude account credentials and revoke any other active sessions to prevent further unauthorized access. Since the malware is unrelated to Claude itself, securing the endpoint is critical to prevent reinfection and session theft. Anthropic's actions stop current unauthorized usage but do not remove the malware from infected devices, so user action is necessary.