Anthropic proactively signed an unknown number of users out of Claude after a threat actor stole their login sessions, accessed their accounts, and consumed their allotted usage.
The attacks came to light via email alerts sent to affected users that were then posted to social media. The theft of login sessions and unauthorized access to Claude accounts resulted from infostealer malware on users' systems, rather than from any malware related to or installed through Claude, Anthropic said in the email notifications.
Claude Accounts Under Attack
The AI company said it had signed affected users out of Claude and removed their saved payment methods after detecting suspicious activity on their accounts. Anthropic's ongoing investigation has found that the threat actor stole Claude login sessions using general-purpose infostealers that had previously been installed on users' systems, likely through a malicious app or unofficial download.
“The malware identified in this campaign so far include Vidar, Lumma (LummaC2), StealC, RedLine and Acreed on Windows, and Atomic Stealer (AMOS) on a small number of Macs,” Anthropic said in its email to affected users. It's likely that affected users have had the infostealers on their system for some time, the company noted.
The Anthropic incident is another example of attackers shifting from stealing passwords to targeting session cookies and authentication tokens. With many organizations implementing stronger password security protocols and multifactor authentication (MFA), traditional credential theft has become harder, so attackers have increasingly begun targeting session artifacts to hijack already-authenticated sessions and bypass MFA altogether.
Experts have described the shift as complicating incident response because resetting a password alone may not cut off an attacker who already has a valid session or refresh token. In Anthropic's case, for instance, infostealers harvested Claude sessions, likely along with other sensitive information such as login cookies, saved passwords and credentials for other apps. The stolen Claude sessions allowed the attackers to access the associated accounts without having to defeat the authentication controls protecting them.
As one user, who posted Anthropic's email communication on Reddit noted, "The hacker stole all my Google Chrome credentials, including cookies and session IDs, which allowed him to bypass all two-factor authentication security measures."