Anthropic Claude accounts compromised by infostealer malware replaying session cookies

Claude Accounts Compromised by Session Cookie Replay

Anthropic informed users that their Claude accounts were compromised by infostealer malware. The attackers used stolen session cookies to access paid accounts, effectively bypassing two-factor authentication (2FA) and single sign-on (SSO) protections. This method allows unauthorized access without needing to interact with the login page.

Mechanism of Attack

Session cookies, which keep users logged in after initial authentication, were copied by general-purpose infostealer malware. By replaying these cookies, attackers appeared to the server as already authenticated users. Anthropic detected the activity through unusual usage patterns, such as limits being refilled and drained while account owners were inactive.

Affected Accounts and Malware Families

The compromised accounts were self-serve, card-billed accounts, which are not managed by corporate identity providers or admin consoles. Anthropic identified six infostealer families involved: Vidar, LummaC2, StealC, RedLine, Acreed on Windows, and Atomic Stealer on macOS. These malware types are known to collect browser login cookies and saved passwords.

Anthropic's Response

Upon discovery, Anthropic notified affected users, signed out the compromised accounts, stripped saved payment methods, and refunded charges incurred by the attackers. While the immediate financial loss from burned usage was small, the primary concern was the potential exposure of data accessible through these unauthorized sessions, none of which were behind enterprise-controlled identities.