Anthropic started reaching out to impacted Claude users on August 30, 2026, after discovering that infostealer malware had compromised their computers and siphoned off active login session cookies. The stolen cookies gave attackers a backdoor into user accounts, letting them burn through paid usage quotas without ever needing a password or cracking two-factor authentication.
What happened and how Anthropic responded
The breach came to light through unusual usage patterns on Claude accounts. Once Anthropic’s security team connected the dots, they moved quickly. All compromised sessions were forcibly signed out. Saved payment methods were stripped from affected accounts. And for users who got hit with unauthorized charges from attackers running up their quotas, Anthropic issued refunds.
Anthropic was also careful to draw a clear line: the malware had nothing to do with Claude itself, its infrastructure, or anything users did on the platform. The infections originated from malicious downloads and compromised software applications that users had installed on their own machines.
The malware families identified in the campaign included Vidar, LummaC2, StealC, RedLine, and Acreed on Windows. A limited number of Mac devices were also affected, primarily through Atomic Stealer, also known as AMOS.
