Infostealer Malware Hijacks Claude AI Sessions and Drains Paid Usage
Infostealer malware on user computers stole active Claude login sessions, allowing attackers to drain paid usage quotas without user knowledge. Anthropic detected the activity, identified six malware families, and revoked sessions while refunding unauthorized charges, though the company noted the malware itself remains a threat if not removed from affected devices.
What happened
Infostealer malware on user computers stole active Claude login sessions without requiring passwords or bypassing multi-factor authentication, allowing attackers to access paid accounts.
Anthropic identified multiple infostealer families affecting Windows and macOS, including Vidar, LummaC2, StealC, RedLine, Acreed on Windows, and Atomic Stealer (AMOS) on macOS.
Reported impact
Attackers used stolen sessions to drain paid usage quotas without user knowledge, with some users reporting their usage limits refilling and then depleting while they were not using Claude.
Anthropic signed affected users out of Claude, removed saved payment cards, and refunded unauthorized charges.
Index analysis
The malware is general-purpose software unrelated to Claude itself, typically arriving through unofficial downloads or malicious apps, and collecting saved passwords, login cookies, and credentials from multiple services.
Infostealer malware bypasses traditional security measures by stealing authenticated browser sessions rather than targeting passwords or multi-factor authentication systems.
Governance controls
Anthropic detected the suspicious activity through its systems and revoked affected sessions.
Anthropic identified the malware families responsible and issued notifications to impacted users, though the company noted that revoking sessions alone is insufficient if malware remains on the user's device.
Classification
Sources and evidence
This record was researched and written by the Index, based on the following original reporting.