Claude session-hijacking infostealer campaign

Infostealers now specifically monetize hijacked Claude sessions: Anthropic revokes sessions compromised via Vidar, LummaC2, StealC, RedLine, Acreed and AMOS

Anthropic sent emails to affected users last week, according to Help Net Security's 2026-08-31 report, to say a threat actor had stolen active Claude (claude.ai) login sessions using general-purpose infostealer malware already resident on those users' machines, then replayed the stolen session cookies to access the accounts and consume paid usage (Help Net Security, 2026-08-31; Anthropic, via BleepingComputer, 2026-08-30). Anthropic names the malware families involved as Vidar, Lumma (LummaC2), StealC, RedLine and Acreed on Windows, and Atomic Stealer (AMOS) on a small number of Macs (Anthropic, via Help Net Security, 2026-08-31); all commodity infostealers whose logs are traded on criminal markets. Anthropic states the infection vector is unrelated to Claude itself, typically an unofficial download or a malicious app; the case that surfaced publicly traced to a pirated-game download (BleepingComputer, 2026-08-30).

Because a stolen browser session cookie authenticates as an already-logged-in user, the attacker bypasses password and two-factor authentication entirely; Dark Reading frames the incident as part of a broader shift, as stronger password policies and wider MFA adoption make traditional credential theft harder, toward attackers targeting session cookies and tokens to hijack already-authenticated sessions instead (Dark Reading, 2026-08-31). Anthropic's remediation was to invalidate the stolen sessions, strip saved payment methods from affected accounts, and refund unauthorized charges; it explicitly warns that signing a user out does not remove the infostealer itself, so an unremediated host will have its next session stolen the same way (Anthropic, via BleepingComputer, 2026-08-30).

Triage: the discriminating signal Anthropic itself points users to is a usage allotment that "refilled and then drained" while the subscriber was not using Claude (Anthropic, via BleepingComputer, 2026-08-30). A legitimate usage spike correlates with the account owner's own activity; a drained allotment with no corresponding use by the account owner is the anomaly infostealer-driven session replay produces.