Anthropic Warns Infostealer Malware Is Hijacking Active Claude Sessions

What Happened

Anthropic has warned some Claude users that infostealer malware running on their PCs has stolen active Claude login sessions, according to a report from BleepingComputer. Attackers who obtain these session tokens can access victims' Claude accounts and consume their usage without needing the victim's password.

Why It Matters for Defenders

This is not a vulnerability in Claude itself — it's a consequence of commodity infostealer malware harvesting browser session tokens/cookies, a technique already widely used against SaaS platforms, cloud consoles, and enterprise SSO sessions. Any organization with users who have infostealer infections on unmanaged or poorly monitored endpoints is potentially exposed, and the same stolen-session pattern likely extends beyond Claude to other AI and SaaS tools employees use. Consumed usage/quota theft is the immediate visible harm, but stolen sessions to AI assistants can also expose whatever sensitive data or conversation history is tied to the account.

What Defenders Should Watch For

• Endpoint telemetry for known infostealer families (browser credential/cookie store access, unusual reads of local session storage or browser profile directories).
• Anomalous login/API usage patterns on SaaS and AI-tool accounts — logins or usage spikes from unfamiliar IPs, ASNs, or geographies inconsistent with the user's normal pattern.
• Sudden, unexplained increases in AI service usage/billing for an account as a proxy signal for session theft.
• General infostealer hunting: unusual outbound connections to known stealer C2/exfil infrastructure, and credential/cookie exfiltration artifacts in EDR telemetry.
• Mitigations: enforce session expiry and re-authentication where supported, monitor for concurrent/impossible-travel sessions, and prioritize infostealer remediation and endpoint hardening (this is fundamentally an endpoint compromise problem, not an AI platform flaw).

Developing Story

Details on scope, affected user counts, and the specific infostealer family (or families) involved have not been confirmed at time of writing. This is net-new intel; see the original report at BleepingComputer for updates.