Infostealers Hijack Active Claude Sessions to Drain Paid Subscriptions
Anthropic disclosed that multiple infostealer malware families are harvesting authenticated browser cookies to hijack Claude accounts across Windows and macOS. Because attackers reuse active session tokens, they bypass two-factor authentication and passwords to consume expensive API usage and subscription compute.
Impact: High
Why it matters
You need to audit active sessions, clear compromised browser tokens, and consider hardware security keys to prevent unauthorized spend on your AI developer accounts.
TL;DR
- 01Six infostealer families are actively exfiltrating Claude session tokens on Windows and macOS.
- 02Cookie hijacking completely bypasses traditional passwords, SSO, and 2FA prompts.
- 03Anthropic has revoked affected sessions and refunded unauthorized compute charges.
Key facts
- Identified Malware Families
- 6 (Vidar, LummaC2, StealC, RedLine, Acreed, Atomic Stealer)
- Targeted Operating Systems
- Windows and macOS
- FBI Initial Access Attribution
- >70% in ransomware campaigns
The Mechanism of Cookie Hijacking
Anthropic's security team identified active credential theft campaigns harvesting session cookies from developer workstations. The identified malware strains include:
- Windows: Vidar, LummaC2, StealC, RedLine, and Acreed.
- macOS: Atomic Stealer.
Because these infostealers capture active session tokens rather than raw passwords, they bypass both standard password authentication and hardware-independent 2FA/SSO checks. The hijacked sessions allow attackers to access paid Claude Pro features and enterprise API quotas without triggering traditional login alerts.
Mitigation and Account Hardening
To remediate compromised developer environments:
1. Revoke Sessions: Navigate to Claude account settings and force-logout all active devices. 2. Scan Host Systems: Remove persistent local malware before entering fresh credentials to prevent immediate re-theft. 3. Hardware Keys: Migrate authentication policies to WebAuthn hardware security keys (FIDO2) which cannot be exfiltrated via simple cookie theft.
✓ When to use
- Auditing developer workstation security policies across teams with shared Claude API keys or seats.
- Investigating unexpected spikes in monthly Claude Pro usage or sudden token limit exhaustion.
What to do today
- Log into Claude settings and revoke all active sessions across web and desktop clients.
- Scan development machines for LummaC2, Atomic Stealer, and related credential-theft binaries.
- Implement WebAuthn hardware security keys for critical AI workspace administrative accounts.
Sources