Full Report
Anthropic is warning some Claude users that infostealer malware on their PCs has stolen active Claude login sessions, allowing attackers to access accounts and consume their usage. [...]
Analysis Summary
# Incident Report: Infostealer Hijacking of Claude Session Tokens
## Executive Summary
Anthropic has identified a targeted campaign where threat actors are utilizing commodity infostealer malware to hijack active Claude session tokens from users' local machines. These stolen sessions allow attackers to bypass passwords and Multi-Factor Authentication (MFA) to access accounts and consume usage quotas. Anthropic has responded by invalidating sessions, removing payment methods, and issuing refunds to affected users.
## Incident Details
- **Discovery Date:** August 2026 (Reported)
- **Incident Date:** Ongoing; publicized August 30, 2026
- **Affected Organization:** Anthropic (Claude AI users)
- **Sector:** Artificial Intelligence / Technology
- **Geography:** Global
## Timeline of Events
### Initial Access
- **Date/Time:** Variable (Pre-dating August 2026)
- **Vector:** Phishing, malicious downloads, and fraudulent applications.
- **Details:** Users inadvertently installed commodity infostealer malware on their local Windows or macOS devices.
### Lateral Movement
- **Details:** Not applicable in a traditional network sense; the malware performed local lateral movement across browser data to locate specific session cookies and credentials.
### Data Exfiltration/Impact
- **Details:** Attackers exfiltrated active browser login cookies and session tokens. These were then used to hijack Claude sessions, allowing unauthorized AI usage and potential exposure of chat history or account details.
### Detection & Response
- **How it was discovered:** Users reported usage limits draining unexpectedly; Anthropic internal monitoring identified unauthorized session activity linked to known malware signatures.
- **Response actions taken:** Anthropic revoked all compromised sessions, removed saved payment methods from affected accounts, and initiated proactive email notifications and refunds.
## Attack Methodology
- **Initial Access:** Delivery of infostealer payloads via malicious web downloads or third-party apps.
- **Persistence:** Malware resides on the host OS (Windows/macOS) to capture future login sessions.
- **Privilege Escalation:** Not required for session theft.
- **Defense Evasion:** Use of diverse, evolving malware strains to bypass local antivirus.
- **Credential Access:** Theft of browser cookies and stored credentials.
- **Discovery:** Scanning local directories for browser profiles and cookie databases.
- **Lateral Movement:** Session hijacking (Pass-the-Cookie).
- **Collection:** Automated harvesting of active session tokens for high-value web services.
- **Exfiltration:** Data sent to Command and Control (C2) servers.
- **Impact:** Financial loss (usage consumption) and unauthorized account access.
## Impact Assessment
- **Financial:** Unauthorized consumption of paid AI usage; Anthropic is processing refunds for affected users.
- **Data Breach:** Compromise of Claude chat histories and account metadata.
- **Operational:** Disruption of service for legitimate users due to drained quotas.
- **Reputational:** Minimal for Anthropic (issue stems from user-side infections), but highlights risks of web-based session persistence.
## Indicators of Compromise
### File Indicators (Known Malware Families)
- **Windows:** Vidar, LummaC2, StealC, RedLine, Acreed
- **macOS:** Atomic Stealer (AMOS)
### Behavioral Indicators
- Unexpected "Usage Limit Reached" notifications.
- Active sessions appearing from unfamiliar IP addresses or geographic locations.
- Unauthorized changes to account settings or payment methods.
## Response Actions
- **Containment:** Revocation of all active session tokens for identified compromised accounts.
- **Eradication:** Users advised to run full system scans and remove malware from local PCs.
- **Recovery:** Proactive removal of saved payment information and refunding of unauthorized charges.
## Lessons Learned
- **Key takeaways:** Infostealers are increasingly moving beyond simple password theft to "Session Hijacking," which renders standard MFA ineffective once a session is established.
- **What could have been done better:** Implementation of shorter session lifetimes or device-bound session tokens (DBSC) could mitigate the utility of stolen cookies.
## Recommendations
- **For Users:**
- Perform a clean reinstallation of the operating system or use reputable antivirus to ensure malware removal.
- Clear browser cookies and cache after a security incident.
- Avoid downloading software from unverified third-party sources.
- **For Organizations:**
- Implement hardware-backed session security or IP-pinning for sensitive sessions.
- Monitor for concurrent sessions from disparate geographic locations.