Anthropic warns infostealer malware hijacks Claude AI sessions, drains user quotas

Anthropic has warned that a bad actor is using common infostealer malware to steal Claude login sessions from users' computers, then using those sessions to access accounts and consume usage. The company is sending emails to affected users, signing them out of Claude, removing saved payment methods, and refunding charges it identifies as unauthorized.

"We have recently become aware of a bad actor that is using common infostealer malware to steal Claude login sessions from people's computers, then using those login sessions to access Claude accounts and consume their usage," Anthropic said in an email shared on Reddit by an affected user.

According to Anthropic, the malware copies authenticated browser sessions, bypassing password and two-factor authentication. The company emphasized that the malware is general-purpose and unrelated to Claude itself: "We have no reason to believe that this malware is related to Claude, installed through Claude, or related to anything you did with Claude."

The malware typically arrives through downloads or malicious apps and collects browser passwords, login cookies, and credentials. The affected Redditor confirmed they downloaded a pirated game, which likely caused the infection.

Anthropic identified multiple types of infostealers involved, including Vidar, LummaC2, StealC, RedLine, and Acreed on Windows, as well as Atomic Stealer (AMOS) on a small number of Macs.

The company warned affected users that signing them out stops the stolen sessions but does not remove the malware. It urged users to change credentials, revoke other sessions, and remove the malware from their PCs.

Why This Matters

Claude users who unknowingly have infostealer malware could lose access to paid subscription quotas or incur unauthorized charges.
The attack highlights how session hijacking bypasses strong authentication, making it a persistent threat for AI service users.
Affected users must clean their systems to prevent recurring theft, as the malware remains active after session revocation.

Background

Infostealer malware has become a widespread threat, often distributed through pirated software, phishing campaigns, or compromised download sites. These programs harvest stored credentials, cookies, and session tokens from browsers, enabling attackers to access accounts without passwords or 2FA. AI platforms like Claude have become attractive targets because attackers seek to consume prepaid usage or extract value from compromised accounts.

Key Perspectives

Anthropic: The company is taking proactive steps to mitigate damage—revoking sessions, removing payment methods, and refunding fraud. It stresses the malware is not related to Claude and urges users to remove infections independently. Affected users: Users may face unexpected usage depletion and need to secure their devices beyond Claude. The burden of cleaning malware falls on them, which may be challenging for less technical users. Security researchers: This incident underscores the importance of session token security and the need for services to detect anomalous usage patterns. Infostealer operators continue to target high-value accounts across platforms.

What to Watch

Whether Anthropic implements additional session protection measures, such as device binding or anomaly detection.
The scale of the attack: how many users were affected and whether refund requests rise.
Potential for other AI services to issue similar warnings as attackers expand to ChatGPT, Gemini, or other platforms.

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