Claude sessions stolen by malware, leading to extra charges

Anthropic has taken security measures after stolen Claude sessions were used to exhaust limits on some accounts and incur unauthorized charges. The company automatically logged affected users out of their accounts. It also removed saved payment cards and refunded additional usage charges. Affected users were informed of the measures in an email sent last week.

According to the company’s investigation, the incident was not caused by a breach of Anthropic’s systems, and the problem originated on users’ computers. Information-stealing malware hijacked active Claude sessions in browsers. After gaining access to the accounts, the attackers quickly exhausted usage limits and incurred unauthorized additional usage charges. One possible sign is that limits reset and are then depleted again while the user is not using Claude.

The problem affected both Windows and macOS users. Anthropic has so far identified 6 malware families. Vidar, Lumma, StealC, RedLine, and Acreed were detected on Windows computers, while Atomic Stealer was found on a small number of Mac devices. These programs were not created specifically for Claude and usually reach computers through malicious downloads, collecting passwords and browser data.

The malware also copied Claude session cookies. Browsers store this information so users do not have to reenter their passwords on every page. Someone using a stolen cookie can continue an already authenticated session. As a result, 2-step verification, which protects passwords and account access with an additional code, is not triggered at that point.

Users are advised to remove the malware from their devices before signing back into their Anthropic accounts. The email address linked to the Claude account should then be secured with a new password and 2-step verification. Payment methods should be added back to the account only after these steps. Otherwise, the session cookie created during the new login could be stolen again by the same malware.

Claude login credentials are valuable because stolen AI accounts are sold among criminals. Credentials for ChatGPT and Gemini accounts are also part of this trade. Some intermediary services aggregate stolen accounts and sell access to AI services at prices far below official rates. When card information remains on an account, attackers can also purchase additional usage packages through the victim’s account.