If your Claude usage quota looked like it got refilled only to mysteriously get burned through again, an email Anthropic sent out last week has the answer: the problem wasn't your password or two-factor authentication—it was something in your browser called a session cookie that got stolen.
The email was first shared by a user on Reddit, and later reported by SecurityWeek. According to the letter, Anthropic has force-logged out all sessions on affected accounts, deleted saved credit cards, and refunded charges from the abnormal usage. The company also clarified that the root cause was info-stealing malware on users' own computers, not a breach of Anthropic's systems.
Anthropic has identified six malware families behind the incidents, including Vidar, Lumma, StealC, RedLine, and Acreed on Windows, and Atomic Stealer (AMOS) on macOS. The company described the number of affected Mac users as "very small." None of these tools were designed specifically to target Claude—they're common generic info-stealers, typically bundled with malicious downloads. Once executed, they harvest passwords and cookies stored in the browser.
The key lies in what a session cookie actually does: once you log into Claude, your browser holds onto this credential so the system doesn't need to ask for your password every time you load the page. Once an attacker copies that cookie, they can simply pick up right where the victim's already-logged-in, already-2FA-verified session left off—no need to trigger a password prompt or two-step verification at all. That's exactly why the account password never leaked, yet the quota mysteriously got burned through.
Anthropic warns that a forced logout only cuts off the currently hijacked session—it doesn't remove the malware itself from the user's computer. The company's recommended order of operations: first remove the malware from your computer, then log back into Claude, set a new password for the linked email account and turn on two-factor authentication, and only after that re-add your payment method. Do the steps out of order, and the same malware could just intercept the new cookie generated by your fresh login.
The email didn't disclose the scale of affected accounts, and Anthropic has not yet responded to Engadget's request for comment.
