Infostealers Are Hijacking Claude Sessions and Quietly Draining Users' Limits

Anthropic has begun sending warnings to a subset of Claude users: infostealers installed on their computers have stolen their active authorization sessions. Using the stolen sessions, attackers gain access to the accounts and spend other people's usage limits.

According to the company, the telltale symptom is when limits “recover” and then immediately vanish at moments when the Claude user themselves hadn't opened it. That's exactly how the activity of an outsider who has logged into the account shows up.

How it works

The danger is that an infostealer is able to copy an already authorized browser session. This means the attacker doesn't need to re-enter a password and pass two-factor authentication — the stolen cookies are enough. Anthropic stresses that the malware has nothing to do with Claude itself: it isn't installed through the service and isn't tied to anything the user does inside it. These are all-purpose malicious programs that land on a computer through downloads and malicious apps, and then collect everything available locally — browser passwords, session cookies and the credentials of other programs. The Claude session, in the company's phrasing, turned out to be “one of many things” they collected, and now someone has begun deliberately fishing out precisely the Claude access from that haul.

One of the victims is telling: he admitted on Reddit that he had downloaded a pirated game — which explains the infection.

Anthropic listed the specific malware families spotted in these attacks: Vidar, LummaC2, StealC, RedLine and Acreed on Windows, as well as Atomic Stealer (AMOS) — on a small number of Mac computers.

What Anthropic is doing and what the user should do

For those affected, the company forcibly terminates Claude sessions, removes saved payment methods to prevent unauthorized purchases, and refunds any charges it deems illegitimate.

At the same time, Anthropic gives an honest warning: logging out stops the stolen sessions, but doesn't remove the malware itself. If it remains on the computer, the next session can be hijacked in exactly the same way. That's why users are advised to take the basic steps: change their credentials, revoke the remaining active sessions and remove the malicious software from the device.

Source: Bleeping Computer