Anthropic has warned some Claude users that criminals are using information stealers to take over their accounts.
Rather than guessing passwords or intercepting two-factor authentication (2FA) codes, the attackers steal the browser sessions that prove a user is already logged in.
According to a warning email shared publicly by an affected user, the attackers used common infostealer malware to copy Claude login sessions from victims’ computers. They then used those sessions to access the accounts and consume their usage.
“We recently signed you out of Claude and removed the payment method saved on your account, so you’ll need to log back in and re-add your card. We’re sorry for the disruption. Here’s what happened and what we’ve done about it.
What happened
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. Our systems detected this activity on your account, and we’ve therefore removed your card on file and signed out the sessions involved to help block further unauthorized access.
If your usage limits looked like they refilled and then drained while you weren’t using Claude, this was likely the cause.”
The message adds that Anthropic has no reason to believe the malware was “related to Claude, installed through Claude, or related to anything you did with Claude.”
To sum this up:
- Cybercriminals are spreading infostealers. How they are doing this and whether they are targeting groups likely to use Claude professionally is unknown.
- Infostealers can bypass standard credentials and multi-factor authentication (MFA) by stealing active browser sessions and session cookies.
- Once they are able to take over a Claude account, they can consume the victim’s usage and potentially incur additional charges.
- Anthropic is signing affected users out of Claude, removing saved payment methods, and refunding charges it identifies as unauthorized.
To better understand this, you should know that paid Claude plans can offer additional “Usage credits.” When a subscriber reaches the plan’s session limit, Claude can allow them to continue using the service through consumption-based billing at standard API rates. The user must enable the feature, configure a monthly spending limit or select unlimited spending, and prepay for credits.
Users can also enable auto-reload, which automatically buys more prepaid credits when the balance falls below a threshold. So, in a session-hijacking scenario, a thief could use up the account’s included allowance and any available Usage credits. If auto-reload is enabled, they could also trigger further purchases.
The criminals’ likely motive is to use paid Claude capacity for free. The account and any exposed data could also be useful for fraud, social engineering, or follow-on attacks.
Stolen Claude capacity could be used to write and refine phishing and scam content, build campaign infrastructure, develop, modify, or obfuscate malware, improve delivery methods, and analyze stolen information. Cybercriminals can use AI to support several parts of an operation, although Claude has safeguards and abuse monitoring, and Anthropic says it has disrupted accounts used for malicious activity.
What to do
Anthropic provided advice for dealing with a possible infostealer infection. After removing the malware, we recommend you install an up-to-date, real-time anti-malware solution to help protect you against new infections.
These steps are good practice when cleaning up after infostealer malware:
- Scan any computer you use with Claude for malware and remove any malware before logging back in or changing passwords.
- Once the malware has been removed, secure the email account you use for Claude by changing its password, signing out of other devices, and enabling two-factor authentication (2FA).
- Change sensitive passwords that were saved in the affected browser, including those for banking, work, and cloud services. Check your card statements if you stored payment details in the browser.
- Only after completing these steps should you add your payment method to Claude again if you want your plan to continue renewing.
If you still see your usage changing while Claude is idle, or notice an unrecognized charge after completing these steps, contact usersafety@anthropic.com.
From reporting threats to removing them.
Cybersecurity risks should never spread beyond a headline. Keep threats off your devices by downloading Malwarebytes today.
