Since August 30, Anthropic has notified some Claude users that infostealing malware was used to steal Claude login session cookies from their computers, bypass 2FA to access their accounts, and consume their usage. Anthropic clarified that the Claude platform itself was not breached; Claude accounts were merely used as a stepping stone for monetization.
Anthropic Notification: Abnormal Sessions Logged Out
In its notification email, Anthropic explained that it took the following measures after detecting the anomaly: logging out of all detected abnormal login sessions, removing saved payment methods from the account, and refunding identified unauthorized charges. Anthropic also reminded affected users that logging out can only sever the stolen connection associated with this incident; it does not remove malware from the computer. If the malware is not removed, information may be stolen again in the future.
Anthropic’s recommended steps include changing the account password, revoking login sessions on other devices, and scanning for and removing malware from the computer.
How the Attack Works: Infostealing Malware Steals Session Cookies
2FA’s protection mechanism verifies the user’s identity “at the moment of login”—the account, password, and second-factor verification code confirm that the user is the one pressing the login button. However, this attack steals not the login credentials but the session cookie saved by the browser after login—the credential that allows the browser to remember the verified identity. Once attackers possess the session cookie, they can directly access the account without going through 2FA.
A user on Reddit posted the notification email they received and explained that they had logged into Claude with a Google account and enabled 2FA, but the attacker stole the Chrome browser’s usernames and passwords, cookies, and session ID all at once, completely bypassing 2FA. After investigating, the user suspected that the infection source was pirated software they had previously downloaded.
Infostealing Malware Families Named
The infostealing malware families named in Anthropic’s notification are as follows:
Windows (5 variants):
· Vidar
· LummaC2
· StealC
· RedLine
· Acreed
macOS (1 variant):
· Atomic Stealer (AMOS)
All of the above are commercial infostealing malware that have circulated in underground markets for years. They have low barriers to purchase and are easy to operate, enabling large-scale deployment and making them one of the reasons this type of attack has proliferated in recent years.
NordVPN Research: More Than 5.8 Million Infected Devices in the First Half of 2025
NordVPN researchers analyzed infostealing records circulating on dark web forums and Telegram marketplaces from June 2025 to June 2026 and found more than 52.4 billion browser cookies. Focusing on the first half of 2025, they found that infostealing malware stole approximately 1.8 billion sets of login credentials from approximately 5.8 million infected devices; about 1.17 million records contained both usernames and passwords and still-valid session cookies, meaning attackers could bypass 2FA to log directly into accounts. These statistics show that this type of attack has reached an industrial scale.
FAQ
When did Anthropic begin notifying affected Claude users?
Starting August 30, 2026, Anthropic sent some Claude users a notification titled “A recent issue affected your Claude account,” explaining that their accounts had been affected by session cookie theft and used to consume usage.
Why couldn’t 2FA prevent this attack?
2FA only verifies identity “at the time of login” and protects the account-password login process. Infostealing malware steals the session cookie saved in the browser after login—the credential representing the verified identity—allowing an attacker who possesses the cookie to directly access the account without going through 2FA.
What measures has Anthropic taken in response to this incident?
According to Anthropic’s notification, the measures included logging out of detected abnormal login sessions, removing saved payment methods from the account, and refunding identified unauthorized charges. It also recommended that affected users change their passwords, revoke login sessions on other devices, and remove malware from their computers.
Which infostealing malware was involved in this incident?
The infostealing malware named by Anthropic includes Vidar, LummaC2, StealC, RedLine, and Acreed on Windows, as well as Atomic Stealer (AMOS) on macOS. All are commercial infostealing malware that have circulated in underground markets.
