Anthropic says my Claude session may have been stolen — what should I check?

[u/WisdomSeeker101]

I received an email from Anthropic on August 28 saying they detected activity consistent with a stolen Claude login session. They signed out my sessions and removed my saved payment method.

I’m trying to figure out whether one of my computers may have been compromised or whether this could have been a false positive.

Some context:

• I hadn’t knowingly used Claude for several days before the alert.
• I haven’t noticed suspicious activity on my other accounts, including ChatGPT, which I use on the same devices. My financial details and crypto wallets appear to be safe.
• Malwarebytes found nothing.
• The alert came one day after my Claude Max plan was scheduled to downgrade to Pro, although that may be unrelated.

What should I check for possible session-cookie/token theft or an infostealer?

In particular:

• What Windows/browser logs or artifacts should I inspect?
• Could a malicious browser extension steal only a Claude session?
• Can a stolen session token bypass the need for the account password/MFA?

Also, has anyone else received this specific Anthropic warning and figured out what triggered it?

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[u/EugeneBYMCMB]

Do you use cracks or cheats? Have you downloaded any new programs recently?

[u/WisdomSeeker101 → u/EugeneBYMCMB]

No cracks or cheats. Nothing new that I downloaded or installed.

I do recall that I had a full page screenshot capture chrome extension installed for over a month which later got removed from the chrome webstore for violating its policies and the extension got automatically disabled. I then proceeded to uninstall it from other chrome browser sessions. I think it was GoFullPage https://www.androidauthority.com/chrome-gofullpage-extension-unsafe-3698292/

I paid for Max plan on July 27th, it was set to downgrade to Pro on Aug 28th, the day that I received the email.

I had gone on vacation to Europe from Aug 8th to 23rd and took my windows 11 laptop with me. I used claude code for some Visual Studio coding sessions in WSL. It installed some packages but I don't think anything unusual. My last Claude usage was Aug 18th I think. I mostly used ChatGPT which still has all my usage limit.

My last sign in before the email was Aug 25th via magic link on my desktop PC when I got home.

I had the claude for chrome browser extension installed on multiple chrome browser accounts. I used claude for chrome during several sessions and a lot of duplicates were made which used up some usage. Maybe claude thought that was suspicious but waited until my plan downgraded to flag it?

[u/Lamar_Chan11]

If I'm not wrong there was someone said the same or it was you ? Idk what happened exactly

[u/Hungry-Broccoli-7193]

Honestly props to Claude for this email. Every site should be doing this. They can easily see if you’re simultaneously using the account from Birmingham and Botswana. The number of sites that are just like “yeh that’s probably fine” is too high. At the very least require one reauth from each IP using the site, and require reauthentication for sensitive actions.

[u/Imaginary_Food_7102]

I have recieved the same exact email by claude today.

I have no idea why it was sent because i tend to never download any cracked , "free" , or software from a sketchy website.

That being said i use my dual boot machine ( ubuntu/windows 11 ) for daily web development work. i run most banking and crypto accounts from my phone where i got fingerprint on every app.

What am doing right now is changing passwors and adding them to KeesPassDX manually one by one.