News
Anthropic Users Hit by Infostealer Attacks, Session Thefts
Publication date not yet evaluated · Added · 2 reports darkreading.com
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Coverage timeline
Why it matters
Anthropic's Claude accounts being hijacked via stolen sessions shows that even when an AI platform itself is not compromised, ordinary infostealer malware can grant attackers full access to users' AI accounts and their resources.
Anthropic proactively signed an unknown number of Claude users out of their accounts after a threat actor used general-purpose infostealer malware to steal login sessions, access accounts, and consume users' allotted usage. Anthropic stated the malware was pre-existing on users' systems (likely via malicious apps or unofficial downloads) and not related to or installed through Claude itself; the company also removed saved payment methods on affected accounts.
Summary
Anthropic has warned some Claude users that commodity infostealer malware running on their computers stole active Claude login sessions, allowing a bad actor to access their accounts and drain their usage limits. Anthropic told affected users that if their usage appeared to refill and then drain while they were not using Claude, session theft was the likely cause.[0]
Anthropic stressed the malware is general-purpose and unrelated to Claude itself, typically arriving via downloads or malicious apps and harvesting locally stored browser passwords, cookies, and other app credentials. The company says a bad actor has now started picking Claude sessions out of that harvested data and reusing them, and that because infostealers can copy already authenticated browser sessions, the attacker may bypass password and 2FA prompts entirely.[0]
In response, Anthropic is revoking compromised sessions, removing saved payment methods to prevent unauthorized purchases, and refunding charges it identifies as unauthorized, while cautioning that signing users out does not remove the underlying malware.[0]
Attack chain
- Initial infection: Victims' Windows or macOS computers are infected with general-purpose infostealer malware, which typically arrives through downloads or malicious apps.[0]
- Credential and session collection: The infostealer harvests locally stored information, including browser passwords, login cookies, and credentials for other apps; the victim's Claude session is one of many items collected.[0]
- Session extraction: A bad actor selectively picks the Claude login sessions out of the broader collected infostealer data.[0]
- Account takeover and abuse: The attacker replays the stolen, already-authenticated session to access the Claude account without needing the password or 2FA, then consumes the victim's usage and can add unauthorized charges via saved payment methods.[0]
Disclosure timeline
| Date | Event |
|---|---|
| 2026-08-30 | BleepingComputer reports Anthropic's warning that infostealer malware is hijacking Claude sessions to drain usage.[0] |
How it works
The abuse relies on session/cookie theft rather than a software flaw. Infostealers copy an already-authenticated browser session token from the victim's machine, which an attacker can replay to access the Claude account without re-entering the password or completing 2FA. Signing a user out revokes the stolen session but does not remove the malware, so a subsequent login can be stolen the same way.[0]
Indicators of Compromise
| Type | Indicator | Context |
|---|---|---|
| other | Vidar | Windows infostealer family identified by Anthropic as involved in stealing Claude sessions.[0] |
| other | LummaC2 | Windows infostealer family identified by Anthropic among malware that collected Claude sessions.[0] |
| other | StealC | Windows infostealer family identified by Anthropic among malware that collected Claude sessions.[0] |
| other | RedLine | Windows infostealer family identified by Anthropic among malware that collected Claude sessions.[0] |
| other | Acreed | Windows infostealer family identified by Anthropic among malware that collected Claude sessions.[0] |
| other | Atomic Stealer (AMOS) | macOS infostealer family identified by Anthropic on a small number of Macs involved in session theft.[0] |
Key takeaways
- Stolen authenticated session cookies let attackers bypass passwords and 2FA, so revoking sessions alone is insufficient—the infostealer must be removed and credentials rotated.[0]
- The Claude account compromises stem from commodity, general-purpose infostealers (Vidar, LummaC2, StealC, RedLine, Acreed, AMOS), not from any flaw in Claude, underscoring that endpoint malware infections directly threaten SaaS and AI accounts.[0]
Defensive actions
- Remove the underlying infostealer malware from the affected computer.: Anthropic warns that signing users out stops the stolen session but does not remove the malware; if it remains, the next login session can be stolen the same way.[0]
- Change credentials and revoke other active sessions.: Anthropic urged affected users to change credentials and revoke other sessions to protect personal information collected by the infostealer.[0]
- Review Claude usage and payment activity for unauthorized refill-then-drain patterns and unexpected charges.: Anthropic said usage that appeared to refill and then drain while the user was not using Claude was likely caused by the stolen session, and it is refunding charges identified as unauthorized.[0]