Malicious Claude Code extension reached up to 5,000 Cursor users
Original title: A Supply Chain Attack Put Malicious Anthropic Claude Extensions on Thousands of Devices
A marketplace backdoor stayed live about 11 hours, but a design flaw stopped it on most devices.
A tampered Anthropic Claude Code extension was available in Cursor’s IDE marketplace for Windows, macOS, and Linux for roughly 11 hours on Thursday. Cursor says at most 5,000 users downloaded it and published remediation steps after briefly closing the marketplace. The extension contained a backdoor, but a flaw prevented it from fully establishing itself on most targets. Anthropic and Cursor say Anthropic’s systems were not compromised.
Unconfirmed: How the malicious extension entered Cursor’s marketplace, and whether Cursor itself was breached, is unconfirmed.
- Phorion first spotted the extension on a customer’s Mac.
- Cursor says no other marketplace extensions were affected.
- Anthropic says official-channel versions were not tampered with.
- Cursor has not explained how the upload was authorized.
Impact: Backdoor access to developer devices running the extension
| Product | Affected |
|---|---|
| Cursor Claude Code extension | — |
- 1.Follow Cursor’s Thursday remediation steps if the extension was installed.
- 2.Use Claude Code only from Anthropic’s official channels.
- Products
- Claude Code
From the original article
2 min read at wired.com
Thanks to a lucky break, flawed code in a tainted Claude extension prevented the attacker from fully worming their way into some 5,000 developer machines.
By Lily Hay Newman · wired.com
A short excerpt; the article and its images belong to the publisher. Summary, key facts and assessment above are AI-written from the article and can be wrong.