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Anthropic: That Claude Code extension compromise… couldn't be us

Security1 min read

Anthropic says it did not cause a malicious version of its Claude Code extension to spread through Cursor's marketplace. Cursor says an outside attacker used access to its marketplace to publish a tampered copy that installed a hidden backdoor on developers' computers.

What happened

A fake version of the Claude Code extension was posted to the marketplace used by Cursor, a code editor. The fake version, numbered 2.1.313, was available for about 11 hours. It could open a secret remote-access path, called an SSH backdoor, on Windows, macOS, and Linux. Anthropic said it did not build or publish the copy. It also said its own systems and employee laptops were not breached.

Key facts

  • Up to 5,000 developers may have been affected.
  • The bad version was online for about 11 hours on Thursday, from 0227 to 1335 PT.
  • The extension tried to send the computer's username, Git email, and public IP address to the attacker.
  • The backdoor usually failed to open its tunnel, because it needed admin rights that most users do not have.
  • Updating to the latest version does not fully remove the backdoor.

Why this is a supply-chain attack

A supply-chain attack hides bad code inside a tool that many people already trust. Here, the attacker did not hack the users directly. They used a trusted marketplace to reach developers. Cursor has not explained yet how the attacker got into its marketplace.

Why it matters

Developers often trust extensions from well-known companies. If a fake copy spreads through an official store, people may install malware without any warning. Developers who used this extension may need to check their computers and remove the backdoor by hand, using Cursor's instructions.

What to watch

  • Whether Cursor explains how the fake extension got into its marketplace.
  • Whether other extensions were affected. Cursor says its checks found none.
  • Whether more developers report signs of the backdoor.
  • Whether Anthropic and Cursor publish more details about the attacker.

Glossary

Cursor
A code editor, a program where people write computer code. It has its own marketplace for add-on tools.
Claude Code
An AI coding tool from Anthropic that helps people write code. Here it is an add-on, called an extension, for Cursor.
SSH backdoor
A hidden way for an attacker to control a computer from far away without the owner knowing.

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