Malicious backdoor found in Claude Code VS Code extension on Cursor marketplace

A backdoored version of the Claude Code VS Code extension appeared on the Cursor marketplace, prompting a security notice from Cursor.

A malicious extension impersonating the Claude Code tool for VS Code was published on the Cursor marketplace, containing a backdoor — a finding that prompted Cursor to issue an official security notice. The extension targeted developers using Cursor, an AI-powered code editor, meaning the potential victims were software engineers who had installed what appeared to be a legitimate productivity tool. The doublepulsar.com report, whose URL slug references a 'remote access trojan' delivered to 'Claude developer laptops worldwide,' suggests the malicious payload may have been designed to give attackers persistent remote access to affected machines. It is not confirmed from available source text whether this involved a compromise of Anthropic's own systems or was a third-party impersonation attack on the marketplace. Cursor published a dedicated security notice at cursor.com/blog/claude-code-extension-security-notice, indicating the company was aware of the incident and took steps to alert its user base. Security researcher GossiTheDog flagged the issue on the Mastodon-based cyberplace.social platform, helping amplify awareness. The scope of impact — how many developers installed the extension and whether any machines were confirmed compromised — is not established in the available source material. The incident highlights ongoing risks posed by malicious packages published to developer tool marketplaces under the names of trusted, well-known software projects.

Why it matters

Developer tool marketplaces are a high-value attack surface because a single malicious extension can compromise thousands of engineer workstations, potentially giving attackers access to source code, credentials, and internal systems. An extension mimicking a popular AI coding tool like Claude Code is especially dangerous given its rapidly growing user base.

What's next

Watch for Cursor's full security notice detailing which versions were affected, how many users were exposed, and what remediation steps are recommended.

Key facts

• A backdoored extension mimicking Claude Code for VS Code was published on the Cursor marketplace
• Cursor issued an official security notice at cursor.com/blog/claude-code-extension-security-notice
• The doublepulsar.com report describes the payload as a remote access trojan (RAT)
• Security researcher GossiTheDog flagged the issue on cyberplace.social (Mastodon)
• It is unconfirmed from available sources whether Anthropic itself was breached or this was a marketplace impersonation attack

Bias & framing notes

Body text was unavailable for all three sources, making factual verification impossible. The doublepulsar.com headline — 'Anthropic hacked to deliver remote access trojan to Claude developer laptops worldwide' — is notably more dramatic and makes a stronger causal claim (Anthropic being hacked) than the other sources, which frame it more neutrally as a backdoor in a marketplace extension. Whether Anthropic's own infrastructure was compromised or this was a supply-chain/impersonation attack on the marketplace is a material factual distinction that cannot be resolved from available text.

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