A Fake Claude Code Extension Slipped Into Cursor's Marketplace for 11 Hours
A tampered Claude Code extension tried to collect machine details and set up remote access for up to 5,000 Cursor users before the marketplace was pulled offline.
Late last week, a tampered version of the Claude Code extension was available in Cursor’s extension marketplace for about eleven hours. Cursor says up to 5,000 users may have been affected and that no other extensions were touched. Anthropic had no part in building it.
An extension is a small add-on that gives an editor like Cursor extra abilities, and the marketplace is the shop where you pick them up. Here, the shop shelf itself carried a lookalike of a popular product. According to Cursor, the fake tried to collect the user’s hostname, username, operating system and public IP address, and attempted to set up remote access for the attacker. Cursor says that last step likely would not have worked on most machines, because it needs administrator privileges.
Once the problem was found, Cursor pulled the extension, took the whole marketplace offline, worked with providers to neutralise the malware and brought the marketplace back after confirming there was no further risk. The details come from Cursor’s own statement as relayed by The Decoder, so treat the scope as the company’s account rather than an independent audit.
What’s behind it: AI coding tools are attractive targets. They sit close to source code, API keys and company networks, and developers install add-ons for them quickly because the names look official. Attackers copy that trust. This pattern is a software supply-chain attack: instead of breaking into you directly, they poison something you were going to install anyway. Earlier this autumn, researchers also warned about leaky sandboxes in several coding agents, so the surface is wide, not a one-off.
The honest caveat is that we do not know yet how the tampered version got onto the marketplace, and the reporting gives no specific advice to users.
What this means for you: If you use Cursor and installed or updated the Claude Code extension on Thursday, check your installed extensions, remove anything you did not intentionally add, and consider changing credentials that live on that machine, such as keys stored in environment files. These are sensible general steps, not instructions from Cursor. For everyone else, three habits go a long way: install extensions only from the publisher you expect (check the name and download count), keep admin rights off your daily account, and treat “official-looking” as a reason to double-check, not a reason to relax.
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