Tampered Claude Code extension in Cursor marketplace hits up to 5,000 users

The incident centers on a supply-chain gap rather than a flaw in Claude Code itself: a tampered build of the extension was published to Cursor's marketplace, and Anthropic had no involvement in developing it. The extension's data-harvesting targets — hostname, username, operating system and public IP address — are the kind of low-level details that help an attacker profile a machine, and it also attempted to set up remote access. That second step likely failed on most machines because it requires admin privileges, Cursor says, which may have limited the damage.

Cursor's response was to pull the extension and take the entire marketplace offline, then reinstate it after working with providers to neutralize the malware and confirming there was no further risk. The company says no other extensions were affected. For developers, the episode shows that the trust placed in a marketplace listing can be misplaced for hours at a time, and that the exposure window — about eleven hours, reaching up to 5,000 users — is short enough to fit inside a normal working day.