A tampered version of the Claude Code extension was available in Cursor’s marketplace on Thursday for approximately eleven hours, affecting up to 5,000 users. This malicious software was developed without any involvement from Anthropic and attempted to harvest user details including hostname, username, operating system, and public IP address. The code also tried to establish remote access for an attacker, although this likely failed on most machines without administrator privileges. Cursor removed the extension and took the entire marketplace offline immediately after discovery. Once providers neutralised the malware and confirmed there was no further risk, the marketplace was reinstated while Cursor stated no other extensions were affected.
The incident highlights the risks inherent in third-party extension ecosystems where malicious actors can deploy code that mimics legitimate tools. Users who installed the extension during the window of availability may have had their system information exposed, even if the remote access component failed. The brief duration of the compromise limits the window for data exfiltration, but the event serves as a reminder to verify extension sources and monitor for unexpected network activity.
- The malicious extension was removed and the marketplace was offline for a short period.
- Anthropic confirmed they had no role in creating the tampered software.
- Cursor stated no other extensions in the marketplace were compromised.
