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What a defender needs to know
A trojanized Claude Code IDE extension installed an SSH backdoor on machines that loaded Open-VSX version 2.1.313, including copies delivered through Cursor’s marketplace. StepSecurity’s analysis found an added loader in extension.js that inserted a key into authorized_keys, installed separate startup entries and opened a reverse tunnel to 134[.]209[.]82[.]225. Removing the extension leaves those entries behind. StepSecurity attributes to Cursor an upper bound of 5,000 Cursor users who received the version during roughly 11 hours on October 29.
Inventory installed anthropic.claude-code versions and check ~/.ssh/authorized_keys or %USERPROFILE%\.ssh\authorized_keys for svc-telemetry. Search relevant startup entries for Linux cron @reboot, macOS com.apple.sysmond.plist, and Windows SysMon or Startup\sysmon.cmd. Cursor’s notice listing names versions 2.1.312 and 2.1.313 in its marketplace; StepSecurity found the Open-VSX 2.1.312 build clean. Include Cursor-delivered 2.1.312 in your version triage rather than applying the Open-VSX finding across channels.
If the malicious build or key appears, preserve evidence and isolate the host. Remove the implanted key and startup mechanism, reinstall 2.1.314 or later from a trusted source, and rotate credentials reachable from that machine using a clean workstation. Review SSH, process and network logs for the reverse tunnel to 134[.]209[.]82[.]225 on TCP 22; rebuild if a successful tunnel is confirmed. Verify that the key and startup entry stay absent after restart and that the outbound connection does not return.