MAL-2026-17567 Malicious code in rate-limit-mini (RubyGems)

RubyGems runs this gem's native extension script ext/req_throttle_mini/extconf.rb during gem install. The script exits without action on CI runners and sandboxes, based on CI environment variables, the hostname, the username and the working directory. It continues if the system uptime is over 30 minutes and the home directory holds .ssh, .gitconfig, .npmrc, .gem/credentials or .bundle. It then decodes a Base64 Ruby payload, forks twice into a detached process, waits 20 to 40 minutes and opens a reverse shell that connects /bin/sh to 45.138.12.177 on TCP port 8090. The shell reconnects every 30 seconds. The Ruby code in lib/ is a decoy. The RubyGems account reqthrottle_3474 published this gem and 41 others with the same loader on 2026-10-05.

The gem's native extension script ext/extconf.rb base64-decodes an embedded Ruby payload, writes it to a randomly named file under the system temporary directory, and spawns it with the Ruby interpreter before invoking the normal mkmf build. The decoded payload opens a TCP connection to 45.138.12.177:8090 and bridges /bin/sh to the socket in a reconnect loop, giving a remote operator interactive shell access on the installer's host at gem install time. The use of a base64-encoded Ruby blob inside extconf.rb has no build-system purpose and serves to evade casual review. The package name presents as a benign rate-limiting utility, which is inconsistent with the shipped install-time behavior.