MAL-2026-17801 Malicious code in web3-signer-kit (RubyGems)

The gem's extconf.rb decodes a base64 literal and eval's it at gem install time inside a double-forked, session-detached child whose stdio is redirected to /dev/null. The decoded payload opens a TCPSocket to 45.138.12.177:8089 and bridges a /bin/sh process over that socket via IO.popen + IO.copy_stream, yielding full-host remote code execution for the operator of that endpoint. Execution is gated by sandbox-evasion checks that skip CI, ephemeral/short-uptime hosts, analyst-style usernames, and analysis paths, and requires developer artifacts (~/.ssh, ~/.gitconfig, ~/.gem/credentials) to be present before firing; a randomized sleep of 1200+rand(1200) seconds further delays detonation. The package has no legitimate functionality: metadata describes it as a 'web3 signer kit' while the README advertises bitcoin-rpc-lite, the library exposes only stub methods (build_id, healthy?), and extconf.rb calls create_makefile('bitcoin_rpc_lite_native') with no C sources shipped. The entire gem is a decoy wrapper around the dropper in extconf.rb.