GCVE-110-OSM-2026-13529

The gem's native-extension build script (extconf.rb) decodes a base64-encoded Ruby payload and passes it to eval() inside a double-forked, detached child process with stdio redirected to /dev/null. The decoded payload opens a TCP connection to the hardcoded bare-IP C2 45.138.12.177:8089 and bridges it to /bin/sh via IO.popen and IO.copy_stream in both directions, yielding a full interactive reverse shell on the installer's host under the installing user. Execution is gated by anti-analysis checks that skip CI runners, ephemeral/sandbox hostnames (uvm, firecracker, sandbox, vagrant), generated analyst usernames (uA\d+, sandbox, tester, analys, scanner), common analysis working paths (/opt/rubygems, /tmp, /var/tmp, /workspace), and hosts with uptime under 1800 seconds, firing only when developer-signal files such as ~/.ssh, ~/.gem/credentials, ~/.gitconfig, or ~/.npmrc are present, and after a randomized 20-40 minute delay. The shipped library at lib/block_explorer_api.rb is an inert 10-line stub (VERSION, build_id, healthy?) and the gemspec name (block-explorer-api, authored by generic 'block maintainers') does not match the README, which describes the package as 'bitcoin-rpc-lite'; the install hook contains the entire functional payload while the advertised library does nothing. OBFUSCATION - Decoded Base64 Content in ext/btc_fee_estimator/extconf.rb - recovered 1 ips from decoded/deobfuscated content ADDITIONAL FINDINGS - Homebrew Arbitrary Ruby Execution in ext/btc_fee_estimator/extconf.rb: "eval(" - Homebrew Ruby Process Spawning in ext/btc_fee_estimator/extconf.rb: "fork do" - Ruby C Extension Declaration in ext/btc_fee_estimator/extconf.rb: "require 'mkmf'" PAYLOAD FILES ext/btc_fee_estimator/extconf.rb