SigThief is a security testing tool that extracts digital signatures from signed Portable Executable (PE) files and appends them to other files. The tool creates files with invalid signatures by copying signature data from legitimate signed executables to unsigned or different files. This process results in files that appear to have digital signatures but fail cryptographic validation. SigThief is designed to test how different Anti-Virus engines handle signature validation and prioritization. Security professionals use it to identify inconsistencies in AV signature checking mechanisms and evaluate detection capabilities. The tool operates by parsing the signature data from source PE files and transferring this information to target files, maintaining the signature structure while breaking the cryptographic chain of trust. SigThief supports testing scenarios where researchers need to understand how security products respond to files with manipulated or invalid digital signatures.