  We present a new type of attack in which source code is maliciously encoded
so that it appears different to a compiler and to the human eye. This attack
exploits subtleties in text-encoding standards such as Unicode to produce
source code whose tokens are logically encoded in a different order from the
one in which they are displayed, leading to vulnerabilities that cannot be
perceived directly by human code reviewers. 'Trojan Source' attacks, as we call
them, pose an immediate threat both to first-party software and of supply-chain
compromise across the industry. We present working examples of Trojan Source
attacks in C, C++, C#, JavaScript, Java, Rust, Go, Python, SQL, Bash, Assembly,
and Solidity. We propose definitive compiler-level defenses, and describe other
mitigating controls that can be deployed in editors, repositories, and build
pipelines while compilers are upgraded to block this attack. We document an
industry-wide coordinated disclosure for these vulnerabilities; as they affect
most compilers, editors, and repositories, the exercise teaches how different
firms, open-source communities, and other stakeholders respond to vulnerability
disclosure.
