A build log is the recorded output from compilation, linking, and related build steps. It usually contains warnings, errors, tool versions, file paths, and other diagnostics that help developers understand why a build succeeded or failed. In secure environments, it may also capture details from failed parsers, crashed tools, or verbose debug output.
Build logs matter because they are often stored for a long time, copied into CI systems, and shared across teams. That makes them a useful target for attackers: if a build process handles untrusted input, sensitive data such as tokens, usernames, source paths, or even fragments of process memory can sometimes appear in the log. Defenders reduce this risk by limiting log verbosity, redacting secrets, restricting access, and reviewing build artifacts as part of the software supply chain.