The "go tool pack" subcommand is a minimal version of the Unix ar utility.
It is used by the compiler as an internal tool with known-good inputs.

The "pack" subcommand did not sanitize output filenames.
When invoked to extract a malicious archive file, it could write
files to arbitrary locations on the filesystem.

The "pack" subcommand now refuses to extract files with names
containing any directory components.

Thanks to Harshit Gupta (Mr HAX) for reporting this issue.

This is CVE-2026-39817 and Go issue https://go.dev/issue/78778.

This was a PUBLIC track issue, tracked in http://b/499265616.