GHSA-FCV2-XGW5-PQXF sigstore legacy TUF client allows for arbitrary file writes with target cache path traversal

Summary The legacy TUF client pkg/tuf/client.go, which supports caching target files to disk, constructs a filesystem path by joining a cache base directory with a target name sourced from signed target metadata, but it does not validate that the resulting path stays within the cache base...