melange's world-writable permissions expose SBOM files to potential image tampering

It was discovered that the SBOM files generated by melange in apks had file system permissions mode 666:

$ apkrane ls https://packages.wolfi.dev/os/x86_64/APKINDEX.tar.gz -P hello-wolfi --full --latest  | xargs wget -q -O  - | tar tzv 2>/dev/null var/lib/db/sbom
drwxr-xr-x root/root         0 2025-06-23 14:17 var/lib/db/sbom
-rw-rw-rw- root/root      3383 2025-06-23 14:17 var/lib/db/sbom/hello-wolfi-2.12.2-r1.spdx.json

This issue was introduced in commit 1b272db ("Persist workspace filesystem throughout package builds (#1836)") (v0.23.0).

Impact

This potentially allows an unprivileged user to tamper with apk SBOMs on a running image, potentially confusing security scanners. An attacker could also perform a DoS under special circumstances.

Patches

This issue was addressed in melange in e29494b ("fix: tighten up permissions for written SBOM files and signature tarballs (#2086)") (v0.29.5).

Acknowledgements

Thanks to Cody Harris H2O.ai and Markus Boehme for independently reporting this issue.