kas's late signature validation may allow unnoticed repository manipulations

Summary

kas is a setup tool for bitbake based projects. Starting in version 4.8 and prior to version 5.3, kas checks out and processes repositories regarding configuration includes prior to validating signatures of those repositories. This may allow to replace on original repository with one under the control of an attacker under very specific conditions. First of all, the attacker must have gained control of a repository that a kas file of the victim is referencing. Furthermore, the following conditions must be fulfilled: the victim's kas configuration must include a configuration file from the attacked repository; the repository state is referenced by tag, and no commit ID is specified (this is triggering a warning, though); the key used for validating the tag or commit signature is stored as file in a repository; no fingerprint for the key is specified; and the `_source_dir` key must not be set by the victim when calling kas (e.g. by avoiding a local `.config.yaml`). Given these conditions, the attacker could modify the included kas configuration in way that the key used to validate the tag signature of the attacker's repository could be replaced by an attacker-chosen key. No other exploit possibilities have been identified so far, but this does not rule out that those may exist. All patches have been released along with kas version 5.3. As a workaround, pin the expected signature key via its fingerprint, also when storing it as file in a repository.