tuf's Metadata API: Targets.get_delegated_role() is missing input validation

The security of both a TUF client and repository implementations depend on the concept of trusted Metadata objects verifying the signatures over other Metadata that it delegates to. This verification process uses Targets.get_delegated_role(delegated_role: str) to find the delegation information.

tuf.api.metadata.Targets.get_delegated_role() should ensure that the given delegated_rolename is actually a name of a role that is delegated by that Targets, but in the case of "succinct delegation" this does not happen.

tuf.ngclient users are not impacted but direct users of tuf.api.metadata could be impacted.

Impact

If an attacker can make a Metadata API user run Targets.get_delegated_role() so that

1. the Targets uses succinct delegation
2. the delegated_role argument is not actually delegated by the Targets

the result will be incorrect.

This also means that if an attacker can make a Metadata API user run Targets.verify_delegate() or Targets.get_verification_result() so that

1. the delegating Targets uses a succinct delegation
2. the delegated_role argument is the name of some unrelated Metadata
3. that other Metadata is correctly signed by the keys defined in the succinct delegation

the result would be a successful verification even though the Targets in question does not actually delegate to delegated_role.

The impact is estimated to be low for following reasons:

• This cannot impact a TUF client that implements the client workflow as specified since the delegated role name is not an input but is collected from the (trusted) delegating Targets itself
• Actual signature verification is not bypassed: The verified metadata must still be correctly signed by the keys specified in the delegating role.
• The described situations are somewhat hypothetical: there does not seem to be any reason for a python-tuf user (whether client or a repository) to use tuf.api.metadata in this way.

All users of tuf.ngclient are specifically not impacted. Users of tuf.api.metadata could be impacted if they use succinct delegations in a way described above.

Patches

A fix is available in python-tuf 3.1.1 as commit 77cb66bc and in later releases as commit eb4834d9._

Workarounds

tuf.api.metadata users should only call Targets.get_delegated_role(), Targets.verify_delegate() or Targets.get_verification_result() with delegated_role argument that is known to be delegated by the Targets in question.