tough cyclic delegation graphs are not detected

Summary

## Summary In a TUF repository, the targets role’s signature indicates which target files are trusted by clients. The role can delegate full or partial trust to other roles, meaning that that role is trusted to sign target file metadata. Delegated roles can further delegate trust to other delegated roles. When searching for metadata about a given target, tough failed to detect cyclical role delegations. ## Impact When interacting with TUF repositories which contain cyclical role delegations, tough will fail to detect the cycles and will exhaust its stack while recursively searching the delegation graph. The exhausted call stack will cause the process to abort. Impacted versions: < v0.20.0 ## Patches A fix for this issue is available in tough version 0.20.0 and later. Customers are advised to upgrade to version 0.20.0 or later and ensure any forked or derivative code is patched to incorporate the new fixes. ## Workarounds There is no recommended work around. Customers are advised to upgrade to version 0.20.0 or the latest version. ## References If you have any questions or comments about this advisory we ask that you contact AWS/Amazon Security via our vulnerability reporting page [1] or directly via email to [[email protected]](mailto:[email protected]). Please do not create a public GitHub issue. [1] Vulnerability reporting page: https://aws.amazon.com/security/vulnerability-reporting ## Acknowledgement These issues were identified by the [TUF-Conformance project](https://github.com/theupdateframework/tuf-conformance). We would like to thank Google for collaborating on this issue through the coordinated vulnerability disclosure process.