awslabs/tough Missing Delegated Metadata Validation

The awslabs/tough library, a Python implementation of The Update Framework (TUF), is vulnerable to a metadata validation bypass. Specifically, versions prior to 0.22.0 and tuftool versions prior to 0.15.0 do not properly enforce expiration, hash, and length checks on delegated metadata. An attacker with delegated signing authority can exploit this vulnerability to poison the local metadata cache. This occurs because the load_delegations function doesn’t apply the same strict validation checks as the top-level targets metadata path. Successful exploitation allows the attacker to serve expired or otherwise invalid targets from a TUF repository, which the tough library will trust instead of rejecting, ultimately compromising the integrity of software updates.

Attack Chain

1. Attacker gains delegated signing authority within a TUF repository.
2. Attacker modifies delegated targets metadata to point to malicious software or manipulated metadata files. This could involve changing file hashes, lengths, or expiration dates to values that would normally be rejected.
3. Attacker hosts the modified delegated targets metadata on their controlled server.
4. A client using a vulnerable version of tough attempts to update its software using the TUF repository.
5. The client downloads the attacker’s modified delegated targets metadata.
6. The vulnerable load_delegations function in tough fails to properly validate the expiration, hash, and length of the delegated targets metadata.
7. The client trusts the malicious delegated targets metadata and proceeds to download the associated malicious software or metadata.
8. The client’s local metadata cache is poisoned, and subsequent updates may be compromised even if the attacker loses control of the delegated signing authority.

Impact

Successful exploitation of this vulnerability allows an attacker to serve malicious software updates to clients using the tough library. This could lead to arbitrary code execution, data theft, or other malicious activities on the client’s system. The number of affected clients depends on the adoption of the tough library and its use in software update mechanisms. This vulnerability primarily impacts software supply chain security, potentially affecting any sector relying on TUF for secure updates.

Recommendation

• Upgrade the tough library to version 0.22.0 or later to patch CVE-2026-6967.
• Upgrade tuftool to version 0.15.0 or later to patch CVE-2026-6967.
• Monitor network traffic for unexpected connections to untrusted or unknown hosts during software update processes. Analyze associated process executions.
• Implement integrity checks on downloaded software packages beyond TUF metadata validation to provide defense in depth.
• Examine application logs for errors related to metadata validation failures or unexpected software installations.