AWS Tough EUVD-2026-25627

Attacker must possess valid authentication credentials for the TUF repository system (PR:L - low privileges required per CVSS vector). Specifically, the attacker needs access to at least one legitimate private signing key for a delegated role within the TUF repository hierarchy. The attack targets delegated role metadata validation, meaning the vulnerable code path is invoked when the client processes metadata for roles that have been delegated signing authority (not root or top-level targets metadata which have different validation). Attack complexity is high (AC:H) because the attacker must understand TUF's threshold signature model, craft syntactically valid metadata with duplicated signatures, and have network access to position the malicious metadata where the Tough client will retrieve it. The vulnerability is limited to signature uniqueness validation on delegated roles - it does not bypass all TUF security mechanisms, and the attacker still needs one valid signature from a trusted key. Organizations not using TUF delegated roles or those with single-signature thresholds (1-of-1) are not affected by this specific bypass vector.

CVSS 7.0 (High) with CVSS:4.0 vector indicating network-accessible attack requiring low privileges, high attack complexity, and present attack technique maturity (AV:N/AC:H/AT:P/PR:L). The High integrity impact to both vulnerable (VI:H) and subsequent (SI:H) systems reflects TUF's role in software supply chain trust - compromised metadata acceptance can poison downstream update chains. However, real-world exploitation requires the attacker to already possess one valid signature from a legitimate delegated role key holder (PR:L authentication requirement), significantly raising the bar beyond simple remote exploitation. AC:H and AT:P indicate the attack is not trivial - the attacker must understand TUF's delegation model, have network access to the repository client, and craft metadata that passes all validation except signature uniqueness. No EPSS score provided in input data. No CISA KEV listing and no public exploit identified at time of analysis, suggesting this is a design flaw discovered through security review rather than active exploitation. The Authentication Bypass and JWT Attack tags in input are somewhat misleading - this is specifically a TUF signature validation bypass, not generic JWT manipulation. Primary risk is to organizations using Tough for software update verification in supply chain scenarios where compromising even one delegation key (through insider threat, key compromise, or social engineering) could be leveraged to forge trusted metadata. Risk mitigated by patch availability from vendor and relatively narrow deployment footprint (Rust/crates.io ecosystem).

An attacker compromises a single delegated role signing key through phishing or insider access. They craft malicious TUF metadata delegating authority to attacker-controlled keys or referencing malicious software packages. Instead of obtaining the required threshold of independent signatures (e.g., 3 of 5 legitimate signers), the attacker signs the metadata once with their compromised key and duplicates that signature three times in the metadata file. The vulnerable Tough client accepts this metadata as validly signed, trusting the forged delegation and allowing installation of attacker-supplied packages in downstream software update processes.