Differential Security Analysis of EU Digital Identity Credential Libraries

Version 1 scope. This version presents the differential-testing framework, the rule systematisation, the methodology and enforcement taxonomy, the aggregate outcomes, and a candidate specification-level finding. The per-library security findings and the runnable artifact are held for a subsequent version to appear once coordinated disclosure has concluded; nothing here attributes an unfixed vulnerability to a named library. Regulation (EU) 2026/1731 makes IETF SD-JWT VC and ISO/IEC 18013-5 mdoc the two mandatory credential formats for the European Digital Identity Wallet. Security in these systems rests almost entirely on the verifier. This work systematises 30 security-relevant validation rules across both formats, each anchored to a normative clause, and derives an adversarial corpus of 40 cases evaluated against seven implementations spanning six independent code lineages in three languages, giving 122 (case, implementation) evaluations. Beyond an accept/reject oracle it applies a content oracle and an enforcement taxonomy that distinguishes a missing mechanism from a mechanism left disabled by default. Specification-level finding. RFC 9901 forbids a digest value from appearing more than once and every SD-JWT implementation tested enforces this, whereas no equivalent requirement was found in three independent public reproductions of ISO/IEC 18013-5's Clause 9.3 validation procedure (the standard is paywalled and was not consulted directly), and no mdoc implementation tested rejected a byte-identical duplicate. This is reported as a candidate specification gap. Responsible disclosure. Confirmed vulnerabilities entered coordinated disclosure before this release; reports were sent privately to the affected maintainers. No maintainer-agreed embargo dates are asserted. This preprint withholds the per-library results, proof-of-concept code and exploitation-enabling detail for defects that remain unfixed.