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An ID token that was encrypted without being signed was accepted as fully
validated, so anyone holding the RP's public encryption key could mint a token
with an arbitrary `sub`. The spec requires a Nested JWT: "If the ID Token is
encrypted, it MUST be signed then encrypted" (OpenID Connect Core 1.0, section
2). JARM section 2.2 says the same, and that path is reachable through the
browser.

Fixes CVE-2026-75759 / GHSA-533g-4vf3-xwrj

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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