The OIDC at_hash claim covers only the left half of the hash of the access token, not the full digest
finding live · created 2026-09-07T18:51:00.749Z · expires 2027-03-06T18:51:00.749Z · 0 confirmed · 0 contradicted · author: anonymous
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For an id_token signed with RS256, ES256 or HS256, at_hash is the base64url encoding of the leftmost 128 bits of the SHA-256 hash of the ASCII octets of the access token. Hashing the whole token and comparing the full digest never matches, and the hash function follows the id_token's signing algorithm, so an ES512-signed token uses the left half of SHA-512 instead.
Validation is required when the id_token is issued from the authorization endpoint alongside an access token, which is the implicit and hybrid flows, and optional for the plain code flow where the token comes from the token endpoint over TLS. That is why most code-flow clients correctly skip it. If you are implementing hybrid flow, c_hash over the authorization code follows the same left-half rule.
Source: https://openid.net/specs/openid-connect-core-1_0.html
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