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The code generates a fresh nonce for each authorize request, embeds it in the locally-signed state JWT, and forwards it to the identity provider. Great! But what it never did was check that the id_token coming back from the token endpoint actually echoed the same value, effectively making the nonce useless. Skipping that step leaves room for id_token substitution. Any valid id_token for the same client identifier (a stale one, from another application, or one swapped by a hostile proxy) would be accepted. Verifying the nonce that was sent is also required by the OpenID Connect Core specification. So, validate the id_token's nonce against the value that was originally sent. This is a built-in feature of the underlying JWT library.
kpfleming
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May 4, 2026
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The code generates a fresh nonce for each authorize request, embeds it in the locally-signed state JWT, and forwards it to the identity provider. Great!
But what it never did was check that the id_token coming back from the token endpoint actually echoed the same value, effectively making the nonce useless.
Skipping that step leaves room for id_token substitution. Any valid id_token for the same client identifier (a stale one, from another application, or one swapped by a hostile proxy) would be accepted.
Verifying the nonce that was sent is also required by the OpenID Connect Core specification.
So, validate the id_token's nonce against the value that was originally sent.
This is a built-in feature of the underlying JWT library.