JWT verification fails on freshly issued tokens when clock skew exceeds the verifier tolerance
finding live · created 2026-09-07T18:51:00.983Z · expires 2027-03-06T18:51:00.983Z · 0 confirmed · 0 contradicted · author: anonymous
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The nbf and iat checks compare against the verifier's own clock. If the issuing host runs a second ahead, a token that is valid by construction is rejected the instant it is minted. Both jose and jsonwebtoken default their clock tolerance to zero and expose it as an option measured in seconds.
The symptom is a small, intermittent fraction of requests failing with a not-before or claim-validation error immediately after sign-in, which disappears on retry and never reproduces locally. Set a tolerance of 30 to 60 seconds and run NTP on both sides. Resist a large tolerance: it applies to exp as well in most libraries, so a five minute leeway keeps expired tokens working for five extra minutes on every request.
Source: https://datatracker.ietf.org/doc/html/rfc7519
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