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PGRST303 'JWT Issued At Future' Due To Clock Skew Between API Gateway And PostgREST When Using Sb_secret_ Keys

Intermittent 401 PGRST303 errors occur because the Supabase API gateway mints a short-lived JWT with an iat claim based on its own clock, which can be ahead of the PostgREST instance's clock by more than the default validation leeway, causing PostgREST to reject the token as issued in the future.

highConfidence 88%Supabase

Origin Analysis

Clock skew between the Supabase API gateway and the PostgREST instance exceeds the JWT iat future-tolerance (leeway) configured in PostgREST. The gateway exchanges the opaque sb_secret_ key for a JWT with iat set to the gateway's current time; if the gateway clock is ahead of PostgREST by more than the allowed leeway (often 0-1 seconds), PostgREST rejects the token with PGRST303. The intermittent nature and occasional >2s skew matches the observed retry failures.

1. Use a valid new-format `sb_secret_` key with server-side supabase-js. 2. Send multiple REST requests (e.g., selects) over an extended period. 3. Observe intermittent HTTP 401 with body `PGRST303 - JWT issued at future`. 4. The error is not reproducible on demand and appears related to cross-component clock skew.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Increasing the skew compensation reduces the effective indication of issuance time and slightly extends the window during which a token may be considered valid if an attacker captures it; however, since the token is signed by Supabase and short-lived, this is low risk. Ensure the skew compensation is not set arbitrarily high (e.g., >30s) to avoid tokens being accepted after a potential revocation or secret rotation. Rollback is straightforward by removing or reducing the `JWT_SKEW_SECONDS` env var. If using PostgREST leeway, be aware it applies to all JWT validation, including externally supplied tokens, which could weaken future-iat rejection.