Gateway Policy Order and the Blocked-Legitimate-Traffic Incident
An insurer's API gateway applies, in this order: (1) IP allowlist, (2) API key validation, (3) OAuth token validation, (4) rate limiting, (5) request transformation, (6) backend routing. A new mobile release routes traffic through a cloud NAT with a shared egress IP range not on the allowlist. Result: 100% of mobile requests return 403 before authentication. Meanwhile, an internal batch job with a valid key sends 50,000 requests in 2 minutes, exhausting the rate limit and causing 429s for portal users. Logs show 403s with no user identity and 429s attributed to the batch key. The team proposes removing the IP allowlist entirely and raising the rate limit.
1. Diagnose both incidents using the policy order. Why does the 403 incident lack user identity, and why did the batch job affect portal users?
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403 incident: the IP allowlist is evaluated first, before API key and OAuth validation. A request from the new NAT range fails step 1 and is rejected before any credential is examined, so logs contain no user identity - that absence is the diagnostic signature of a pre-authentication rejection. It also means the allowlist, not authentication, is the effective gate for mobile, which is fragile: any egress change silently locks out all users.
429 incident: rate limiting at step 4 is applied to a shared budget (or the batch key's consumption draws from a pool portal users also use). A single key sending 50,000 requests in 2 minutes consumes the allowance, and portal users hit 429s they did not cause. The 429s attributed to the batch key confirm the source; the collateral 429s on portal traffic confirm the limit is not partitioned per consumer.
Root cause in both cases: policy ordering and scoping are doing security work they were not designed for. The allowlist is being used as an access control where authentication should decide, and the rate limit is being used as a global governor where per-consumer quotas should isolate traffic.
2. Evaluate the proposed fixes (remove the allowlist, raise the rate limit) and recommend an alternative.
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Removing the allowlist fixes the symptom but discards a defense-in-depth control that may be required for admin or partner endpoints. Raising the rate limit likewise treats the symptom: a larger pool is still shared, so a bigger batch job recreates the incident. Neither addresses ordering or scoping.
Recommended alternative:
- Reorder policies: validate API key and OAuth token before IP filtering, so rejections carry identity and the allowlist becomes a secondary control rather than the primary gate. If IP restriction is required for specific operations, scope it to those operations or to admin/partner plans rather than all traffic.
- Update the allowlist deliberately: add the cloud NAT range with a documented owner and review date; treat egress ranges as configuration that changes, not a static truth.
- Partition rate limits per consumer: give the batch job its own quota sized to its legitimate peak (and schedule it off-peak), and give portal users a separate allowance. This isolates blast radius regardless of total volume.
- Add observability: alert when 403 rates spike with no identity (pre-auth rejection) and when any single key exceeds a share of total traffic.
This preserves the allowlist as a targeted control, keeps authentication authoritative, and prevents one consumer from degrading others.