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6/30/2026
My Next.js 16 Auth Passed Every Test. Five Bugs That Only Showed Up When I Wired It Together.

My Next.js 16 Auth Passed Every Test. Five Bugs That Only Showed Up When I Wired It Together.

Short summary

A developer debugged five integration bugs in Next.js auth that passed unit tests but failed end-to-end. Root causes: client-side cookies vanishing due to timing, client-side routing bypassing proxy checks, JWT claims stored as numbers instead of strings, and misaligned auth layer validation. Each bug reveals critical gaps between isolated testing and integrated systems.

  • •Set cookies server-side with httpOnly to prevent both XSS theft and client-side race conditions
  • •Client-side navigation skips network requests, making proxy auth checks invisible — use window.location.href for real HTTP
  • •Type mismatches in JWT claims (Postgres integer IDs vs string expectations) fail silently in development, break in production

Generated with AI, which can make mistakes.

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The original title is: "4 Security Bugs I Found in My Own Auth System (And How I Fixed Them)"
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