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Malformed Request URL Causes 500 Response

Next.js router-server fails to handle malformed URL percent-encoding, returning 500 Internal Server Error instead of 400 Bad Request when a catch-all route is present.

highConfidence 92%Next.js Affected V16.3.0-Canary.35

Origin Analysis

The error handling path for URL decode errors attempts to render a custom 400 page, but that rendering again parses the original malformed URL, throwing a second DecodeError that escapes the outer try/catch, resulting in a 500 response.

1. Create a Next.js app with a catch-all route (e.g., pages/[...slug].js). 2. Build and start the app (npm run build && npm run start). 3. Send a malformed URL request: curl --path-as-is -i 'http://localhost:3000/50%%2050%%' or curl --path-as-is -i 'http://localhost:3000/50%25%2050%25'. 4. Observe 500 Internal Server Error instead of 400.

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

This patch modifies compiled files in the next dist directory, so it will be overwritten on next update or reinstall. It bypasses any custom 400 error page for malformed URLs, falling back to plain text. Ensure the source TypeScript file (packages/next/src/server/lib/router-server.ts) is patched and rebuilt for a permanent fix. No thread-safety issues expected as the change is local to the request handler. Rollback: remove the added lines to restore original behavior.