Forcing 304 Not Modified

Mechanism: Per RFC 9110 §13.1.2, when a HEAD request includes If-None-Match: *, the condition evaluates to false if any current representation exists — the server responds 304. If the resource doesn't exist, the server returns 404.

Isolated Variable: Only the If-None-Match: * header is added.

Oracle Signal: 304 (exists) vs 404 (does not exist).

HEAD — Existing Resource

HEAD /api/users/1001 HTTP/1.1
Host: target.com
If-None-Match: *

HTTP/1.1 304 Not Modified
ETag: "a1b2c3d4"

HEAD — Non-Existing Resource

HEAD /api/users/9999 HTTP/1.1
Host: target.com
If-None-Match: *

HTTP/1.1 404 Not Found

💡 Bandwidth efficiency: 304 responses have no body, and HEAD responses never have a body. This makes If-None-Match: * + HEAD the most bandwidth-efficient existence oracle — the entire response is just the status line and a few headers.

Mitigation: Inherent to HTTP conditional request semantics. Require authentication before evaluating conditional headers.