creds-safe fetch guard: resolve relative fetch vs credential-free location (creds-in-URL trap)

8868f2586a8efa9efff735f0aa96018680dcc46c · 2026-08-25 10:49:02 -0700 · Steve

Fleet inoculation — opening this basic-auth app with credentials in the URL
poisoned document.baseURI and made relative fetch('/api/…') throw. Guard resolves
non-absolute request URLs against location instead. Ref: creds-in-url-fetch-guard-fleet-pattern.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Files touched

M public/index.html

Diff

commit 8868f2586a8efa9efff735f0aa96018680dcc46c
Author: Steve <steve@designerwallcoverings.com>
Date:   Tue Aug 25 10:49:02 2026 -0700

    creds-safe fetch guard: resolve relative fetch vs credential-free location (creds-in-URL trap)
    
    Fleet inoculation — opening this basic-auth app with credentials in the URL
    poisoned document.baseURI and made relative fetch('/api/…') throw. Guard resolves
    non-absolute request URLs against location instead. Ref: creds-in-url-fetch-guard-fleet-pattern.
    
    Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 public/index.html | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/public/index.html b/public/index.html
index 6c68bbf..79b4041 100644
--- a/public/index.html
+++ b/public/index.html
@@ -1,4 +1,28 @@
 <!doctype html><html lang="en"><head>
+<script>
+// Credential-safe fetch guard (fleet drop-in) — if this page is opened with
+// credentials in the URL (a saved bookmark / Chrome-remembered basic-auth:
+// https://user:pass@host/…), the browser poisons document.baseURI, so any
+// relative or root-relative fetch('/api/…') throws "Request cannot be constructed
+// from a URL that includes credentials" and callers silently fall to an empty
+// state. Resolve every non-absolute request URL against the credential-free
+// location instead of baseURI. Placed first so it wraps window.fetch before any
+// app script runs. Ref: creds-in-url-fetch-guard-fleet-pattern.
+(function () {
+  var _fetch = window.fetch.bind(window);
+  var cleanBase = function () { return location.origin + location.pathname; };
+  window.fetch = function (input, init) {
+    try {
+      if (typeof input === 'string' && !/^[a-z]+:\/\//i.test(input) && input.indexOf('//') !== 0) {
+        input = new URL(input, cleanBase()).href;
+      } else if (input instanceof Request && !/^[a-z]+:\/\//i.test(input.url)) {
+        input = new Request(new URL(input.url, cleanBase()).href, input);
+      }
+    } catch (_) { /* fall through to native */ }
+    return _fetch(input, init);
+  };
+})();
+</script>
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