Add GPT-domain fast-path guard and fix entity-encoded URL bypass in rewriteHtmlString

ChristianPavilonis · ChristianPavilonis · commit c6c5b2f2b792 · 2026-03-18T09:19:39.000-05:00
diff --git a/crates/js/lib/src/integrations/gpt/script_guard.ts b/crates/js/lib/src/integrations/gpt/script_guard.ts
@@ -204,16 +204,25 @@ function rewriteLinkHref(
  * `document.write` / `document.writeln`.
  *
  * Uses `DOMParser` for robust HTML parsing instead of regex so that
- * edge-cases (unquoted attributes, unusual spacing, mixed quote styles)
- * are handled by the browser's native parser. The raw `getAttribute`
- * value is swapped in the original HTML string so the surrounding markup
- * is preserved verbatim.
+ * edge-cases (unquoted attributes, unusual spacing, mixed quote styles,
+ * HTML-entity-encoded query parameters) are handled by the browser's
+ * native parser. GPT script `src` attributes are mutated in the parsed
+ * DOM and the result is serialized back to HTML.
  *
  * If the GPT domain is present in the HTML but `DOMParser` is
  * unavailable or throws, the function **fails closed** (returns an
  * empty string) rather than passing the unproxied URL through.
+ *
+ * Non-GPT HTML is always passed through unchanged regardless of
+ * `DOMParser` availability.
  */
 function rewriteHtmlString(html: string): string {
+  // Fast-path: if the HTML does not reference the GPT domain at all,
+  // pass it through unchanged.  This avoids unnecessary DOMParser
+  // overhead and, critically, prevents non-GPT document.write calls
+  // from being silently dropped when DOMParser is unavailable.
+  if (!html.includes(GPT_DOMAIN)) return html;
+
   if (typeof DOMParser === 'undefined') {
     log.warn(
       `${LOG_PREFIX}: DOMParser unavailable, blocking document.write HTML that references GPT domain`
@@ -224,7 +233,7 @@ function rewriteHtmlString(html: string): string {
   try {
     const doc = new DOMParser().parseFromString(html, 'text/html');
     const scripts = doc.querySelectorAll('script[src]');
-    let result = html;
+    let didRewriteAny = false;
 
     for (const script of scripts) {
       const rawSrc = script.getAttribute('src') ?? '';
@@ -235,10 +244,17 @@ function rewriteHtmlString(html: string): string {
         original: rawSrc,
         rewritten: rewrittenUrl,
       });
-      result = result.replaceAll(rawSrc, rewrittenUrl);
+      // Mutate the parsed DOM so that HTML-entity-encoded attribute
+      // values (e.g. `&amp;`) are handled correctly.  Serializing the
+      // DOM back to HTML avoids the mismatch between decoded
+      // `getAttribute()` values and the raw HTML string.
+      script.setAttribute('src', rewrittenUrl);
+      didRewriteAny = true;
     }
 
-    return result;
+    // DOMParser wraps input in <html><head>…</head><body>…</body></html>.
+    // Bare <script> tags land in <head>, so we serialize from both.
+    return didRewriteAny ? (doc.head?.innerHTML ?? '') + (doc.body?.innerHTML ?? '') : html;
   } catch (err) {
     log.warn(
       `${LOG_PREFIX}: failed to parse document.write HTML containing GPT domain, blocking`,
diff --git a/crates/js/lib/test/integrations/gpt/script_guard.test.ts b/crates/js/lib/test/integrations/gpt/script_guard.test.ts
@@ -303,4 +303,46 @@ describe('GPT script guard', () => {
       globalThis.DOMParser = originalDOMParser;
     }
   });
+
+  it('passes non-GPT HTML through unchanged when DOMParser is unavailable', () => {
+    const nativeWriteSpy = vi.fn<(...args: string[]) => void>();
+    document.write = nativeWriteSpy as unknown as typeof document.write;
+
+    const originalDOMParser = globalThis.DOMParser;
+    // @ts-expect-error — simulating an environment without DOMParser
+    delete globalThis.DOMParser;
+
+    try {
+      installGptGuard();
+
+      const html = '<p>Hello, world!</p>';
+      document.write(html);
+
+      expect(nativeWriteSpy).toHaveBeenCalledTimes(1);
+      expect(nativeWriteSpy).toHaveBeenCalledWith(html);
+    } finally {
+      globalThis.DOMParser = originalDOMParser;
+    }
+  });
+
+  // -----------------------------------------------------------------------
+  // HTML-entity-encoded URLs
+  // -----------------------------------------------------------------------
+
+  it('rewrites GPT URLs that contain HTML-escaped entities like &amp;', () => {
+    const nativeWriteSpy = vi.fn<(...args: string[]) => void>();
+    document.write = nativeWriteSpy as unknown as typeof document.write;
+
+    installGptGuard();
+
+    document.write(
+      '<script src="https://securepubads.g.doubleclick.net/pagead/managed/js/gpt/current/pubads_impl.js?x=1&amp;y=2"></script>'
+    );
+
+    expect(nativeWriteSpy).toHaveBeenCalledTimes(1);
+    const [writtenHtml] = nativeWriteSpy.mock.calls[0] ?? [];
+    expect(writtenHtml).toContain(window.location.host);
+    expect(writtenHtml).toContain('/integrations/gpt/pagead/managed/js/gpt/current/pubads_impl.js');
+    expect(writtenHtml).not.toContain('securepubads.g.doubleclick.net');
+  });
 });
