diff --git a/build/tasks/esbuild.js b/build/tasks/esbuild.js
index 546c9317e..a358986ff 100644
--- a/build/tasks/esbuild.js
+++ b/build/tasks/esbuild.js
@@ -56,6 +56,11 @@ module.exports = function (grunt) {
             minify: false,
             format: 'esm',
             bundle: true,
+            // [a11y-core]: inline bundled plain-text data assets (e.g. the COI
+            // ad-iframe denylist committed in a11y-engine-core/config) as raw
+            // strings so the client parses them at scan start with no runtime
+            // fetch. Only affects `.txt` imports; JS bundling is unchanged.
+            loader: { '.txt': 'text' },
             plugins: [fingerprintFallback]
           })
             .then(done)
diff --git a/lib/core/base/audit.js b/lib/core/base/audit.js
index 3353545af..092963b95 100644
--- a/lib/core/base/audit.js
+++ b/lib/core/base/audit.js
@@ -180,6 +180,18 @@ export default class Audit {
       }
     }
   }
+  /**
+   * [a11y-core]: Store the cross-origin iframe ad denylist Set (COI denylist).
+   * Built by a11y-engine-core from its bundled list and injected via
+   * axe.configure({ crossOriginDenylist }); consumed at frame enumeration
+   * (pushUniqueFrameSelector) to skip denied ad frames. Stored by reference —
+   * never copied — so every hop points at the same Set.
+   *
+   * @param {Set<string>} crossOriginDenylist
+   */
+  setCrossOriginDenylist(crossOriginDenylist) {
+    this.crossOriginDenylist = crossOriginDenylist;
+  }
   /**
    * Initializes the rules and checks
    */
@@ -195,6 +207,10 @@ export default class Audit {
     this.tagExclude = ['experimental', 'deprecated'];
     this.noHtml = audit.noHtml;
     this.allowedOrigins = audit.allowedOrigins;
+    // [a11y-core]: carry the COI denylist Set across re-init (mirrors
+    // allowedOrigins) so child-frame audits built from the default config
+    // inherit it. Undefined when no cross-origin flag armed it.
+    this.crossOriginDenylist = audit.crossOriginDenylist;
     unpackToObject(audit.rules, this, 'addRule');
     unpackToObject(audit.checks, this, 'addCheck');
     this.data = {};
diff --git a/lib/core/base/context/parse-selector-array.js b/lib/core/base/context/parse-selector-array.js
index 4435664f4..d8af58b69 100644
--- a/lib/core/base/context/parse-selector-array.js
+++ b/lib/core/base/context/parse-selector-array.js
@@ -1,5 +1,9 @@
 import { createFrameContext } from './create-frame-context';
-import { getNodeFromTree, shadowSelectAll } from '../../utils';
+import {
+  getNodeFromTree,
+  isDeniedFrameOrigin,
+  shadowSelectAll
+} from '../../utils';
 
 /**
  * Finds frames in context, converts selectors to Element references and pushes unique frames
@@ -48,7 +52,19 @@ function pushUniqueFrameSelector(context, type, selectorArray) {
 
   const frameSelector = selectorArray.shift();
   const frames = shadowSelectAll(frameSelector);
+  // [a11y-critical]: read the COI ad denylist once for this selector group.
+  // Undefined unless a11y-engine-core armed Type A cross-origin and injected it
+  // via axe.configure — off => enumeration is byte-identical to main.
+  const crossOriginDenylist = axe._audit && axe._audit.crossOriginDenylist;
   frames.forEach(frame => {
+    // Skip ad iframes on the denylist BEFORE they enter context.frames, so they
+    // are never enumerated or traversed by either the sync or async frame path
+    // (this is the single frame-enumeration chokepoint). The iframe element and
+    // its readable src are still in hand here — the postMessage layer one level
+    // down cannot gate on origin.
+    if (isDeniedFrameOrigin(frame, crossOriginDenylist)) {
+      return;
+    }
     let frameContext = context.frames.find(result => result.node === frame);
     if (!frameContext) {
       frameContext = createFrameContext(frame, context);
diff --git a/lib/core/public/configure.js b/lib/core/public/configure.js
index 1cf73d642..b520bac4d 100644
--- a/lib/core/public/configure.js
+++ b/lib/core/public/configure.js
@@ -126,6 +126,14 @@ function configure(spec) {
 
     audit.setAllowedOrigins(spec.allowedOrigins);
   }
+
+  // [a11y-critical]: cross-origin iframe ad denylist (COI denylist). Sibling to
+  // allowedOrigins — a11y-engine-core builds the Set from its bundled list and
+  // passes it here so frame enumeration can skip denied ad frames. The Set is
+  // stored by reference (never serialized cross-frame). Absent/off => unchanged.
+  if (spec.crossOriginDenylist) {
+    audit.setCrossOriginDenylist(spec.crossOriginDenylist);
+  }
 }
 
 export default configure;
diff --git a/lib/core/utils/index.js b/lib/core/utils/index.js
index 3c9b7db27..56b39cc19 100644
--- a/lib/core/utils/index.js
+++ b/lib/core/utils/index.js
@@ -51,6 +51,7 @@ export {
   isLabelledShadowDomSelector,
   isLabelledFramesSelector
 } from './is-context';
+export { default as isDeniedFrameOrigin } from './is-denied-frame-origin'; // [a11y-core]: COI denylist Type A gate
 export { default as isHidden } from './is-hidden';
 export { default as isHtmlElement } from './is-html-element';
 export { default as isNodeInContext } from './is-node-in-context';
diff --git a/lib/core/utils/is-denied-frame-origin.js b/lib/core/utils/is-denied-frame-origin.js
new file mode 100644
index 000000000..d84bbe429
--- /dev/null
+++ b/lib/core/utils/is-denied-frame-origin.js
@@ -0,0 +1,54 @@
+// [a11y-core]: Cross-Origin Iframe ad denylist (COI denylist) — Type A gate.
+//
+// a11y-engine-core builds the ad-hostname Set from its bundled config/coi-denylist.txt
+// and hands it to the fork via axe.configure({ crossOriginDenylist }) ->
+// audit.setCrossOriginDenylist (mirroring allowedOrigins). This fork is a separate
+// submodule and CANNOT import a11y-engine-core, so it carries its own copy of the
+// tiny suffix matcher here. When the Type A cross-origin flag is off, the Set is
+// never set and this returns false, keeping frame enumeration byte-identical to main.
+
+/**
+ * True when a frame element's src origin hostname is on the ad denylist
+ * (suffix rule: host === entry OR host.endsWith('.' + entry)). Scheme and port
+ * are stripped before compare. srcless / srcdoc / relative / about:blank frames
+ * resolve to a same-origin or opaque host that is never on the ad list, so they
+ * are never denied. Any parse error fails open (not denied).
+ *
+ * @param {Element} frame the iframe element (its src is readable even cross-origin)
+ * @param {Set<string>} denylistSet ad-hostname Set, or undefined when the flag is off
+ * @returns {boolean}
+ */
+export default function isDeniedFrameOrigin(frame, denylistSet) {
+  if (
+    !denylistSet ||
+    typeof denylistSet.has !== 'function' ||
+    denylistSet.size === 0
+  ) {
+    return false;
+  }
+  let host;
+  try {
+    const url = new URL(frame.src, window.location.href);
+    if (url.protocol !== 'http:' && url.protocol !== 'https:') {
+      return false;
+    }
+    host = url.hostname.toLowerCase();
+  } catch {
+    return false;
+  }
+  if (!host) {
+    return false;
+  }
+  if (denylistSet.has(host)) {
+    return true;
+  }
+  // Walk parent suffixes: a.b.doubleclick.net -> b.doubleclick.net -> doubleclick.net
+  let idx = host.indexOf('.');
+  while (idx !== -1) {
+    if (denylistSet.has(host.slice(idx + 1))) {
+      return true;
+    }
+    idx = host.indexOf('.', idx + 1);
+  }
+  return false;
+}
diff --git a/test/core/utils/is-denied-frame-origin.js b/test/core/utils/is-denied-frame-origin.js
new file mode 100644
index 000000000..c3a238092
--- /dev/null
+++ b/test/core/utils/is-denied-frame-origin.js
@@ -0,0 +1,61 @@
+describe('axe.utils.isDeniedFrameOrigin', () => {
+  const { isDeniedFrameOrigin } = axe.utils;
+  const denylist = new Set(['doubleclick.net', 'adnxs.com']);
+
+  it('is false when no denylist Set is provided (flag off => byte-identical)', () => {
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'https://ads.doubleclick.net/x' })
+    );
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'https://ads.doubleclick.net/x' }, undefined)
+    );
+  });
+
+  it('is false for an empty denylist Set', () => {
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'https://ads.doubleclick.net/x' }, new Set())
+    );
+  });
+
+  it('denies an exact-origin frame', () => {
+    assert.isTrue(
+      isDeniedFrameOrigin({ src: 'https://doubleclick.net/tag' }, denylist)
+    );
+  });
+
+  it('denies a subdomain frame (dot-boundary suffix)', () => {
+    assert.isTrue(
+      isDeniedFrameOrigin({ src: 'https://a.b.adnxs.com/px' }, denylist)
+    );
+  });
+
+  it('does NOT deny a non-dot-boundary substring host (no false positive)', () => {
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'https://notdoubleclick.net/x' }, denylist)
+    );
+  });
+
+  it('does NOT deny an unrelated origin', () => {
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'https://example.com/x' }, denylist)
+    );
+  });
+
+  it('is false for non-http(s) / srcless / about:blank frames (never denied)', () => {
+    assert.isFalse(isDeniedFrameOrigin({ src: '' }, denylist));
+    assert.isFalse(isDeniedFrameOrigin({ src: 'about:blank' }, denylist));
+    assert.isFalse(
+      isDeniedFrameOrigin({ src: 'data:text/html,<b>x</b>' }, denylist)
+    );
+  });
+
+  it('fails open on an unparseable src', () => {
+    assert.isFalse(isDeniedFrameOrigin({ src: 'http://' }, denylist));
+  });
+
+  it('works on a real iframe element', () => {
+    const iframe = document.createElement('iframe');
+    iframe.src = 'https://ads.doubleclick.net/pagead';
+    assert.isTrue(isDeniedFrameOrigin(iframe, denylist));
+  });
+});