diff --git a/scripts/validate-architecture-assets.mjs b/scripts/validate-architecture-assets.mjs
index 9a197cdd..cc90a82e 100644
--- a/scripts/validate-architecture-assets.mjs
+++ b/scripts/validate-architecture-assets.mjs
@@ -50,7 +50,9 @@ const SITE_CHROME_EXTENSIONS = new Set([...ALLOWED_ASSET_EXTENSIONS, '.ico']);
 // favicons - rather than imported assets, but they are served from the same
 // origin as everything else, so they carry the same security gate. static/img
 // is walked shallowly because its image subdirectories are listed above, each
-// with its own quality setting.
+// with its own quality setting; walk() rejects any *other* subdirectory it
+// finds there, since a shallow walk would otherwise publish its contents with
+// no gate at all.
 //
 // static/fonts is deliberately outside the gate (it holds no SVG, and its
 // extensions are legitimately outside the image allow-list) via the
@@ -165,7 +167,23 @@ function walk(dir, recurse = true) {
       record(path, 'error', 'is a symbolic link; symlinks are not allowed');
       return [];
     }
-    if (entry.isDirectory()) return recurse ? walk(path) : [];
+    if (entry.isDirectory()) {
+      if (recurse) return walk(path);
+      // A shallow walk covers only the files sitting directly in the
+      // directory, so every subdirectory below it must be an asset root with
+      // its own assetDirs entry (returned above via assetRootPaths). One that
+      // is not is published at the site origin with no gate at all -- the same
+      // silent gap checkForUngatedStaticEntries() closes at the static/ root,
+      // one level down.
+      record(
+        path,
+        'error',
+        'is a subdirectory of a shallowly walked asset directory and is not ' +
+          'covered by the asset security gate; add it to assetDirs in ' +
+          'scripts/validate-architecture-assets.mjs',
+      );
+      return [];
+    }
     return [path];
   });
 }
diff --git a/tests/validate-architecture-assets.test.mjs b/tests/validate-architecture-assets.test.mjs
index 9e7235fc..8e3e72b7 100644
--- a/tests/validate-architecture-assets.test.mjs
+++ b/tests/validate-architecture-assets.test.mjs
@@ -524,16 +524,44 @@ test('accepts an allowed image sitting directly in static/', () => {
   assert.equal(result.status, 0, result.stderr);
 });
 
-test('does not descend into subdirectories of the shallow static/img walk', () => {
+test('rejects a subdirectory of the shallow static/img walk', () => {
   // static/img is walked with recurse: false because it holds site chrome
-  // sitting directly in the directory. Its subdirectories are either asset
-  // roots with their own entry or -- as here -- out of the gate's reach, so
-  // an asset nested inside one is neither validated nor counted.
+  // sitting directly in the directory. A shallow walk covers only those
+  // files, so every subdirectory below it must be an asset root with its own
+  // assetDirs entry; one that is not would otherwise ship to the site origin
+  // with no gate at all.
   const result = runScriptWithFixtures(SCRIPT, {
     'static/img/architectures/example/diagram.svg': VALID_SVG,
     'static/img/illustrations/nested.svg': '<svg><rect/></svg>',
   });
+  assert.equal(result.status, 1);
+  assert.match(
+    result.stderr,
+    /static\/img\/illustrations: is a subdirectory of a shallowly walked asset directory and is not covered by the asset security gate/,
+  );
+});
+
+test('rejects markup in a subdirectory of the shallow static/img walk', () => {
+  // The extension allow-list is the only thing that keeps a .html off the
+  // site origin, and it never ran below static/img: the file was published
+  // verbatim with the validator, the unit suite and the build all green.
+  const result = runScriptWithFixtures(SCRIPT, {
+    'static/img/architectures/example/diagram.svg': VALID_SVG,
+    'static/img/blog/pwn.html': '<html><script>alert(1)</script></html>',
+  });
+  assert.equal(result.status, 1);
+  assert.match(result.stderr, /static\/img\/blog: is a subdirectory/);
+});
+
+test('does not flag the static/img subdirectories that are asset roots', () => {
+  // The three real subdirectories each have their own assetDirs entry, so
+  // they are returned by assetRootPaths before the new subdirectory check.
+  const result = runScriptWithFixtures(SCRIPT, {
+    'static/img/architectures/example/diagram.svg': VALID_SVG,
+    'static/img/cncf-projects/helm-helm-icon-color.svg': VALID_SVG,
+    'static/img/awards/example.svg': VALID_SVG,
+    'static/img/cncf_logo_white.svg': VALID_SVG,
+  });
   assert.equal(result.status, 0, result.stderr);
-  assert.match(result.stdout, /Validated 1 architecture asset/);
-  assert.doesNotMatch(result.stderr, /illustrations/);
+  assert.match(result.stdout, /Validated 4 architecture asset/);
 });