diff --git a/scripts/lib/mdx-active-content.mjs b/scripts/lib/mdx-active-content.mjs
index ff0eb671..b36bc649 100644
--- a/scripts/lib/mdx-active-content.mjs
+++ b/scripts/lib/mdx-active-content.mjs
@@ -62,6 +62,7 @@ import {
   isUnwrappedCodeBlock,
   unwrapMdxCodeBlocks,
 } from './mdx-code-blocks.mjs';
+import { rewriteAfterUnwrap } from './mdx-preprocess.mjs';
 import {
   describeTarget,
   disallowedScheme,
@@ -355,15 +356,29 @@ function mapUnwrappedLines(text, unwrapped) {
  * Scans a Markdown/MDX body for anything outside the allowlists, and for
  * images loaded from another host.
  *
- * The body is scanned as written and again as `unwrapMdxCodeBlocks()` leaves
- * it, because that rewritten text is what the MDX compiler is handed. The
- * per-node `mdx-code-block` check only sees the span mdast gave the node, and
- * the fence grammars disagree about where it ends: CommonMark closes a
- * three-backtick fence with any run of three or more, while Docusaurus
- * requires exactly three and pairs the opener with a later bare fence. A
- * payload between those two closers sits inside a `code` node the scan trusts.
- * Scanning the unwrapped text closes that gap for any such disagreement. This
- * mirrors `hasActiveMdx()` in ./mdx-escape.mjs.
+ * The body is scanned as written and again in each form Docusaurus's
+ * `preprocessContent()` produces, because that rewritten text, not the file,
+ * is what the MDX compiler is handed.
+ *
+ * `unwrapMdxCodeBlocks()` is the first of those rewrites. The per-node
+ * `mdx-code-block` check only sees the span mdast gave the node, and the fence
+ * grammars disagree about where it ends: CommonMark closes a three-backtick
+ * fence with any run of three or more, while Docusaurus requires exactly three
+ * and pairs the opener with a later bare fence. A payload between those two
+ * closers sits inside a `code` node the scan trusts. Scanning the unwrapped
+ * text closes that gap for any such disagreement. This mirrors
+ * `hasActiveMdx()` in ./mdx-escape.mjs.
+ *
+ * `rewriteAfterUnwrap()` (./mdx-preprocess.mjs) is the rest of the chain. The
+ * admonition rewrite in it inserts a bracket pair around a `:::note` title,
+ * which is the one character a run of literal text needs to its left to become
+ * a link: `:::note x](javascript:alert(1))` carries no link until Docusaurus
+ * supplies the `[`.
+ *
+ * Every form is scanned and the findings unioned rather than the last form
+ * being scanned alone, because a rewrite can also take capability away -- the
+ * heading-id escape inerts a `{...}` expression -- and a construct that is
+ * active in any form the compiler sees has to be reported.
  *
  * @param {string} markdown - Document body to scan.
  * @param {{ mdastExtensions?: Array<object> }} [options] - Extra mdast
@@ -376,11 +391,23 @@ export function findActiveContent(markdown, { mdastExtensions = [] } = {}) {
   const text = String(markdown ?? '');
   const findings = scanSource(text, mdastExtensions);
 
+  // Both rewrites below operate line by line and neither adds nor removes a
+  // line, so a line of either rewritten form is the same line of `unwrapped`
+  // and maps back through the same table. The snippet is re-read from the
+  // source as well, so a finding quotes the text a reader can open the file
+  // to rather than the rewritten line it was spotted in.
+  const sourceLines = text.split('\n');
   const unwrapped = unwrapMdxCodeBlocks(text);
-  if (unwrapped !== text) {
-    const sourceLine = mapUnwrappedLines(text, unwrapped);
-    for (const finding of scanSource(unwrapped, mdastExtensions)) {
-      findings.push({ ...finding, line: sourceLine[finding.line - 1] });
+  const sourceLine = mapUnwrappedLines(text, unwrapped);
+  for (const form of new Set([unwrapped, rewriteAfterUnwrap(unwrapped)])) {
+    if (form === text) continue;
+    for (const finding of scanSource(form, mdastExtensions)) {
+      const line = sourceLine[finding.line - 1];
+      findings.push({
+        ...finding,
+        line,
+        snippet: sourceLines[line - 1].trim().slice(0, 120),
+      });
     }
   }
 
diff --git a/scripts/lib/mdx-preprocess.mjs b/scripts/lib/mdx-preprocess.mjs
new file mode 100644
index 00000000..7ca7193d
--- /dev/null
+++ b/scripts/lib/mdx-preprocess.mjs
@@ -0,0 +1,68 @@
+/**
+ * The rewrites Docusaurus applies to a page before the MDX compiler sees it.
+ *
+ * Both MDX gates in this directory rest on the premise that the text they scan
+ * is the text that gets compiled. It is not: `preprocessContent()` in
+ * @docusaurus/mdx-loader runs a chain of string rewrites over every page
+ * first, and only their output reaches MDX. A construct that is inert on disk
+ * and active after a rewrite is therefore invisible to a scan of the file
+ * alone.
+ *
+ * `unwrapMdxCodeBlocks()` is one link in that chain and already has a module of
+ * its own (./mdx-code-blocks.mjs). This one covers the rest of it, in the order
+ * the loader applies them:
+ *
+ * 1. `escapeMarkdownHeadingIds()` backslash-escapes `{#id}` in an ATX heading.
+ * 2. `admonitionTitleToDirectiveLabel()` turns the legacy `:::note Title`
+ *    syntax into the container-directive form `:::note[Title]`.
+ *
+ * The second is the one that creates capability rather than removing it. It
+ * inserts a `[` after the directive token and a `]` at end of line, and a
+ * Markdown link needs exactly a `[` to its left and a `](url)` to its right --
+ * so `:::note x](javascript:alert(1))`, which is literal text in the file, is
+ * parsed as a link to `javascript:alert(1)` once the `[` is inserted. Neither
+ * rewrite adds or removes a line, so a finding's line number in the rewritten
+ * text is its line number in the text that went in.
+ *
+ * As with ./mdx-code-blocks.mjs, the upstream functions are called rather than
+ * their patterns restated, so where a bracket lands, which quote and
+ * indentation prefixes are accepted and how many colons open a directive
+ * cannot drift from the installed version.
+ *
+ * The one thing deliberately not taken from upstream is the keyword list.
+ * Docusaurus rewrites only the admonition names it knows, and a site may add
+ * more through `markdown.admonitions`; those names live in
+ * @docusaurus/mdx-loader, which is not a direct dependency and exports no entry
+ * point for them. Passing a name pattern instead of a list means the rewrite is
+ * applied to every directive name. That scans a form Docusaurus does not
+ * produce for a name it does not know, which can only report a finding that
+ * would not have shipped -- an over-rejection, never a miss -- and it holds if
+ * the keyword list is ever extended upstream or by configuration.
+ */
+
+import {
+  admonitionTitleToDirectiveLabel,
+  escapeMarkdownHeadingIds,
+} from '@docusaurus/utils';
+
+/**
+ * Stands in for the admonition keyword list, matching any directive name.
+ *
+ * `admonitionTitleToDirectiveLabel()` joins its keywords with `|` and drops
+ * them into a capture group, so a pattern here is interpolated into the same
+ * position a literal name would occupy.
+ */
+const ANY_DIRECTIVE_NAME = '[A-Za-z][A-Za-z0-9-]*';
+
+/**
+ * Apply the preprocessor rewrites that follow `unwrapMdxCodeBlocks()`.
+ *
+ * @param {string} unwrapped - A document, already unwrapped.
+ * @returns {string} The text the MDX compiler is handed. Line count is
+ *   unchanged, so line numbers carry over.
+ */
+export function rewriteAfterUnwrap(unwrapped) {
+  return admonitionTitleToDirectiveLabel(escapeMarkdownHeadingIds(unwrapped), [
+    ANY_DIRECTIVE_NAME,
+  ]);
+}
diff --git a/tests/mdx-active-content.test.mjs b/tests/mdx-active-content.test.mjs
index 4cb68ef6..273db7a6 100644
--- a/tests/mdx-active-content.test.mjs
+++ b/tests/mdx-active-content.test.mjs
@@ -596,3 +596,84 @@ test('leaves ordinary fenced and inline code alone', () => {
   ].join('\n');
   assert.deepEqual(reasons(body), []);
 });
+
+// Docusaurus hands the MDX compiler the output of preprocessContent(), not the
+// file. unwrapMdxCodeBlocks() is only its first rewrite; the admonition
+// rewrite that follows turns ":::note Title" into ":::note[Title]", and the
+// bracket it inserts is the one character a run of literal text needs to its
+// left to become a link. Each body below carries no link on disk and a link to
+// a disallowed scheme once that bracket is supplied.
+const PREPROCESSED = [
+  [
+    'admonition title rewrite forms a javascript: link',
+    ':::note x](javascript:alert(document.domain))',
+    'disallowed URL scheme javascript:',
+  ],
+  [
+    'admonition title rewrite forms a data:text/html link',
+    ':::danger c](data:text/html;base64,PHNjcmlwdD4x)',
+    'disallowed URL scheme data:text/html',
+  ],
+  [
+    'the rewrite applies inside a blockquote',
+    '> :::tip c](javascript:alert(1))',
+    'disallowed URL scheme javascript:',
+  ],
+  [
+    'the rewrite applies to a four-colon directive',
+    '::::warning c](javascript:alert(1))',
+    'disallowed URL scheme javascript:',
+  ],
+  [
+    'the rewrite is not limited to the upstream keyword list',
+    ':::housekeeping c](javascript:alert(1))',
+    'disallowed URL scheme javascript:',
+  ],
+];
+
+for (const [name, body, reason] of PREPROCESSED) {
+  test(`rejects a scheme that only appears after preprocessing: ${name}`, () => {
+    assert.deepEqual(reasons(body), [reason]);
+  });
+}
+
+test('reports a preprocessing-only finding at its true source line', () => {
+  // Neither rewrite after unwrapMdxCodeBlocks() adds or removes a line, so the
+  // line a reader opens the file to is the unwrapped line mapped back to the
+  // source -- here past a deleted mdx-code-block fence.
+  const body = [
+    '## Overview',
+    '',
+    '```mdx-code-block',
+    ':::note x](javascript:alert(1))',
+    '```',
+  ].join('\n');
+  assert.deepEqual(
+    findActiveContent(body).find(
+      (finding) => finding.reason === 'disallowed URL scheme javascript:',
+    ),
+    {
+      line: 4,
+      reason: 'disallowed URL scheme javascript:',
+      snippet: ':::note x](javascript:alert(1))',
+    },
+  );
+});
+
+test('leaves an ordinary admonition and a fenced copy of the payload alone', () => {
+  // The rewrite is line-based and runs inside fenced code too, but a fence the
+  // unwrap step leaves in place still makes its body code to the MDX compiler,
+  // so neither body below publishes a link.
+  const body = [
+    ':::note A normal title',
+    '',
+    'Body prose.',
+    '',
+    ':::',
+    '',
+    '```',
+    ':::note x](javascript:alert(1))',
+    '```',
+  ].join('\n');
+  assert.deepEqual(reasons(body), []);
+});