DOMPurify: CUSTOM_ELEMENT_HANDLING bypasses afterSanitizeElements for allowed custom elements.
Summary
There is a possible hook-policy inconsistency in DOMPurify 3.4.11 involving CUSTOM_ELEMENT_HANDLING.
When a custom element is allowed via CUSTOM_ELEMENT_HANDLING.tagNameCheck, it appears that the element does not go through afterSanitizeElements in the same way as a normal element. As a result, an application that relies on afterSanitizeElements as a security policy layer to strip sensitive attributes from all elements may see those attributes removed from normal elements but preserved on allowed custom elements.
This does not appear to be a direct DOMPurify XSS or a case where DOMPurify directly allows executable payloads. The preserved value is still inert at sanitize time. The issue becomes relevant when the allowed custom element later re-injects that attribute value into an HTML sink such as innerHTML, creating a second-order XSS gadget.
Details
The issue appears to originate from the control flow in src/purify.ts: line 1672~1691
```tsx
const _sanitizeDisallowedNode = function (
currentNode: any,
tagName: string
): boolean {
/* Check if we have a custom element to handle */
if (!FORBID_TAGS[tagName] && _isBasicCustomElement(tagName)) {
if (
CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof RegExp &&
regExpTest(CUSTOM_ELEMENT_HANDLING.tagNameCheck, tagName)
) {
return false;
}
if (
CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof Function &&
CUSTOM_ELEMENT_HANDLING.tagNameCheck(tagName)
) {
return false;
}
}
```
CUSTOM_ELEMENT_HANDLING is parsed from user configuration at src/purify.ts: line 741~748
```tsx
const customElementHandling =
objectHasOwnProperty(cfg, 'CUSTOM_ELEMENT_HANDLING') &&
cfg.CUSTOM_ELEMENT_HANDLING &&
typeof cfg.CUSTOM_ELEMENT_HANDLING === 'object'
? clone(cfg.CUSTOM_ELEMENT_HANDLING)
: create(null);
CUSTOM_ELEMENT_HANDLING = create(null);
```
In particular, tagNameCheck, attributeNameCheck, and allowCustomizedBuiltInElements are copied into the internal CUSTOM_ELEMENT_HANDLING object there.
During element sanitization, _sanitizeElements() checks whether a node is forbidden or not allowlisted at src/purify.ts: line 1805~1814
```tsx
/* Remove element if anything forbids its presence */
if (
FORBID_TAGS[tagName] ||
(!(
EXTRA_ELEMENT_HANDLING.tagCheck instanceof Function &&
EXTRA_ELEMENT_HANDLING.tagCheck(tagName)
) &&
!ALLOWED_TAGS[tagName])
) {
return _sanitizeDisallowedNode(currentNode, tagName);
}
```
If so, it immediately delegates to _sanitizeDisallowedNode(currentNode, tagName) and returns its boolean result.
Inside _sanitizeDisallowedNode(), the custom-element-specific allow path is implemented at src/purify.ts: line 1672~1692
```tsx
const _sanitizeDisallowedNode = function (
currentNode: any,
tagName: string
): boolean {
/* Check if we have a custom element to handle */
if (!FORBID_TAGS[tagName] && _isBasicCustomElement(tagName)) {
if (
CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof RegExp &&
regExpTest(CUSTOM_ELEMENT_HANDLING.tagNameCheck, tagName)
) {
return false;
}
if (
CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof Function &&
CUSTOM_ELEMENT_HANDLING.tagNameCheck(tagName)
) {
return false;
}
}
```
If the node is treated as a basic custom element and CUSTOM_ELEMENT_HANDLING.tagNameCheck matches, the function returns false immediately at line 1682 or 1689, meaning “do not remove this node”.
That early return false is significant because control returns directly to _sanitizeElements() via the return _sanitizeDisallowedNode(...) at line 1813. As a result, the later logic in _sanitizeElements() is skipped for that custom element instance, including:
- the namespace validation at
src/purify.ts: line 1816~1826
```tsx
- Check whether element has a valid namespace.
Realm-safe check (GHSA-hpcv-96wg-7vj8): use the cached Node.prototype
nodeType getter rather than instanceof Element, which is realm-
bound and short-circuits to false for any node minted in a different
realm — letting a foreign-realm element with a forbidden namespace
slip past the namespace check entirely. */
const nt = getNodeType ? getNodeType(currentNode) : currentNode.nodeType;
if (nt === NODE_TYPE.element && !_checkValidNamespace(currentNode)) {
_forceRemove(currentNode);
return true;
}
```
- the fallback-tag mXSS check at
src/purify.ts: line 1828~1837
```tsx
/* Make sure that older browsers don't get fallback-tag mXSS */
if (
(tagName === 'noscript' ||
tagName === 'noembed' ||
tagName === 'noframes') &&
regExpTest(EXPRESSIONS.FALLBACK_
