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FF139 ExprFeat: Escape < and > in attributes when serialize HTML #39363

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hamishwillee merged 2 commits into from
May 5, 2025
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FF139 ExprFeat: Escape < and > in attributes when serialize HTML #39363

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FF139 ExprFeat: Escape < and > in attributes when serialize HTML
hamishwillee May 2, 2025
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hamishwillee May 5, 2025
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43 changes: 43 additions & 0 deletions files/en-us/mozilla/firefox/experimental_features/index.md
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Expand Up @@ -1108,6 +1108,49 @@ The [HTML Sanitizer API](/en-US/docs/Web/API/HTML_Sanitizer_API) allow developer
</tbody>
</table>

### Escape < and > in attributes when serializing HTML

Firefox replaces the `<` and `>` characters with `&lt;` and `&gt;` (respectively) in attributes when serializing HTML.
This prevents certain exploits where HTML is serialized and then injected back into the DOM.
The affected methods and properties are: {{domxref("Element.innerHTML")}}, {{domxref("Element.outerHTML")}}, {{domxref("Element.getHTML()")}}, {{domxref("ShadowRoot.innerHTML")}}, and {{domxref("ShadowRoot.getHTML()")}}.
([Firefox bug 1941347](https://bugzil.la/1941347)).

<table>
<thead>
<tr>
<th>Release channel</th>
<th>Version added</th>
<th>Enabled by default?</th>
</tr>
</thead>
<tbody>
<tr>
<th>Nightly</th>
<td>139</td>
<td>Yes</td>
</tr>
<tr>
<th>Developer Edition</th>
<td>139</td>
<td>No</td>
</tr>
<tr>
<th>Beta</th>
<td>139</td>
<td>No</td>
</tr>
<tr>
<th>Release</th>
<td>139</td>
<td>No</td>
</tr>
<tr>
<th>Preference name</th>
<td colspan="2"><code>dom.security.html_serialization_escape_lt_gt</code></td>
</tr>
</tbody>
</table>

### Removal of MutationEvent

{{domxref("MutationEvent")}} and its associated events (`DOMSubtreeModified`, `DOMNodeInserted`, `DOMNodeRemoved`, `DOMCharacterDataModified`, `DOMAttrModified`) are on the path for removal, and have been disabled on nightly.
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