SunEditor: Critical XSS vulnerability - sanitizer bypass

Summary

SUNEDITOR v2.47.10 appears to allow JavaScript execution through crafted namespaced HTML elements.

The sanitization logic does not fully remove executable event-handler attributes from certain custom/namespaced tags. As a result, an attacker may be able to inject HTML content that executes JavaScript when the rendered element is interacted with.

This behavior was observed after the changes introduced in this commit:

https://github.com/JiHong88/suneditor/commit/9ed405fb0de676e56cd0e6a13c19c103ad5948d3

Affected Version

Confirmed vulnerable:

• SUNEDITOR =< 2.47.10

Potentially affected:

• Previous versions, if the same sanitization logic is still present

Proof of Concept

The following payload preserves an executable event handler:

<a:b src="/x"><iframe src=javascript:alert(1)></iframe></a:b>

<p><a:b src="/x" onclick="document.body.style.background='red'">click</a:b></p>

Simplified PoC:

<a:b src="/x" onclick="console.log('XSS:',document.domain,document.cookie)">click</a:b>

<p><a:b src="/x" onmouseover="alert('XSS — '+document.domain)">📎 Click here for solutions</a:b></p>

Steps to Reproduce

1. Open SUNEDITOR using version 2.47.10.
2. Insert the payload above into the editor.
3. Save or render the generated content.
4. Click the generated element.
5. Observe that JavaScript is executed.

Impact

This may allow an attacker to inject arbitrary JavaScript into rendered editor content.

Depending on how SUNEDITOR is integrated into an application, this could lead to:

• Stored XSS
• DOM manipulation
• Session theft
• Credential theft
• Account takeover actions performed in the victim’s browser context

Technical Details

The issue appears to be related to incomplete handling of namespaced/custom HTML elements such as:

<a:b>

Event-handler attributes such as onclick can remain attached to these crafted elements and execute when interacted with.

This suggests that the sanitization process may not consistently normalize and validate custom or namespaced elements before applying attribute filtering.

Suggested Remediation

Possible mitigations include:

• Normalize DOM elements before sanitization.
• Explicitly reject or unwrap unknown/custom/namespaced tags.
• Strip all event-handler attributes from all elements, including unknown/custom elements.
• Re-validate sanitized output after browser DOM parsing.
• Add regression tests for namespaced/custom tag payloads.