DOM-based XSS in SunEditor Embed Plugin

A DOM-based Cross-Site Scripting (XSS) vulnerability (CVE-2026-54606) affects the SunEditor Embed plugin in versions 3.1.3 and earlier. The vulnerability arises from improper handling of user-supplied HTML within the Embed plugin. When the plugin processes raw embed HTML, it parses the content and identifies DOM nodes. Specifically, the plugin logic fails to adequately validate or discard sibling <script> elements that follow an <iframe>. Instead, it explicitly recreates these script elements using an attacker-controlled src attribute and appends them to the live DOM. This process effectively bypasses existing sanitization mechanisms, causing the browser to execute the referenced external JavaScript in the security context of the editor page. This issue is particularly dangerous in applications where SunEditor content is persisted in a backend and later rendered for other users or administrators, facilitating stored XSS attacks.

Attack Chain

1. The attacker prepares an external malicious JavaScript file and hosts it on a server under their control.
2. The attacker gains access to the SunEditor interface, either as a legitimate user or through an application-level injection vector.
3. The attacker utilizes the SunEditor Embed modal to submit a crafted HTML payload containing a legitimate iframe followed by the malicious script tag.
4. The application saves the malicious payload into the backend database.
5. A victim, such as an administrator, opens the SunEditor instance to edit or preview the stored content.
6. The Embed plugin parses the stored content, extracts the script element, and recreates the node with the attacker's src.
7. The plugin appends the malicious script node to the document body.
8. The victim's browser executes the external JavaScript, resulting in session hijacking, data exfiltration, or UI manipulation.

Impact

Successful exploitation allows for arbitrary JavaScript execution within the context of the user's session. This could lead to account takeover, unauthorized actions performed as the victim, theft of session tokens or sensitive data, and the modification of editor content. The impact is elevated in multi-user environments where content is shared or reviewed by privileged users.

Recommendation

1. Upgrade SunEditor to a patched version that resolves CVE-2026-54606.
2. If patching is not immediately feasible, implement the suggested fix to explicitly discard script elements during the parsing phase of the Embed plugin.
3. Implement strict Content Security Policy (CSP) headers on the host application to prevent the execution of scripts from unauthorized or untrusted domains.
4. Sanitize all stored content on the backend before rendering it in the browser to prevent stored XSS.