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The vulnerability exploits a legacy feature in DNN that allows loading a specific theme via query parameters like “&theme=MyVulnerableTheme”. This functionality was intended for design previews. An attacker can craft a malicious URL specifying the path to an unused, outdated theme installed on the server. When a user visits this URL, the DNN server processes and serves the page using the specified theme’s files (e.g., .ascx controls), even if it is not active. If the loaded theme contains a separate, unpatched vulnerability—such as a server-side code injection flaw—the attacker can trigger it. This effectively bypasses the site administrator’s security posture, as an outdated, vulnerable theme that is installed but not actively used becomes a weaponized entry point, potentially leading to remote code execution.
Platform: DNN (DotNetNuke)
Version: < 10.1.0
Vulnerability: Arbitrary Theme Load
Severity: Critical
date: 2023-07-20
Prediction: Patch expected 2023-07-27
What Undercode Say:
`curl -s “http://vuln-dnn-site.com/Default.aspx?theme=../../App_Data/old_theme” | grep -i “execute”`
`find /var/www/dnn/ -name “.ascx” -exec grep -l “CodeBehind” {} \;`
How Exploit:
Attacker identifies an unused, vulnerable theme on the target DNN instance. A malicious URL is crafted with the “theme” parameter pointing to the vulnerable theme’s directory. The link is sent to users. Upon visit, the server loads the malicious theme, executing its flawed code and granting attacker control.
Protection from this CVE:
Upgrade to DNN 10.1.0 or later, where the feature is disabled by default. If using an older version, apply available patches. In the Security module, disable the “Allow Arbitrary Theme Load” setting. Regularly audit and remove unused themes/skins from the installation.
Impact:
Remote Code Execution, Privilege Escalation, Information Disclosure by exploiting vulnerabilities in unused, outdated themes.
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Sources:
Reported By: github.com
Extra Source Hub:
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