CVE-2020-5809: 
C# vulnerability analysis and mitigation

A stored Cross-Site Scripting (XSS) vulnerability exists in Umbraco CMS versions 8.9.1 and earlier, identified as CVE-2020-5809. The vulnerability allows authenticated users to inject arbitrary JavaScript code into iframes when editing content using the TinyMCE rich-text editor, as TinyMCE is configured to allow iframes by default in Umbraco CMS. The vulnerability was discovered in September 2020 and publicly disclosed on December 30, 2020 (Tenable Research).

The vulnerability stems from a misconfiguration in the TinyMCE rich-text editor within Umbraco CMS that allows iframes by default. This configuration enables authenticated users to inject malicious JavaScript code through iframe elements when editing content. The vulnerability has been assigned a CVSS v3.1 Base Score of 5.4 (Medium) with the vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N, indicating network accessibility with low attack complexity but requiring user interaction (NVD).

If exploited, this vulnerability could allow attackers to execute arbitrary JavaScript code in the context of other users' browsers who view the compromised content. In a worst-case scenario, if triggered by an admin, the attacker could potentially escalate privileges to gain administrative access and subsequently install malicious Umbraco packages, potentially leading to remote code execution (Tenable Research).

The vulnerability requires authentication to exploit, as the attacker must have access to content editing capabilities within the Umbraco CMS. A proof of concept exists demonstrating how an attacker could exploit this vulnerability to escalate privileges by adding their user ID to the admin group when the payload is triggered by an administrator (Tenable Research).

Administrators can mitigate this vulnerability by removing iframe[*] from validElements in the tinyMceConfig.config file. This configuration change prevents the injection of iframe elements through the TinyMCE editor, effectively blocking the attack vector (Tenable Research).