CVE-2022-39270 is a medium-severity cross-site scripting (XSS) vulnerability affecting the DiscoTOC theme component of Discourse, an open-source discussion platform widely used for online forums and communities. DiscoTOC generates a table of contents for topics within Discourse forums. The vulnerability arises due to improper neutralization of user input during web page generation (CWE-79), allowing users with the ability to create topics in categories where DiscoTOC is enabled—and who meet a configured trust level—to inject arbitrary HTML content into the topic page. This injection can lead to the execution of malicious scripts in the context of other users viewing the topic, potentially compromising session tokens, user credentials, or enabling further attacks such as phishing or defacement. The issue affects DiscoTOC versions prior to 2.1.0 and has been fixed in the main branch. Administrators can remediate the vulnerability by updating the DiscoTOC component via the Discourse admin UI or temporarily disabling the component. No known exploits have been reported in the wild, but the vulnerability's presence in a widely deployed forum component means it could be leveraged by attackers targeting communities that rely on Discourse for communication and collaboration.

For European organizations, the impact of this vulnerability depends on their use of Discourse forums with the DiscoTOC component enabled. Organizations using Discourse for internal collaboration, customer support, or public community engagement could face risks including unauthorized script execution leading to session hijacking, data theft, or defacement of forum content. This could result in reputational damage, loss of user trust, and potential data breaches involving user information. Given that the vulnerability requires users to have topic creation privileges and a certain trust level, the risk is somewhat mitigated by access controls, but insider threats or compromised accounts could still exploit it. Additionally, the vulnerability could be used as a foothold for further attacks within the organization’s network if forum users have elevated privileges or reuse credentials. The absence of known exploits reduces immediate risk, but the potential for targeted attacks remains, especially against organizations with active online communities or public-facing forums.

European organizations should take the following specific steps to mitigate this vulnerability: 1) Immediately verify if DiscoTOC is in use within their Discourse installations and identify the version. 2) Update the DiscoTOC component to version 2.1.0 or later via the Discourse admin UI (Customize -> Themes -> Components -> DiscoTOC -> Check for Updates) to apply the official fix. 3) If immediate updating is not feasible, temporarily disable the DiscoTOC component to prevent exploitation. 4) Review and tighten category permissions and trust level configurations to restrict topic creation privileges to trusted users only, minimizing the attack surface. 5) Monitor forum activity for unusual topic creation or content injection attempts. 6) Educate forum moderators and administrators on recognizing signs of XSS exploitation and maintaining secure configurations. 7) Consider implementing Content Security Policy (CSP) headers on the Discourse server to limit the impact of potential XSS attacks by restricting script execution sources. These measures go beyond generic patching by emphasizing access control and monitoring tailored to the Discourse environment.