CVE-2021-25984 is a stored Cross-Site Scripting (XSS) vulnerability identified in the FactorJS Factor forum plugin, specifically affecting versions from v1.3.3 up to v1.8.30. This vulnerability resides in the 'post reply' section of the forum plugin, where user input is insufficiently sanitized or escaped before being stored and subsequently rendered in the web application. An unauthenticated attacker can exploit this flaw by submitting malicious JavaScript code as part of a forum reply. Because the malicious script is stored on the server and served to other users, it can execute in the context of their browsers when they view the affected forum threads. The primary impact of this vulnerability is the theft of session cookies, which can lead to session hijacking, allowing attackers to impersonate legitimate users. The CVSS 3.1 base score is 6.1 (medium severity), reflecting that the attack vector is network-based (AV:N), requires no privileges (PR:N), but does require user interaction (UI:R) such as viewing the malicious post. The scope is changed (S:C), indicating that the vulnerability affects resources beyond the initially vulnerable component. The impact on confidentiality and integrity is low (C:L, I:L), with no impact on availability (A:N). No known exploits in the wild have been reported, and no official patches are linked in the provided data, suggesting that organizations may need to verify if updates or mitigations have been released by FactorJS. This vulnerability is categorized under CWE-79, which is a common and well-understood class of web application security issues related to improper input validation and output encoding.

For European organizations using the FactorJS Factor forum plugin, this vulnerability poses a risk primarily to the confidentiality and integrity of user sessions. Exploitation could allow attackers to hijack user accounts, potentially gaining unauthorized access to sensitive information or administrative functions if the compromised accounts have elevated privileges. This can lead to data breaches, unauthorized content manipulation, or further exploitation within the affected web application. Given that the attack requires user interaction (viewing a malicious post), the impact is somewhat limited to active forum users, but the risk remains significant in environments where the forum is used for internal communication, customer support, or community engagement. The absence of authentication requirements for the attacker lowers the barrier to exploitation, increasing the threat surface. While availability is not impacted, the reputational damage and potential regulatory consequences under GDPR for data breaches involving session hijacking could be substantial for European entities. Organizations relying on this plugin should consider the sensitivity of data accessible through the forum and the privileges of forum users to assess their risk exposure.

1. Immediate mitigation should include disabling or restricting access to the vulnerable 'post reply' functionality until a patch or update is applied. 2. Implement robust input validation and output encoding on all user-supplied content in the forum plugin, specifically sanitizing HTML and JavaScript inputs to prevent script injection. 3. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers accessing the forum. 4. Enforce HttpOnly and Secure flags on session cookies to reduce the risk of cookie theft via XSS. 5. Monitor forum posts for suspicious content and implement automated scanning tools to detect potential XSS payloads. 6. Educate users to be cautious when interacting with forum content and report suspicious posts. 7. Regularly review and update the FactorJS Factor plugin to the latest secure versions once patches are available. 8. Consider deploying Web Application Firewalls (WAFs) with rules tailored to detect and block XSS attack patterns targeting the forum plugin. 9. Conduct security assessments and penetration testing focused on the forum component to identify any residual or related vulnerabilities.