CVE-2024-21641 is a medium-severity security vulnerability classified as CWE-601 (URL Redirection to Untrusted Site, commonly known as an 'Open Redirect') affecting the Flarum open-source discussion platform framework versions prior to 1.8.5. The vulnerability resides in the /logout route of Flarum, which accepts a redirect parameter. This parameter can be manipulated by an attacker to redirect users from a trusted Flarum domain to any arbitrary external URL. For logged-in users, logout requires confirmation before redirection, but guests are redirected immediately without confirmation. This behavior can be exploited by attackers, such as spammers or phishers, to leverage the trusted domain of a legitimate Flarum installation to redirect users to malicious websites. This can facilitate phishing attacks, social engineering, or distribution of malware by exploiting the trust users place in the original domain. The vulnerability does not directly compromise confidentiality or availability but impacts integrity by enabling attackers to manipulate user navigation flow. The issue has been fixed in Flarum core version 1.8.5. No known exploits are currently reported in the wild. Some third-party extensions that modify the logout route may mitigate the issue if implemented securely. The CVSS v3.1 base score is 6.5 (medium), reflecting network attack vector, low attack complexity, no privileges required, user interaction required, unchanged scope, no confidentiality impact, high integrity impact, and no availability impact.

For European organizations using Flarum as a discussion or community platform, this vulnerability could lead to reputational damage and user trust erosion if attackers exploit the open redirect to conduct phishing or malware distribution campaigns. Users redirected from a trusted domain may be more likely to fall victim to social engineering attacks. While the vulnerability does not allow direct data theft or system compromise, the indirect impact on user security and organizational credibility can be significant, especially for sectors handling sensitive discussions or customer interactions. Additionally, regulatory compliance under GDPR may be implicated if user data or trust is compromised through phishing attacks facilitated by this vulnerability. The impact is more pronounced for organizations with large user bases or those in regulated industries where user trust and data protection are critical.

European organizations should promptly upgrade all Flarum installations to version 1.8.5 or later to apply the official patch that removes the unsafe redirect parameter behavior. Until upgrading is possible, administrators should audit and, if necessary, disable or carefully review any third-party extensions that modify the logout route to ensure they do not introduce similar open redirect risks. Implementing strict validation or whitelisting of redirect URLs can prevent arbitrary redirection. Additionally, organizations should educate users about the risks of unexpected redirects and encourage vigilance against phishing attempts. Web application firewalls (WAFs) can be configured to detect and block suspicious redirect parameters targeting the logout route. Monitoring logs for unusual redirect patterns can help detect exploitation attempts early. Finally, organizations should review their incident response plans to address potential phishing campaigns leveraging this vulnerability.