CVE-2026-21664 is a reflected Cross-Site Scripting (XSS) vulnerability identified in the afr.php delivery script of Revive Adserver version 6. The vulnerability arises because the script improperly sanitizes user-supplied input parameters, allowing an attacker to embed malicious HTML or JavaScript code within a crafted URL. When a logged-in administrator clicks this URL, the malicious payload is reflected in the HTTP response and executed in the administrator's browser context. This can lead to theft of session cookies, unauthorized actions performed with administrator privileges, or redirection to malicious sites. The vulnerability requires that the victim be authenticated as an administrator and interact by clicking the malicious link, which limits the attack surface but still poses a significant risk due to the high privileges of the targeted user. The CVSS 3.0 score of 6.1 reflects a medium severity, with network attack vector, low attack complexity, no privileges required, but user interaction necessary, and impacts on confidentiality and integrity but not availability. No patches or known exploits are currently reported, but the vulnerability is publicly disclosed and thus may be targeted in the future. The underlying issue corresponds to CWE-79, a common web application security weakness related to improper input validation and output encoding.

For European organizations, this vulnerability can lead to unauthorized access to administrative functions within Revive Adserver, potentially allowing attackers to manipulate ad campaigns, steal sensitive data, or pivot to other internal systems. Given that Revive Adserver is widely used by digital marketing agencies, media companies, and publishers across Europe to manage online advertising, exploitation could disrupt advertising operations and damage brand reputation. The confidentiality of administrator credentials and session tokens is at risk, which could lead to broader compromise if attackers leverage stolen credentials elsewhere. While availability is not directly impacted, the integrity of advertising content and reporting data could be compromised, affecting business decisions and revenue. Organizations with strict data protection regulations, such as GDPR, must consider the implications of such breaches on personal data processed via the platform. The requirement for administrator interaction reduces the likelihood of mass exploitation but targeted spear-phishing or social engineering attacks could be effective.

European organizations should immediately review their use of Revive Adserver version 6 and monitor for any vendor patches or updates addressing CVE-2026-21664. In the absence of an official patch, implement input validation and output encoding on the afr.php script parameters to neutralize malicious payloads. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in the browser context. Educate administrators on the risks of clicking unsolicited or suspicious URLs, especially those purporting to relate to adserver management. Employ multi-factor authentication (MFA) for administrator accounts to reduce the impact of session hijacking. Monitor web server logs for unusual URL access patterns that may indicate exploitation attempts. Consider isolating the adserver administration interface behind VPN or IP whitelisting to limit exposure. Regularly back up configuration and campaign data to enable recovery in case of compromise.