CVE-2026-21664 is a reflected cross-site scripting (XSS) vulnerability identified in the afr.php delivery script of Revive Adserver version 6, an open-source ad serving platform widely used for managing and delivering online advertisements. The vulnerability arises because the afr.php script improperly sanitizes user-supplied input parameters, allowing an attacker to inject malicious HTML or JavaScript code into a crafted URL. When a logged-in administrator clicks or visits this malicious URL, the injected script executes within the context of the administrator's browser session. This can lead to session hijacking, unauthorized actions performed with administrator privileges, or theft of sensitive information such as authentication tokens. The vulnerability requires no prior authentication by the attacker but does require that the victim administrator interacts with the malicious URL, making social engineering or phishing a likely attack vector. The CVSS 3.0 base score of 6.1 reflects a medium severity, with network attack vector, low attack complexity, no privileges required, but user interaction needed, and a scope change due to potential impact on administrative privileges. Currently, there are no publicly known exploits in the wild, and no official patches have been linked yet. However, the presence of this vulnerability in a critical administrative component of Revive Adserver necessitates prompt attention from users of version 6 to prevent exploitation.

For European organizations using Revive Adserver version 6, this vulnerability poses a significant risk to the confidentiality and integrity of their advertising infrastructure. Successful exploitation could allow attackers to hijack administrator sessions, manipulate ad delivery configurations, or inject malicious content into ad campaigns, potentially damaging brand reputation and causing financial loss. Since the vulnerability targets administrators, it could lead to broader compromise of the ad server environment, affecting availability indirectly through administrative disruption. The reflected XSS nature means that exploitation requires tricking administrators into clicking malicious links, which could be facilitated by phishing campaigns targeting marketing or IT staff. Given the widespread use of Revive Adserver in digital marketing across Europe, especially among small to medium enterprises and agencies relying on open-source solutions, the impact could be widespread. Additionally, compromised ad servers could be used to distribute malware or conduct further attacks on end users, amplifying the threat. The medium CVSS score indicates moderate risk, but the potential for privilege escalation and session hijacking elevates the concern for organizations with high-value advertising assets or sensitive customer data.

Organizations should immediately audit their use of Revive Adserver version 6 and monitor for any suspicious administrator activity or phishing attempts targeting their staff. Although no official patch links are provided yet, users should apply any forthcoming security updates from the Revive project as a priority. In the interim, administrators should be trained to recognize and avoid clicking on suspicious URLs, especially those received via email or messaging platforms. Implementing web application firewalls (WAFs) with rules to detect and block reflected XSS payloads targeting afr.php can provide a protective layer. Additionally, enforcing multi-factor authentication (MFA) for administrator accounts can reduce the risk of session hijacking leading to full compromise. Organizations should also consider isolating administrative interfaces behind VPNs or IP whitelisting to limit exposure. Regularly reviewing server logs for unusual URL access patterns and conducting security awareness training focused on phishing can further reduce exploitation likelihood. Finally, organizations should prepare incident response plans specific to web application attacks to quickly contain any breach resulting from this vulnerability.