CVE-2026-21642 is a reflected Cross-Site Scripting (XSS) vulnerability identified in Revive Adserver version 6, a widely used open-source ad serving platform. The vulnerability exists in the banner-acl.php and channel-acl.php scripts, which handle access control lists for banners and channels. An attacker can craft a specially constructed URL embedding malicious HTML or JavaScript code within a parameter. When a logged-in administrator clicks or visits this URL, the server reflects the injected payload back in the HTTP response without proper sanitization or encoding, causing the administrator's browser to execute the malicious script. This can lead to session hijacking, credential theft, or unauthorized actions performed with administrator privileges. The vulnerability does not require the attacker to be authenticated but does require the administrator to interact with the malicious link, making social engineering a likely attack vector. The CVSS 3.0 score of 6.1 reflects a medium severity level, with network attack vector, low attack complexity, no privileges required, but user interaction necessary. The scope is changed (S:C), indicating that the vulnerability can affect resources beyond the initially vulnerable component. No patches or exploits are currently publicly available, but the risk remains significant due to the administrative access level targeted. The vulnerability is classified under CWE-79, which covers improper neutralization of input leading to XSS. Organizations using Revive Adserver 6 should be aware of this threat and implement mitigations promptly.

For European organizations, this vulnerability poses a risk primarily to the confidentiality and integrity of administrative sessions within Revive Adserver. Successful exploitation could allow attackers to hijack administrator sessions, steal credentials, or perform unauthorized administrative actions such as modifying ad campaigns or redirecting ad traffic. This can lead to financial loss, reputational damage, and potential data leakage. Since Revive Adserver is used by many digital marketing and advertising companies, media agencies, and publishers across Europe, the impact could disrupt advertising operations and compromise sensitive business data. The reflected XSS nature means the attack requires social engineering to trick administrators into clicking malicious links, but once successful, the attacker gains significant control. The vulnerability does not affect availability directly but can indirectly cause service disruption if administrative controls are compromised. Given the medium CVSS score and the administrative level targeted, the impact is moderate but non-negligible, especially for organizations heavily reliant on digital advertising infrastructure.

1. Immediately apply any available patches or updates from Revive Adserver addressing this vulnerability once released. 2. If patches are not yet available, implement web application firewall (WAF) rules to detect and block suspicious requests containing script tags or unusual parameters targeting banner-acl.php and channel-acl.php. 3. Educate administrators about the risk of clicking untrusted URLs, especially those received via email or messaging platforms, to reduce the likelihood of social engineering exploitation. 4. Enforce multi-factor authentication (MFA) for administrative accounts to limit the impact of stolen credentials. 5. Review and restrict administrative access to trusted networks or VPNs to reduce exposure. 6. Monitor logs for unusual access patterns or repeated requests to the vulnerable scripts. 7. Implement Content Security Policy (CSP) headers to limit the execution of unauthorized scripts in the browser. 8. Regularly audit and sanitize all user inputs and URL parameters in custom deployments or plugins related to Revive Adserver. These steps collectively reduce the risk of exploitation until a vendor patch is applied.