CVE-2026-21642 is a reflected Cross-Site Scripting (XSS) vulnerability identified in Revive Adserver version 6, specifically within the banner-acl.php and channel-acl.php scripts. The vulnerability arises because these scripts improperly handle 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 URL, the malicious payload is reflected back and executed in the administrator's browser context. This execution can lead to session hijacking, theft of authentication tokens, or unauthorized actions performed with administrative privileges. The attack vector is remote and requires no prior authentication, but successful exploitation depends on social engineering to convince an administrator to visit the malicious link. The CVSS 3.0 base score of 6.1 reflects the attack vector as network-based (AV:N), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). 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 have been reported in the wild as of the publication date. The vulnerability was responsibly disclosed by a HackerOne community member and is publicly documented in the CVE database. Given the nature of the vulnerability, it primarily threatens the security of administrative users and the integrity of the adserver management interface.

For European organizations using Revive Adserver version 6, this vulnerability poses a risk of administrative account compromise through session hijacking or unauthorized actions performed by an attacker exploiting the reflected XSS. This can lead to manipulation of advertising campaigns, unauthorized data access, or insertion of malicious content into ad delivery. The impact on confidentiality and integrity is moderate, as attackers can potentially steal credentials or alter configurations. However, availability is not affected. The requirement for an administrator to interact with a malicious link reduces the likelihood of widespread exploitation but does not eliminate risk, especially in environments with less stringent user awareness or phishing defenses. Organizations heavily reliant on digital advertising infrastructure may face reputational damage and financial loss if attackers manipulate ad content or redirect traffic. Additionally, compromised administrative accounts could serve as a foothold for further network intrusion. The threat is more pronounced in sectors with high-value advertising operations or where Revive Adserver is integrated with other critical systems.

To mitigate CVE-2026-21642, organizations should immediately update Revive Adserver to a patched version once available from the vendor. In the absence of an official patch, administrators should implement input validation and output encoding on the affected scripts to neutralize malicious payloads. Employ Content Security Policy (CSP) headers to restrict script execution origins and reduce the impact of XSS attacks. Educate administrators on phishing risks and the dangers of clicking untrusted links, especially those targeting management interfaces. Restrict access to the administrative interface by IP whitelisting or VPN-only access to reduce exposure. Monitor web server logs for suspicious URL patterns targeting banner-acl.php and channel-acl.php. Implement multi-factor authentication (MFA) for administrative accounts to limit the impact of credential theft. Regularly audit and review adserver configurations and user activities to detect anomalies. Finally, consider deploying web application firewalls (WAFs) with rules to detect and block reflected XSS attempts targeting known vulnerable endpoints.