CVE-2026-26861 is a Cross-Site Scripting (XSS) vulnerability identified in the CleverTap Web SDK, specifically in versions 1.15.2 and earlier. The vulnerability is located in the handleCustomHtmlPreviewPostMessageEvent function within the src/util/campaignRender/nativeDisplay.js file. This function is responsible for handling messages sent via the window.postMessage API, which is commonly used for cross-origin communication between windows or iframes. The root cause of the vulnerability is insufficient origin validation: the function uses the JavaScript includes() method to verify the origin of incoming messages. This method is inadequate because it can be bypassed by an attacker who controls a subdomain of a trusted domain, allowing malicious messages to be accepted as legitimate. By exploiting this flaw, an attacker can inject and execute arbitrary JavaScript code within the context of the vulnerable web application. This can lead to theft of sensitive information such as cookies or session tokens, manipulation of the DOM, or redirection to malicious sites. The vulnerability does not require user interaction to be exploited once the attacker can deliver a crafted postMessage event. Although no known exploits have been reported in the wild, the vulnerability is publicly disclosed and should be treated as a significant risk. No official patch or CVSS score has been published as of the date of this report, but the nature of the vulnerability and its potential impact warrant urgent attention from developers and security teams using the affected SDK.

The impact of CVE-2026-26861 on organizations worldwide can be substantial. Successful exploitation allows attackers to execute arbitrary scripts in the security context of the affected web application, potentially leading to the compromise of user credentials, session hijacking, unauthorized actions on behalf of users, and data exfiltration. This undermines the confidentiality and integrity of user data and can damage the reputation and trustworthiness of affected organizations. Additionally, attackers could use the vulnerability to deliver further malware or conduct phishing attacks by manipulating the web application's content. Since CleverTap is a widely used customer engagement and analytics platform integrated into many web applications, the scope of affected systems can be broad, impacting industries such as e-commerce, finance, media, and technology. The vulnerability could also be leveraged as a foothold for more advanced attacks within enterprise environments. The absence of a patch increases the risk window, and organizations that do not implement mitigations remain exposed. Overall, the vulnerability poses a high risk to the security posture of organizations relying on the CleverTap Web SDK.

To mitigate CVE-2026-26861, organizations should first verify if their web applications use CleverTap Web SDK version 1.15.2 or earlier. If so, they should seek an updated SDK version from CleverTap that addresses the origin validation flaw or apply vendor-provided patches once available. In the absence of an official patch, developers should implement strict origin validation by replacing the includes() method with exact origin matching, such as using strict equality checks against a whitelist of trusted origins. Additionally, web applications should employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. Developers can also sanitize and validate all inputs and messages received via postMessage to ensure they conform to expected formats. Monitoring and logging postMessage events can help detect suspicious activity. Finally, educating development teams about secure handling of cross-origin communications and conducting regular security code reviews focused on message handling functions will help prevent similar vulnerabilities.