CVE-2026-77115 in Popup Builder Plugin
Summary
by MITRE • 08/23/2026
Brave Popup Builder (brave-popup-builder) up to version 0.8.5 reflects UTM query parameters into popup form HTML without escaping them.
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in Brave Popup Builder versions prior to 0.8.5 constitutes a classic Cross-Site Scripting flaw rooted in the improper handling of user-supplied input within server-side rendering processes. Specifically, the application accepts Uniform Resource Locator query parameters from the HTTP request and directly injects these values into the HTML structure of generated popups without applying any form of encoding or sanitization. This architectural oversight allows an attacker to craft a malicious URL containing embedded JavaScript code as part of the query string. When a victim user visits this crafted link, the vulnerable application retrieves the unescaped parameter value and renders it directly within the popup's DOM tree. Consequently, the browser interprets the injected script as legitimate content belonging to the site, executing it in the context of the Brave Popup Builder domain rather than an external origin.
From a technical perspective, this flaw aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-Site Scripting or XSS. The root cause lies in the failure to implement output encoding mechanisms such as HTML entity encoding before inserting dynamic data into the markup structure. By reflecting UTM parameters directly into the form HTML, the application violates fundamental security principles regarding input validation and output sanitization. This type of reflected XSS is particularly dangerous because it does not require persistent storage on the server; instead, exploitation relies entirely on social engineering or link manipulation to trigger the execution payload in the victim's browser session.
The operational impact of this vulnerability extends beyond simple script execution. An attacker leveraging this flaw can perform a variety of malicious actions depending on the capabilities granted by the popup context. These include stealing sensitive cookies and session tokens, thereby facilitating account takeover attacks if authentication data is accessible within that domain. Furthermore, the attacker could manipulate the DOM to phish for credentials or other confidential information presented in the form fields, redirect users to phishing sites, or deface the application interface. In environments where Brave Popup Builder manages critical user interactions or collects personal data, this vulnerability poses a significant risk to both end-user privacy and organizational reputation due to potential data breaches and loss of trust.
To mitigate this vulnerability, developers must implement strict input validation and output encoding strategies immediately upon upgrading past version 0.8.5. The primary remediation involves ensuring that all dynamic content inserted into HTML is properly encoded using context-aware methods such as HTML entity encoding for text nodes or attribute values. Additionally, implementing Content Security Policy headers can provide an additional layer of defense by restricting the sources from which scripts are allowed to execute, thereby neutralizing many XSS payloads even if they manage to bypass input filters. Regular security audits and static code analysis tools configured to detect unescaped output patterns should also be employed during the development lifecycle to prevent similar issues in future releases.