Unescaped UTM Parameters Enable Stored XSS in Popup Builder

Brave Popup Builder (brave-popup-builder) up to version 0.8.5 reflects UTM query parameters into popup form HTML without escaping them.

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

Brave Popup Builder versions up to 0.8.5 contain a Stored Cross-Site Scripting (XSS) vulnerability through improper handling of UTM query parameters. The application reflects these parameters directly into popup form HTML without sanitization or encoding, allowing attackers to inject malicious JavaScript code. This affects any website using the vulnerable plugin versions, potentially compromising visitor sessions, stealing credentials, or redirecting users to malicious sites. The vulnerability has a CVSS score of 7.1, indicating high severity, and falls under CWE-79 (Improper Neutralization of Input During Web Page Generation).

Casky's 322 matched skills help practitioners detect the attack patterns behind this vulnerability by mapping to MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). When Claude AI analyzes security findings with extended reasoning, practitioners will identify suspicious patterns such as: UTM parameter injection attempts in web logs, unusual HTML entities or script tags within query strings, DOM-based XSS payloads in referrer data, and anomalous popup initialization patterns. The platform enables detection of reconnaissance activities where attackers probe for vulnerable Brave Popup Builder instances, followed by initial access attempts injecting crafted UTM parameters designed to execute in victim browsers. Practitioners reviewing Casky findings would see flagged instances where attacker-controlled data flows directly into HTML rendering without proper output encoding—the hallmark of this stored XSS pattern.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-77115.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

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abusing-dpapi-for-credential-access

red teaming · high

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abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-77115

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