WordPress QuickCal Plugin Stored XSS via Unvalidated Custom Fields

The QuickCal plugin for WordPress is vulnerable to Stored Cross-Site Scripting via Custom Field Parameters in all versions up to, and including, 1.0.20 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The nonce guarding the unauthenticated booked_add_appt AJAX action is publicly embedded on any page rendering the booking calendar shortcode, making it trivially obtainable by unauthenticated attackers without any prior account or privilege.

Casky was already ahead

This CVE exploits attack patterns that Casky's 323matched 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

The QuickCal WordPress plugin contains a stored cross-site scripting (XSS) vulnerability affecting all versions through 1.0.20. Attackers can inject malicious scripts through custom field parameters in the booking calendar, which are then permanently stored and executed when legitimate users access the affected pages. This vulnerability is particularly dangerous because it requires no authentication—any visitor can exploit it—and affects any WordPress site running the vulnerable plugin. The persistent nature of stored XSS means the attack payload remains active indefinitely until manually removed, potentially compromising user sessions, stealing credentials, or redirecting visitors to malicious sites.

Casky's 323 matching skills help practitioners detect the attack patterns associated with TA0001 (Initial Access) and TA0043 (Reconnaissance) techniques by identifying insufficient input sanitization in form handlers and output escaping failures in rendered content. When analyzing the QuickCal plugin, Casky's Claude AI-powered reasoning identifies suspicious patterns: unvalidated user input flowing directly into database storage, absence of nonce validation on critical AJAX actions (like booked_add_appt), and script tags or event handlers appearing in saved custom field data. Practitioners would see findings flagging the publicly-embedded nonce weakness, detecting injection vectors in calendar rendering functions, and identifying stored payloads that execute in page context—enabling teams to prioritize patching before exploitation occurs at scale.

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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 →

323 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-15984.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

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