Stored XSS in Relevanssi WordPress Search Plugin

The Relevanssi – A Better Search plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'comment' parameter in all versions up to, and including, 4.28.5 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. This requires the site to have both the comment indexing and comment highlighting options enabled, and the payload only fires when a visitor reaches a singular page via a Relevanssi search-result permalink carrying the highlight query parameter.

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

Relevanssi – A Better Search is a popular WordPress plugin that enhances site search functionality. Versions up to 4.28.5 contain a Stored Cross-Site Scripting (XSS) vulnerability in the comment parameter when both comment indexing and highlighting are enabled. Because this is a stored XSS vulnerability, malicious scripts are permanently injected into the WordPress database and execute for any user viewing the affected page—not just the attacker. This affects all WordPress sites using the vulnerable plugin versions, regardless of authentication status, making it particularly dangerous as unauthenticated attackers can compromise any visitor's session. The vulnerability stems from insufficient input sanitization and output escaping, allowing arbitrary JavaScript to persist and run in users' browsers.

Casky's 323 matching skills detect this vulnerability pattern by mapping to MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky would identify reconnaissance activities where attackers probe for comment indexing/highlighting configurations, followed by initial access attempts through comment parameter injection. Extended reasoning capabilities help security teams recognize the subtle difference between reflected and stored XSS patterns—in this case, the storage mechanism creates a persistent threat. Casky's skills would flag anomalous comment submissions containing script tags or event handlers, unusual database modifications in WordPress comment tables, and suspicious page rendering behavior. Practitioners would see detailed findings showing how user input flows through the plugin without sanitization checks, CWE-79 classification details, and specific exploitation chains that demonstrate the vulnerability's attack surface across both authenticated and unauthenticated attack vectors.

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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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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-2026-116522.

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

Investigate the attack patterns behind CVE-2026-116522

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