Invisible Anti-Spam & CAPTCHA <= 5.1 - Unauthenticated Stored Cross-Site Scripting via 'action' Parameter
Strategic Overview
- Status
- Patched in 5.1.1
- Affected Version
<= 5.1- CVSS
- 7.2High
- Weakness type
- CWE-79 · Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
- CVE
CVE-2026-16145
At a glance
CVE-2026-16145 is a high-severity Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') vulnerability in the Invisible Anti-Spam & CAPTCHA WordPress plugin, affecting versions <= 5.1. It carries a CVSS score of 7.2 (reachable over the network; low attack complexity). Exploitation requires no authentication. The issue is fixed in version 5.1.1; sites on affected versions should update now. Disclosed August 2026, reported by Ivaylo.
Vulnerability Overview
The Invisible Anti-Spam & CAPTCHA — reCAPTCHA Alternative for All Forms plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'action' parameter in all versions up to, and including, 5.1 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 stored payload is written by any unauthenticated admin-ajax.php request whose action value matches an entry in the plugin's explicit-actions list, which is auto-populated for common form builders at activation and requires no authentication gate to reach the save path.
Technical Analysis
The vector marks this flaw as remotely reachable over the network, with low attack complexity — no special timing or configuration is needed, and no privileges on the target site, and no interaction from a victim user.
CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Invisible Anti-Spam & CAPTCHA <= 5.1 carries this weakness at action, and reaching it takes no account at all. Cross-site scripting happens when input from a request is written into a page without being escaped for the context it lands in, so the browser parses attacker-supplied text as markup or script.
Injected script runs with the privileges of whoever views the affected page, which is how these flaws turn into administrator session theft, silent account creation or persistent backdoors in page content. For Invisible Anti-Spam & CAPTCHA the fix is 5.1.1: builds <= 5.1 are affected, anything from 5.1.1 onward is not.
Remediation
Update to version 5.1.1, or a newer patched version
How does WordSec protect against this?
Requests carrying this pattern arrive without a session, so nothing upstream of the firewall gets a chance to reject them: WordSec's web application firewall inspects request payloads before WordPress loads them. None of that substitutes for the fix: Invisible Anti-Spam & CAPTCHA 5.1.1 closes this, and updating the plugin is the step that ends it.
- Firewall
- Login Security
- Alerts
External References
Related records
Other vulnerabilities in Invisible Anti-Spam & CAPTCHA — reCAPTCHA Alternative for All Forms
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