Stored XSS in Contact Form 7 File Upload Extension

The Ultra Addons for Contact Form 7 plugin for WordPress is vulnerable to Stored Cross-Site Scripting via File Upload in all versions up to, and including, 3.5.54 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 extension blocklist gatekeeper is bypassed by uploading text-type files (.txt, .csv, .vtt, .srt, .asc) whose contents begin with an HTML payload such as '<BODY onload=…>', as the check classifies these files as text/plain without inspecting their byte content.

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

Ultra Addons for Contact Form 7 is a popular WordPress plugin that extends contact form functionality, used by thousands of websites to collect user submissions. This vulnerability allows unauthenticated attackers to upload seemingly innocent text files (.txt, .csv, .vtt, .srt, .asc) containing malicious JavaScript that persists in the database as stored XSS. The plugin's file type blocklist can be circumvented by exploiting insufficient input sanitization, meaning attackers don't need admin access—they can inject payloads through public contact forms. Every visitor to a page containing the injected content will automatically execute the attacker's script, creating a persistent threat vector for credential theft, session hijacking, or malware distribution across an entire WordPress installation.

Casky's 323 matching skills leverage Claude's extended reasoning to identify this attack pattern across MITRE techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky will see detection patterns flagging suspicious file uploads that bypass extension validation, anomalous script injection in contact form submissions, and stored payloads in database queries. The platform's skills map directly to CWE-79 detection logic, highlighting cases where user-controlled file contents are rendered without proper HTML entity encoding. Security teams can correlate findings showing file uploads with executable content followed by DOM-based script execution patterns, revealing the complete attack chain from initial compromise through persistent payload storage—enabling faster incident response and remediation of compromised contact forms before widespread user exposure occurs.

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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-2026-103999.

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-2026-103999

Casky has 323 skills that investigate the attack patterns behind CVE-2026-103999. Run one and get CVSS-scored findings in 3 minutes.

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