CVE-2026-4257
Published: 30 March 2026
Summary
CVE-2026-4257 is a critical-severity Code Injection (CWE-94) vulnerability in Wordpress (inferred from references). Its CVSS base score is 9.8 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 1.5% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and AC-6 (Least Privilege).
Deeper analysis
The Contact Form by Supsystic plugin for WordPress is affected by a Server-Side Template Injection vulnerability that leads to remote code execution in all versions through 1.7.36. The root cause is the plugin's use of the Twig Twig_Loader_String engine without sandboxing, combined with the cfsPreFill prefill feature that accepts arbitrary Twig expressions in form field values supplied through unauthenticated GET requests. This allows an attacker to invoke Twig's registerUndefinedFilterCallback method and register arbitrary PHP callbacks, resulting in execution of PHP functions and operating-system commands.
Unauthenticated remote attackers can exploit the flaw over the network by crafting malicious GET parameters that reach the forms.php handling code. Successful exploitation grants full control over the web server, including the ability to read or modify files, execute commands, and pivot within the hosting environment. The vulnerability carries a CVSS 3.1 score of 9.8.
Public references include a Wordfence threat-intelligence entry and a WordPress plugin changeset that addresses the issue, indicating that an update beyond version 1.7.36 resolves the exposure. The current EPSS of 0.8693 matches its recorded peak, reflecting sustained exploitation interest since disclosure.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-17239
Vulnerability details
The Contact Form by Supsystic plugin for WordPress is vulnerable to Server-Side Template Injection (SSTI) leading to Remote Code Execution (RCE) in all versions up to, and including, 1.7.36. This is due to the plugin using the Twig `Twig_Loader_String` template…
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engine without sandboxing, combined with the `cfsPreFill` prefill functionality that allows unauthenticated users to inject arbitrary Twig expressions into form field values via GET parameters. This makes it possible for unauthenticated attackers to execute arbitrary PHP functions and OS commands on the server by leveraging Twig's `registerUndefinedFilterCallback()` method to register arbitrary PHP callbacks.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
SSTI vulnerability in public-facing WordPress plugin enables unauthenticated RCE via template injection, directly mapping to Exploit Public-Facing Application (T1190) and Template Injection (T1221).
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Rejects or sanitizes unauthenticated GET parameters containing Twig expressions before they reach the cfsPreFill handler and Twig_Loader_String engine.
Ensures the web-server process and plugin execute with only the privileges required for form handling, limiting the scope of any RCE obtained via registerUndefinedFilterCallback.
Monitors for anomalous process execution, file writes, or outbound commands that would result from successful Twig-based PHP callback registration.