Attack Chain

1. Attacker identifies a Craft CMS instance utilizing the vulnerable Formie plugin (versions >= 3.0.0-beta.1 and <= 3.1.26).
2. Attacker scans for public-facing forms on the target instance that contain a Hidden field configured to use a request-derived default value (e.g., HTTP User Agent, Referer URL, Query Parameter, or Cookie Value).
3. Attacker crafts a specially malformed HTTP request targeting the identified form.
4. The malicious request embeds Twig template syntax within the value of the request-derived field (e.g., User-Agent: {{_self.env.getconfig()}} or ?param={{7*7}}).
5. The Formie plugin, specifically within the Hidden::getFrontEndInputOptions() function, copies the attacker-controlled input directly into the defaultValue for the Hidden field.
6. During the subsequent front-end rendering process of the form, Craft CMS's Twig rendering engine evaluates the malicious defaultValue server-side, executing the injected Twig syntax.
7. Successful exploitation results in Server-Side Template Injection, potentially leading to remote code execution, disclosure of sensitive application or system information, or unauthorized modification of application state.

Impact

Successful exploitation of CVE-2026-52889 allows an unauthenticated attacker to trigger server-side template evaluation simply by visiting a crafted URL. Depending on the injected payload and the site's Craft CMS configuration, this can lead to severe consequences including arbitrary remote code execution on the server hosting the Craft CMS instance. Other potential impacts include the disclosure of sensitive application data, database credentials, or system configurations, and unauthorized modification of the application's state, potentially defacing websites or disrupting services. The unauthenticated nature of the vulnerability means any public-facing vulnerable form is at risk.

Recommendation

• Immediately update verbb/formie to version 3.1.27 or later to patch CVE-2026-52889.
• As a temporary workaround until patching, audit all public forms and remove or reconfigure any Hidden fields using request-derived default values, specifically: HTTP User Agent, HTTP Refer URL, Current URL, Current URL without Query String, Query Parameter, or Cookie Value, as noted in the summary.
• Deploy the provided Sigma rule "Detects CVE-2026-52889 Exploitation — Formie SSTI via Request-Derived Hidden Fields" to your SIEM/detection platform and tune it for your environment to identify attempted exploitation.