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Unlock with Pro - from £29/user/moDetect Formie Hidden Field Default Server-Side Template Injection (CVE-2026-52889) in Sumo Logic CSE
Detects exploitation attempts against CVE-2026-52889, a Server-Side Template Injection (SSTI, CWE-1336) vulnerability in the Formie plugin (verbb/formie) for Craft CMS. The vulnerability exists in how Formie processes 'default value' expressions for Hidden form fields, allowing an unauthenticated or low-privileged attacker to inject Twig template syntax that is evaluated server-side, potentially leading to remote code execution. Affects verbb/formie < 3.1.27. Detection focuses on Twig/Craft template injection payload patterns in form submission requests, anomalous POST bodies to Formie submission endpoints, and resulting web shell / RCE indicators (process spawning from PHP-FPM/Craft, outbound connections, file writes to web root).
MITRE ATT&CK
- Tactic
- Initial Access Execution
Sumo Detection Query
_sourceCategory="web/access"
| where method matches "POST"
| where uri matches "*actions/formie*" or uri matches "*formie*"
| parse regex "(?<decoded_query>.*)"
| where decoded_query matches "*{{*" or decoded_query matches "*_self.env*" or decoded_query matches "*system(*" or decoded_query matches "*proc_open(*" or decoded_query matches "*getFilter*"
| count by src_ip, uri
| where _count >= 1Sumo Logic search over web access logs identifying POST requests to Formie form submission paths containing Twig/PHP SSTI markers for CVE-2026-52889.
Data Sources
Required Tables
False Positives & Tuning
- Synthetic monitoring bots submitting templated test payloads to uptime-check forms
- Marketing automation tools posting merge-tag-like syntax (e.g. double braces) to embedded forms
- Security researchers submitting responsible-disclosure test payloads to a bug bounty scope
Other platforms for CVE-2026-52889
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Formie Hidden Field SSTI Payload Submission
Expected signal: Web server access log entry recording a POST to /actions/formie/submissions/submit containing the Twig SSTI payload string in the request body.
- Test 2Formie SSTI to Command Execution Simulation
Expected signal: EDR/auditd process creation events showing 'bash' spawned as a child of a process running under the www-data (or equivalent web server) user, executing 'id', 'whoami', and 'uname -a'.
- Test 3Formie Malicious Webshell Drop Simulation
Expected signal: File integrity monitoring / EDR file-creation event for a new .php file written under the web root by the web server process user shortly after a Formie form submission.
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