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Upgrade to ProDetect Mautic Server-Side Template Injection (SSTI) in Theme Templates in Elastic Security
Detects exploitation of CVE-2026-9558, a critical (CVSS 9.9) Server-Side Template Injection vulnerability in Mautic's theme template engine (CWE-1336, Twig-based SSTI). An authenticated or in some deployments unauthenticated attacker can inject malicious Twig template syntax through theme customization, email/landing page builder, or theme upload/import functionality, achieving remote code execution on the underlying PHP host. Affects mautic/core versions >=1.3.0 <4.4.13, >=5.0.0 <5.2.11, >=6.0.0 <6.0.9, and >=7.0.0 <7.1.2. A public PoC/advisory exists (GHSA-9fx4-7cmj-47vg).
MITRE ATT&CK
- Tactic
- Initial Access Execution
Elastic Detection Query
sequence by http.client_ip with maxspan=5m
[network where event.dataset == "nginx.access" and http.request.method in ("POST", "PUT") and url.path : ("*/s/themes*", "*/s/emails*", "*/s/pages*", "*/theme/upload*", "*/api/themes*") and (url.query : "*%7B%7B*" or url.query : "*__construct*" or url.query : "*system(*")]
[process where event.type == "start" and process.parent.name : ("php-fpm*", "php*", "apache2*", "nginx*") and process.name in ("sh", "bash", "whoami", "id", "curl", "wget", "python*", "perl")]Correlates a suspicious Twig-injection HTTP request against Mautic theme/email endpoints followed within 5 minutes by an unexpected child process spawned from the PHP/web server process, indicating successful SSTI-to-RCE exploitation of CVE-2026-9558.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate admin scripts or cron jobs invoked by the web server user around the same time as unrelated web traffic
- Deployment/CI automation that runs shell commands via a web-triggered webhook
- Health-check or monitoring agents spawning short-lived shell utilities from php-fpm workers
Other platforms for CVE-2026-9558
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 1Twig SSTI probe against theme endpoint
Expected signal: Web access log entry for POST /s/themes/edit/1 containing the literal string {{7*7}}; if vulnerable, the rendered theme output contains '49'.
- Test 2Twig-to-PHP function call injection
Expected signal: Web log entry showing POST to /s/emails/edit/1 with __construct/getFilter/system( markers; process telemetry showing php-fpm spawning 'id' if the gadget chain succeeds in the lab sandbox.
- Test 3Post-exploitation shell spawn simulation from PHP-FPM
Expected signal: Process creation events showing bash/id/whoami/curl spawned with parent process php-fpm running as www-data, plus an outbound connection to the test callback listener.
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