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Upgrade to ProDetect Mautic Server-Side Template Injection (SSTI) in Theme Templates in Splunk
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
SPL Detection Query
index=web sourcetype=access_combined OR sourcetype=mautic:access
(uri_path="*/s/themes*" OR uri_path="*/s/emails*" OR uri_path="*/s/pages*" OR uri_path="*/theme/upload*" OR uri_path="*/api/themes*")
method IN ("POST","PUT")
| eval decoded_uri=urldecode(uri_query)
| regex decoded_uri="(\{\{|\{%|_self|getFilter|__construct|system\(|exec\(|passthru\(|proc_open\(|shell_exec\()"
| table _time, clientip, method, uri_path, decoded_uri, status, http_user_agent
| sort -_timeSearches Mautic web access logs for POST/PUT requests to theme, email, or landing page endpoints containing Twig SSTI payload indicators, correlating with CVE-2026-9558 exploitation attempts.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Internal QA teams stress-testing custom Twig theme snippets
- Marketing content containing literal curly-brace text (e.g., mail-merge examples) that is URL-encoded
- Third-party integrations posting JSON payloads with braces that are misidentified as Twig delimiters
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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