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Upgrade to ProDetect Mautic Server-Side Template Injection (SSTI) in Theme Templates in Google Chronicle
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
YARA-L Detection Query
rule mautic_ssti_cve_2026_9558 {
meta:
author = "df00tech"
description = "Detects Twig SSTI payload markers in HTTP requests to Mautic theme/email endpoints (CVE-2026-9558)"
severity = "CRITICAL"
events:
$http.metadata.event_type = "NETWORK_HTTP"
$http.network.http.method = "POST" or $http.network.http.method = "PUT"
re.regex($http.target.url, `(?i)/s/(themes|emails|pages)|/theme/upload|/api/themes`)
re.regex($http.target.url, `(?i)(%7B%7B|__construct|system\(|getFilter|proc_open)`)
outcome:
$risk_score = max(85)
condition:
$http
}Chronicle YARA-L rule matching HTTP POST/PUT requests to Mautic theme, email, or landing page endpoints whose URL/query contains Twig SSTI injection markers consistent with CVE-2026-9558 exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Security researchers or internal red teams testing the Mautic staging environment
- Legitimate template import/export operations that include escaped Twig-like text
- Proxy re-logging of the same request causing duplicate low-confidence matches
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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