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Upgrade to ProDetect Mautic Server-Side Template Injection (SSTI) in Theme Templates in Sumo Logic CSE
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
Sumo Detection Query
_sourceCategory=web/access OR _sourceCategory=mautic/app
| where method matches "POST" or method matches "PUT"
| where uri_path matches "*\/s\/themes*" or uri_path matches "*\/s\/emails*" or uri_path matches "*\/s\/pages*" or uri_path matches "*\/theme\/upload*" or uri_path matches "*\/api\/themes*"
| parse regex "(?<decoded_query>.*)" from uri_query
| where decoded_query matches "*{{*" or decoded_query matches "*__construct*" or decoded_query matches "*system(*" or decoded_query matches "*getFilter*"
| timeslice 5m
| count by _timeslice, clientip, uri_path
| sort by _count descAggregates suspicious requests to Mautic theme/email/page endpoints containing Twig SSTI payload signatures over 5-minute windows to surface exploitation attempts of CVE-2026-9558.
Data Sources
Required Tables
False Positives & Tuning
- Internal developers customizing Mautic themes and legitimately using Twig syntax
- Automated web scanners generating high volumes of probe requests unrelated to real exploitation
- Load balancer or CDN logs duplicating the same request multiple times, inflating counts
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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