Summary
Mautic has Server-Side Template Injection (SSTI) in Theme Templates
A Server-Side Template Injection (SSTI) vulnerability exists in Mautic's theme engine. The platform renders uploaded Twig templates without a sandbox or strict function restrictions. Authenticated users with permissions to create or upload themes can abuse this to execute arbitrary code.
Patched Versions
This security issue has been addressed in the following releases:
- 7.1.2
- 6.0.9
- 5.2.11
For users on Mautic 4.x, this fix is available in:
- 4.4.20 via ELTS
Mautic strongly recommend upgrading to a patched version immediately.
Workarounds
There are no official workarounds. To mitigate this vulnerability without upgrading, restrict theme upload and creation permissions (core:themes:create) to only highly trusted administrators.
Impact
An authenticated user with theme upload and creation privileges can bypass boundaries to execute arbitrary system commands on the hosting server (Remote Code Execution) or access restricted system files and configuration settings.
CVE-2026-9558 has a CVSS score of 9.9 (Critical). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.2.11, 6.0.9, 7.1.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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mautic/core to 5.2.11 or later; mautic/core to 6.0.9 or later; mautic/core to 7.1.2 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-9558? CVE-2026-9558 is a critical-severity security vulnerability in mautic/core (composer), affecting versions >= 1.3.0, < 4.4.13. It is fixed in 5.2.11, 6.0.9, 7.1.2.
- How severe is CVE-2026-9558? CVE-2026-9558 has a CVSS score of 9.9 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of mautic/core are affected by CVE-2026-9558? mautic/core (composer) versions >= 1.3.0, < 4.4.13 is affected.
- Is there a fix for CVE-2026-9558? Yes. CVE-2026-9558 is fixed in 5.2.11, 6.0.9, 7.1.2. Upgrade to this version or later.
- Is CVE-2026-9558 exploitable, and should I be worried? Whether CVE-2026-9558 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-9558 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-9558?
- Upgrade
mautic/coreto 5.2.11 or later - Upgrade
mautic/coreto 6.0.9 or later - Upgrade
mautic/coreto 7.1.2 or later
- Upgrade