dbcve.org
CWE-1336Weakness · CWE-1336

CVE-2026-9558

CRITICAL · 9.9 CVSS v3.1 Published 2026-05-29
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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 on the hosting server (Remote Code Execution) or access restricted system files and configuration settings.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Mautic's theme engine suffers from a Server-Side Template Injection (SSTI) vulnerability where uploaded Twig templates are rendered without sandboxing or function restrictions. Authenticated users with theme creation/upload permissions can craft malicious templates to achieve Remote Code Execution on the server or read sensitive system files and configuration.

MitigationImplement Twig's built-in sandboxing mechanism to restrict template capabilities, or create a strict allowlist of safe functions. Additionally, limit theme upload permissions to only highly trusted users and audit existing theme files.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify users with theme permissions
    In Mautic admin, go to Users > Roles and check which roles have 'Themes' or 'Theme (create/upload)' permissions enabled. Also check Users > Users directly to see assigned roles.
    Affected if Any authenticated user account is assigned a role with theme creation or upload permissions enabled
  2. Check Mautic version
    Access your Mautic installation and navigate to Settings > System Info > Version, or check the version file in your Mautic root directory (version.txt or in the admin dashboard). Compare this version against the affected range for CVE-2026-9558.
    Affected if Installed Mautic version falls within the vulnerable version range and has not been patched
  3. Verify custom themes exist
    Navigate to the themes directory (typically /themes or app/themes in Mautic root). List all directories and check for any themes that are not part of the default Mautic installation or were manually uploaded.
    Affected if Non-default or custom-uploaded themes are present in the themes directory
  4. Check Twig sandbox configuration
    Examine the theme rendering code in Mautic (typically in the theming service or template engine). Look for Twig Environment initialization and check if the sandbox extension is loaded and applied: $twig->addExtension(new \Twig\Extension\SandboxExtension()). Also check if templates are rendered with the sandbox policy.
    Affected if The Twig sandbox extension is not loaded or not enforced when rendering themes
  5. Verify Twig function restrictions
    In the Twig environment configuration, check if a SandboxSecurityPolicy is defined with restricted functions. Look for allowed functions whitelist or blocked functions like system, exec, shell_exec, passthru, file_get_contents, include, require in the security policy.
    Affected if No function restrictions are configured or dangerous functions (system, exec, file operations) are allowed in Twig policy

User is affected if they have any user with theme upload/create permissions AND are running a vulnerable Mautic version AND Twig sandbox isolation or function restrictions are not properly configured.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Implement Twig's built-in sandboxing mechanism to restrict template capabilities, or create a strict allowlist of safe functions. Additionally, limit theme upload permissions to only highly trusted users and audit existing theme files.

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation16.0 h
  • Testing8.0 h
  • Review / QA4.0 h
32.0 hours of engineering $5,600
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,960.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-9558 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-9558 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data