# CVE-2026-9558: Mautic has Server-Side Template Injection (SSTI) in Theme Templates

> Authenticated users can execute arbitrary code or read system files via Server-Side Template Injection in Mautic's theme engine.

- **Severity:** Critical 9.9 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 May 2026
- **Updated:** 29 May 2026
- **Also known as:** GHSA-9fx4-7cmj-47vg

## What is CVE-2026-9558?

CVE-2026-9558 is a critical code injection vulnerability in mautic/core, published on 29 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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.

Weakness: CWE-1336 (Improper Neutralization of Special Elements Used in a Template Engine)

## Which versions are affected by CVE-2026-9558?

8 open source packages ship an affected version, including mautic/core.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mautic/core` | Packagist | From 1.3.0 | None |
| `mautic/core` | Packagist | From 1.3.0 | None |
| `mautic/core` | Packagist | From 5.0.0 | 5.2.11 |
| `mautic/core` | Packagist | From 5.0.0 | 5.2.11 |
| `mautic/core` | Packagist | From 6.0.0 | 6.0.9 |
| `mautic/core` | Packagist | From 6.0.0 | 6.0.9 |
| `mautic/core` | Packagist | From 7.0.0 | 7.1.2 |
| `mautic/core` | Packagist | From 7.0.0 | 7.1.2 |

## How do I fix CVE-2026-9558?

Upgrade mautic/core to a fixed version for your release line: 5.2.11, 6.0.9 or 7.1.2.

## Is CVE-2026-9558 being exploited?

There is no known exploitation of CVE-2026-9558 in the wild and no public exploit code.

Public exploit code:

- Exploitability PoC for CVE-2026-9558 (SSTI Mautic Theme) (github_poc): https://github.com/covepseng/cve-2026-9558-poc
- https://github.com/Cimihan123/CVE-2026-9558-lab-poc-bundle (github_poc): https://github.com/Cimihan123/CVE-2026-9558-lab-poc-bundle

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9558 and fix it if it is affected.

What it is: CVE-2026-9558 is a critical code injection vulnerability in mautic/core, published on 29 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-9558 in the wild and no public exploit code.

Affected packages and fixed versions:
- mautic/core (Packagist): affected from 1.3.0; no fix listed
- mautic/core (Packagist): affected from 1.3.0; no fix listed
- mautic/core (Packagist): affected from 5.0.0; fixed in 5.2.11
- mautic/core (Packagist): affected from 5.0.0; fixed in 5.2.11
- mautic/core (Packagist): affected from 6.0.0; fixed in 6.0.9
- mautic/core (Packagist): affected from 6.0.0; fixed in 6.0.9
- mautic/core (Packagist): affected from 7.0.0; fixed in 7.1.2
- mautic/core (Packagist): affected from 7.0.0; fixed in 7.1.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-9558/ (Markdown: https://insights.sbomify.com/CVE-2026-9558/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9558
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9558
- OSV: https://osv.dev/vulnerability/CVE-2026-9558

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-9558/
Summaries are generated and may contain mistakes; the official description above is the record.