How severe is CVE-2026-9558?

CVE-2026-9558 has a CVSS 3.x base score of 9.9, rated critical severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-9558 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (35th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-9558?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-9558 have an EU (EUVD) identifier?

Yes. CVE-2026-9558 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-33276.

When was CVE-2026-9558 published?

CVE-2026-9558 was published on 2026-05-29 and last updated on 2026-06-17.

CVE-2026-9558

CVE-2026-9558 is a critical-severity vulnerability with a CVSS 3.x base score of 9.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-1336.

Key facts

Severity: Critical (CVSS 3.x base score 9.9)
EPSS exploit prediction: 0% (35th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-33276
Weakness: CWE-1336
Published: 2026-05-29
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-9558?: 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.
How severe is CVE-2026-9558?: CVE-2026-9558 has a CVSS 3.x base score of 9.9, rated critical severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-9558 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (35th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-9558?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-9558 have an EU (EUVD) identifier?: Yes. CVE-2026-9558 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-33276.
When was CVE-2026-9558 published?: CVE-2026-9558 was published on 2026-05-29 and last updated on 2026-06-17.

References

https://github.com/mautic/mautic/security/advisories/GHSA-9fx4-7cmj-47vg

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