What is CVE-2026-9558?

CVE-2026-9558 is a critical-severity vulnerability, classified under Improper Neutralization of Special Elements Used in a Template Engine. CVSS score: 9.9/10. Published 2026-05-29.

How severe is CVE-2026-9558?

Critical severity. CVSS v3 base score is 9.9 out of 10.

Is CVE-2026-9558 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

CVE-2026-9558

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…

EPSS: 0.004 (35.6th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 9.9 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H.

Weakness classification (CWE)

CWE-1336 — Improper Neutralization of Special Elements Used in a Template Engine

Public proof-of-concept exploits

covepseng/cve-2026-9558-poc

References

security@mautic.org

Frequently asked questions

What is CVE-2026-9558?: CVE-2026-9558 is a critical-severity vulnerability, classified under Improper Neutralization of Special Elements Used in a Template Engine. CVSS score: 9.9/10. Published 2026-05-29.
How severe is CVE-2026-9558?: Critical severity. CVSS v3 base score is 9.9 out of 10.
Is CVE-2026-9558 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.