CVE-2026-53909

Description

MCO does not correctly validate types of uploaded files. File upload validation functionality relies only on client-side checks, which can be bypassed. An authorized, low-privileged attacker can upload files with arbitrary types to the server. Because vendor contact attempts were unsuccessful, the vulnerability has only been confirmed in version 25.3.3.1 but may also affect other versions.

References

• https://cert.pl/en/posts/2026/07/CVE-2026-53902
• https://mco.mycomplianceoffice.com/