CVE-2026-53909: MCO does not correctly validate types of uploaded files

Routine

MEDIUM 6.5DOWNGRADE

CVE-2026-53909 is a medium-severity flaw (CVSS 6.5) affecting mycomplianceoffice. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

No elevated risk signals yet

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.4% (top 67%)
CISA exploited list	No
Weakness	CWE-434
Published	2026-07-01
Last modified	2026-10-25

Publisher (CNA): CERT.PL · Vendor: mycomplianceoffice

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-01 13:17 UTCPublished CVE record published
2026-10-25 17:10 UTCUpdated Record last modified

mycomplianceoffice mycomplianceoffice DOWNGRADE

Affected: 25.3.3.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 25.3.3.1

1. Check installed version

apt-cache policy mycomplianceoffice || rpm -q mycomplianceoffice || mycomplianceoffice --version

2. No fixed release: DOWNGRADE to a release before 25.3.3.1 (Debian / Ubuntu)

apt-cache madison mycomplianceoffice sudo apt install mycomplianceoffice=<VERSION_BEFORE_25.3.3.1> sudo apt-mark hold mycomplianceoffice

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list mycomplianceoffice sudo dnf downgrade mycomplianceoffice-<VERSION_BEFORE_25.3.3.1> sudo dnf versionlock add mycomplianceoffice

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Advisories and coverage

Official advisories

CERT Polska: Vulnerabilities in MyComplianceOffice MCO software 2026-07-01

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-53909?

mycomplianceoffice: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-53909 being exploited?

CVE-2026-53909 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-53909 affect?

CVE-2026-53909 affects mycomplianceoffice. Severity is medium (CVSS 6.5). 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 serv

How urgent is CVE-2026-53909?

Priority 31/100. No elevated risk signals yet.

Related CVEs

CVE-2026-53906 HIGH 8.2 DOWNGRADE MCO is vulnerable to Path Disclosure and Path Traversal in file handling functionality related to data export
CVE-2026-53903 HIGH 8.1 DOWNGRADE MCO is vulnerable to an Insecure Direct Object Reference (IDOR) vulnerability in the /customer/servlet/mco/web
CVE-2026-53905 HIGH 7.1 DOWNGRADE MCO does not properly enforce authorization checks in the /customer/servlet/mco/webapi/admin-view-hierarchy/ge
CVE-2026-53902 MEDIUM 6.5 DOWNGRADE MCO does not properly enforce authorization checks in the /customer/servlet/mco/webapi/profile-sections/group-
CVE-2026-53907 MEDIUM 5.4 DOWNGRADE MCO is vulnerable to Stored Cross‑Site Scripting (XSS) via the application logo upload functionality

References

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