# CVE-2026-53909: Arbitrary File Upload in MCO

> An authorized low-privileged attacker can bypass client-side checks to upload arbitrary file types, potentially leading to code execution or data modification.

- **Severity:** Medium 5.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 1 Jul 2026
- **Updated:** 1 Jul 2026
- **Also known as:** GHSA-5v97-p7p8-r448

## What is CVE-2026-53909?

CVE-2026-53909 is a medium-severity vulnerability in MyComplianceOffice MCO, published on 1 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official 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.

Weakness: CWE-434 (Unrestricted Upload of File with Dangerous Type)

## Which versions are affected by CVE-2026-53909?

It affects 1 product.


Products: Mycomplianceoffice Mycomplianceoffice.

## How do I fix CVE-2026-53909?

No fix has been published for CVE-2026-53909 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-53909 being exploited?

There is no known exploitation of CVE-2026-53909 in the wild and no public exploit code.

## Other vulnerabilities in MyComplianceOffice MCO

- [CVE-2026-53902](https://insights.sbomify.com/CVE-2026-53902/): Privilege Escalation in MCO (high 7.1)
- [CVE-2026-53908](https://insights.sbomify.com/CVE-2026-53908/): User Enumeration in MCO (medium 6.9)
- [CVE-2026-53903](https://insights.sbomify.com/CVE-2026-53903/): Insecure Direct Object Reference in MCO (medium 5.3)
- [CVE-2026-53905](https://insights.sbomify.com/CVE-2026-53905/): Unauthorized Access to Administrator ACL View in MCO (medium 5.3)
- [CVE-2026-53906](https://insights.sbomify.com/CVE-2026-53906/): Path Disclosure and Path Traversal in MCO (medium 5.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53909 and fix it if it is affected.

What it is: CVE-2026-53909 is a medium-severity vulnerability in MyComplianceOffice MCO, published on 1 Jul 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-2026-53909 in the wild and no public exploit code.

Affected product: MyComplianceOffice MCO. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-53909/ (Markdown: https://insights.sbomify.com/CVE-2026-53909/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53909
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53909
- OSV: https://osv.dev/vulnerability/CVE-2026-53909

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-53909/
Summaries are generated and may contain mistakes; the official description above is the record.