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Actively exploited

CVE-2021-3199 ONLYOFFICE Docs Server Path Traversal Vulnerability

ONLYOFFICE Docs contains a path traversal vulnerability that can occur when JWT is used, via a /.. sequence in an image upload parameter and could allow for remote code execution.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 8 Oct 2026.
  • Rated Critical severity (CVSS 9.8, scored by CISA).
The Vulnerability

What It Is

Directory traversal with remote code execution can occur in /upload in ONLYOFFICE Document Server before 5.6.3, when JWT is used, via a /.. sequence in an image upload parameter.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for ONLYOFFICE Docs. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 11 Oct 2026 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-3199
  3. CVE record: CVE-2021-3199
  4. FIRST EPSS
  5. github.com/ONLYOFFICE/DocumentServer/blob/903fe5ab7a275bd69c3c3346af2d21cf87ebeabf/CHANGELOG.md#563
  6. github.com/nola-milkin/poc_exploits/blob/master/CVE-2021-3199/poc_uploadImageFile.py
  7. github.com/moehw/poc_exploits/tree/master/CVE-2021-3199/poc_uploadImageFile.py