ONLYOFFICE Docs is affected by CVE-2021-3199, rated 9.8 (critical). CISA lists it as a Known Exploited Vulnerability, added on 2026-10-08, which means attacks against it have been observed.
What the flaw is
The NIST National Vulnerability Database describes it as follows:
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.
At a glance
| Field | Detail |
|---|---|
| CVE | CVE-2021-3199 |
| Affected product | ONLYOFFICE Docs |
| Weakness | CWE-22 (Path Traversal) |
| CVSS | 9.8 CRITICAL — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| Added to CISA KEV | 2026-10-08 |
| Federal remediation due | 2026-10-11 |
| Known ransomware use | Unknown |
What defenders should do
- CISA required action: 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.
- Patch: apply the fixed release from ONLYOFFICE; check the vendor advisory linked below for the exact versions.
- Inventory: identify every instance of ONLYOFFICE Docs, prioritising internet-facing ones.
- Hunt: review access and application logs on exposed instances for activity predating the patch, and treat unpatched internet-facing instances as potentially compromised.
