How to fix CVE-2021-3199: ONLYOFFICE Docs (exploited in the wild)

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.

• Severity: CRITICAL (CVSS 3.1 9.8, vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H).
• Weakness: CWE-22.
• NVD published 2021-01-26, status Analyzed.

Anyone running ONLYOFFICE Docs in the version ranges below. Internet-facing instances are the priority.

• onlyoffice document server: up to (not including) 5.6.3.

Version ranges come from the NVD CPE configuration. Confirm against the vendor advisory before you rely on them.

• Exploited in the wild: added to the CISA Known Exploited Vulnerabilities catalog on 2026-10-08; federal remediation due date 2026-10-11.

• EPSS (probability of exploitation activity in the next 30 days): 20.14%, higher than 97% of all scored CVEs (as of 2026-10-24).
• CVERadar threat level: 70/100 (see methodology).
• Likely ATT&CK techniques: T1190 Exploit Public-Facing Application (inferred); T1083 File and Directory Discovery (inferred); T1203 Exploitation for Client Execution (inferred).

1. Find every instance. List where ONLYOFFICE Docs runs, including test, DR and forgotten internet-facing systems, and record the installed version.
2. Upgrade to a fixed release. onlyoffice document server: 5.6.3 or later. Get the build from the vendor advisory.
3. If you can't patch today, follow CISA's 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.
4. Verify the fix. Re-run the version check on every instance and confirm it reports the fixed build (5.6.3 or later). Re-scan from outside if the service is exposed.
5. Hunt for signs of earlier compromise. If the system was exposed before you patched, assume it may have been hit and check the items below.
6. Rotate credentials stored on or used by the affected system if you cannot rule out compromise, and keep the logs you reviewed.

Detection hints

• T1190 Exploit Public-Facing Application: Review WAF, reverse-proxy and application logs for unusual requests to Docs (odd paths, oversized or encoded payloads, bursts of errors), going back to at least 2021-01-26.
• T1203 Exploitation for Client Execution: Exploitation of client or server software usually shows as crashes of the affected process followed by unexpected child processes; check EDR alerts for both.

Detection commands are generic starting points, not CVE-specific signatures. Use the indicators from the vendor advisory when it publishes them.

• NVD: CVE-2021-3199 nvd.nist.gov/vuln/detail/CVE-2021-3199
• CVE.org record www.cve.org/CVERecord?id=CVE-2021-3199
• Vendor link from CISA KEV github.com/ONLYOFFICE/DocumentServer/blob/903fe5ab7a275bd69c3c3346af2d
• CISA KEV catalog www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (2026-10-24) api.first.org/data/v1/epss?cve=CVE-2021-3199