CVE-2021-3199: ONLYOFFICE Docs Server Path Traversal Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run ONLYOFFICE Docs. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-10-08, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-10-11; treat that as the outer limit, not the target.
• EPSS 20.1%, percentile 97: FIRST’s model estimates a 20.1% chance of exploitation activity in the next 30 days, higher than 97% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Editorial verdict: Check exposure. ONLYOFFICE Docs CVE-2021-3199: Old Upload RCE Now in KEV — Only Document Server installs older than 5.6.3, a release from August 2020, are at risk, but any you find should be treated as exposed and upgraded or pulled offline today.

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.

CISA’s summary

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.

Weakness type: CWE-22.

Product: ONLYOFFICE Docs · Assigned by: mitre · Published: 2021-01-22

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-10-08; federal due date 2026-10-11. Known ransomware use: Unknown.
• 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.
• CISA SSVC assessment: exploitation active, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 20.14% probability of exploitation activity in the next 30 days; percentile 97.

Timeline

• 2021-01-22 — CVE record published by mitre
• 2021-01-22 — erroneo.us brief: ONLYOFFICE Docs CVE-2021-3199: Old Upload RCE Now in KEV
• 2026-10-08 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-10-09 — CVE record last updated
• 2026-10-11 — CISA remediation deadline for US federal agencies

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Listed in CISA KEV notes (github.com) github.com

Vulnerability records

1. CVE.org record CVE-2021-3199 cve.org
2. NVD entry CVE-2021-3199 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
2. Listed in CISA KEV notes (cisa.gov) cisa.gov
3. Listed in CISA KEV notes (cisa.gov) cisa.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

All exploited ONLYOFFICE vulnerabilities · Today’s Security Digest