FalehSec Exploited-vulnerability triage · joined from CISA KEV, NVD CVSS and EPSS
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CVE-2021-3199

ONLYOFFICE · Docs · ONLYOFFICE Docs Server Path Traversal Vulnerability

Verdict
Patch first · actively exploited, reachable from the internet without credentials, and in the top 10% of CVEs by near-term exploitation probability

Added to CISA KEV 2026-10-08 · exploited in the wild for 1 day

How a test catches it

Test
Request a file that sits outside the served root through the parameter that names a path, percent-encoded, then double-encoded.
Passes when
Every encoding depth returns an error or an empty body, never file content. A single depth that resolves is the finding.
What an attacker can reach How likely Time What CISA says to do Sources All 550

What an attacker can reach

CVSS 9.8 CRITICAL. The score is a summary; these three components are the part that decides whether something scanning the internet can use it.

Attack vectornetworkreachable over the network, with no local access needed
Privileges requirednoneneeds no credentials at all
User interactionnonecan be triggered with nobody interacting with the page or request
Base vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
WeaknessCWE-22

This describes the vulnerability. It says nothing about whether your systems run it.

How likely

EPSS puts the probability of exploitation in the next 30 days at 0.0822, which places it in the top 10% of all tracked CVEs. The percentile is the more useful half: 0.18 in absolute terms sounds low until you know most of the internet-facing software in the world scores under that.

EPSS probability0.0822
Percentile0.948 — top 10%

Time

Added to KEV2026-10-08
Days exploited1
Federal remediation duenot set
Ransomware useno known ransomware campaign use

What CISA says to do

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.

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.

Sources

This page describes the bug, not your estate. It cannot tell you whether you run it, or whether your own systems carry the same class of flaw. That question is what an assessment answers.

Web application penetration testing · API security testing · every service