CVE-2021-3199
CVE-2021-3199 is a security flaw in ONLYOFFICE Docs. Attackers can run their own code on ONLYOFFICE Docs. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 8 Oct 2026.[1]
ONLYOFFICE Docs is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff.
Description
CISA calls it a server path traversal in ONLYOFFICE Docs. In its words: “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.”[1]
The underlying weakness is path traversal (CWE-22), in MITRE’s list of common software weaknesses.[2]
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it. CISA names the product as ONLYOFFICE Docs.
Most families won’t have ONLYOFFICE Docs; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 8 Oct 2026, 2,081 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 11 Oct 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 30%. That’s in the top 2% of the flaws it scores.[3]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
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).”[1]
- Check whether you use it.ONLYOFFICE Docs is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 11 Oct 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 30%.[3] |
See also
Other path traversal flaws
- CVE-2025-61624, Fortinet products: path traversal ·
- CVE-2026-104286, Fortinet FortiMail: path traversal ·
- CVE-2026-93616, Check Point products: path traversal ·
- CVE-2026-66384, JFrog Artifactory: improper limitation of a pathname to a restricted directory ·
- CVE-2026-59310, Broadcom VMware vCenter: path traversal ·
Added the same day
- CVE-2023-22894, Strapi: cleartext storage of sensitive information ·
- CVE-2016-3081, Apache Struts: command injection ·
- CVE-2015-5477, ISC BIND: data processing errors ·
- CVE-2015-3306, ProFTPD: improper access control ·
References
- ^ Cybersecurity and Infrastructure Security Agency. “ONLYOFFICE Docs Server Path Traversal Vulnerability”. Known Exploited Vulnerabilities Catalog, added 8 Oct 2026.
- ^ National Institute of Standards and Technology. “CVE-2021-3199 Detail”. National Vulnerability Database, published 26 Jan 2021.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.302, percentile 98.2, 31 Oct 2026.
- CVE Program. “CVE-2021-3199”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.