CVE-2021-3199 ONLYOFFICE Docs: file path flaw
In plain words: The program can be tricked into reading or writing files outside the folder it should stay in. Leaks files such as passwords and settings, or overwrites important ones.
Confirmed exploited by CISA on · US federal patch deadline Oct 11, 2026
What to do
- Install ONLYOFFICE's security update for Docs. If no update exists, follow the vendor's workaround or stop using the product until one ships.
- Patch edge devices first — attackers often start scanning for them within hours of a fix.
- Never expose the device's admin page to the internet.
- After patching a device that was exposed, follow the vendor's compromise checks, end active sessions and change passwords.
Who is affected
Work IT. Aimed mostly at company systems — you are affected through the companies you use, not your own phone.
What's at risk: They face the internet and guard the whole network, so a flaw here can hand attackers a way in — and these devices usually run no antivirus, so break-ins go unnoticed.
The details
- Official name
- ONLYOFFICE Docs Server Path Traversal Vulnerability
- Product
- ONLYOFFICE Docs
- CISA description
- 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.
- Type of flaw
- File path flaw (CWE-22)
- Confirmed exploited
- Oct 8, 2026
- US federal patch deadline
- Oct 11, 2026
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Answering about CVE-2021-3199 ·
Questions about CVE-2021-3199
Is CVE-2021-3199 being exploited?
Yes. CISA added CVE-2021-3199 to its Known Exploited Vulnerabilities catalog on Oct 8, 2026, which means there is reliable evidence of real-world attacks.
How do I fix CVE-2021-3199?
Install ONLYOFFICE's security update for Docs. If no update exists, follow the vendor's workaround or stop using the product until one ships. Patch edge devices first — attackers often start scanning for them within hours of a fix.
Am I affected by CVE-2021-3199?
Only if you use ONLYOFFICE Docs. Aimed mostly at company systems — you are affected through the companies you use, not your own phone.