CVE-2021-3199

What this means for your business

• It affects ONLYOFFICE Docs. It matters if your company, or a supplier that handles your data, runs it.
• Attackers are already using it. CISA added it to its list of flaws used in real attacks on 8 October 2026.
• An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
• FIRST's prediction model gives it a 19% chance of attack attempts being seen in the next 30 days, ranking above 97% of all known flaws.

What to do

1. 1Check whether your company or your suppliers run ONLYOFFICE Docs, and which version. The affected versions are listed further down this page.
2. 2If you do, follow CISA's required action below. No patch reference has been published yet.
3. 3Treat it as urgent. US federal agencies must fix it by 11 October 2026. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
4. 4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.