CVE-2021-25831: ONLYOFFICE Document Server

Critical severity, CVSS 9.8. EPSS: 11.5% chance of exploitation in the next 30 days.

A file extension handling issue was found in [core] module of ONLYOFFICE DocumentServer v4.0.0-9-v5.6.3. An attacker must request the conversion of the crafted file from PPTT into PPTX format. Using the chain of two other bugs related to improper string handling, a remote attacker can obtain remote code execution on DocumentServer.

Affected products

• ONLYOFFICE Document Server: from 4.0.0-9, up to and including 5.6.3

Published 2021-03-01. Last modified 2026-06-17.