CVE-2021-3199: ONLYOFFICE Docs exploitation alert

CVE-2021-3199: what changed

CISA added this vulnerability to its Known Exploited Vulnerabilities catalog on 2026-10-08. Inclusion indicates evidence of exploitation; it does not establish that your own systems have been compromised.

Affected product

ONLYOFFICE Docs. Confirm affected versions against the vendor advisory.

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.

Recommended response

Inventory affected deployments, check vendor guidance for fixes or mitigations, and verify the update was applied. Review available logs and incident-response guidance when exploitation is suspected.

Catalog context

CISA lists a remediation due date of 2026-10-11. Federal directive deadlines have a defined scope; other organizations should prioritize according to their exposure and risk. Ransomware campaign use is listed as unknown; “unknown” does not mean exploitation is absent.

Sources

• CISA Known Exploited Vulnerabilities catalog
• CVE-2021-3199 reference

Automated security brief based on CISA catalog data. Catalog addition date: 2026-10-08.