CVE-2021-3199
Directory traversal with remote code execution can occur in /upload in ONLYOFFICE Document Server before 5.6.3, when JWT is used, via a /.. sequence in an image upload parameter.
do firstApply 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). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
Can a WAF block it? Yes. The attack uses a directory traversal sequence (/..) in an image upload parameter sent via an HTTP request to the /upload endpoint, which is unusual and can be detected by a WAF rule matching such sequences in that parameter. A WAF rule can match "/.." sequences in the image upload parameter of /upload. No tracked WAF vendor has published a rule for this CVE yet, so you would write it yourself.
What a custom rule would match: "/.." sequences in the image upload parameter of /upload
Also consider: patch · custom WAF rule · canonical path validation
Based on an AI reading of the CVE description, and no vendor rule found yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Is CVE-2021-3199 being exploited?
Yes. CVE-2021-3199 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2021-3199?
CVE-2021-3199 is rated Critical with a CVSS score of 9.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2021-3199?
Yes. A fix has been recorded for CVE-2021-3199. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2021-3199 affect?
CVE-2021-3199 affects onlyoffice:document_server. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
cisa ssvc
- exploitation
- active
- automatable
- yes
- technical impact
- total
Assessed by CISA 8 Oct 2026 (SSVC 2.0.3).
timeline
- 26 Jan 2021Published
- 8 Oct 2026Added to CISA KEV (exploited in the wild) · 2,080 days (about 6 years) later
- 8 Oct 2026CISA SSVC assessment: exploitation active · same day later
- 9 Oct 2026CVE record last updated · 1 day later
- 11 Oct 2026CISA remediation due date (US federal agencies) · 2 days later
- 2 Nov 2026Analysed by vulneraibility.com · 23 days later
- Linkgithub.com /ONLYOFFICE/DocumentServer/blob/903fe5ab7a275bd69c3c3346af2d21cf87ebeabf/CHANGEL
- Linkgithub.com /nola-milkin/poc_exploits/blob/master/CVE-2021-3199/poc_uploadImageFile.py
- Linkgithub.com /moehw/poc_exploits/tree/master/CVE-2021-3199/poc_uploadImageFile.py
- Linkcisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-3199
sources
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.