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FortiosOperating system · Fortinet

CVE-2020-12820

HIGH · 8.8 CVSS v3.1 Published 2024-12-19
Fix available
A fix is available. Upgrade to 5.6.13 / 6.0.11 or later.
See remediation →
94/100
Remediation priority · Urgent
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Under non-default configuration, a stack-based buffer overflow in FortiOS version 6.0.10 and below, version 5.6.12 and below may allow a remote attacker authenticated to the SSL VPN to crash the FortiClient NAC daemon (fcnacd) and potentially execute arbitrary code via requesting a large FortiClient file name. We are not aware of proof of concept code successfully achieving the latter.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

A stack-based buffer overflow in FortiOS SSL VPN (versions 6.0.10 and below, 5.6.12 and below) under non-default configuration allows an authenticated remote attacker to crash the FortiClient NAC daemon (fcnacd) and potentially execute arbitrary code by requesting an excessively large FortiClient file name.

MitigationUpgrade FortiOS to a patched version beyond 6.0.10/5.6.12. Until then, disable the vulnerable non-default SSL VPN configuration and restrict SSL VPN access to trusted users only.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FortiosOperating system
Affected:< 5.6.13>= 6.0.0, < 6.0.11

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify FortiOS version
    Run 'get system status' via FortiOS CLI or check the web UI dashboard. Locate the Firmware Version field.
    Affected if Version is 5.6.0 through 5.6.12, or 6.0.0 through 6.0.10 (versions below 5.6.13 or 6.0.11). Versions 6.2.0 and later are not affected.
  2. Confirm SSL VPN is configured
    Run 'show vpn ssl settings' via FortiOS CLI to view SSL VPN configuration.
    Affected if SSL VPN is enabled and actively listening (status shows 'enabled' or port is configured). If SSL VPN is disabled, the attack surface does not exist.
  3. Check FortiClient NAC daemon status
    Run 'diagnose system fcnacd status' or 'get system fcnacd' via FortiOS CLI to check if the FortiClient NAC daemon is running.
    Affected if The fcnacd daemon is enabled and running. This is the component vulnerable to the buffer overflow when handling FortiClient file names.
  4. Review NAC policy or profile binding
    Run 'get firewall policy' or check SSL VPN authentication policy settings to see if FortiClient NAC enforcement is linked to SSL VPN access.
    Affected if FortiClient NAC is enforced as a requirement for SSL VPN connections. This non-default configuration links the vulnerable fcnacd to SSL VPN traffic.

You are affected if your FortiOS version is 5.6.x below 5.6.13 or 6.0.x below 6.0.11 AND SSL VPN is enabled AND FortiClient NAC daemon is actively enforcing policies for SSL VPN users.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.6.13 / 6.0.11 or later
Fixed in 5.6.136.0.11
Interim mitigation

Upgrade FortiOS to a patched version beyond 6.0.10/5.6.12. Until then, disable the vulnerable non-default SSL VPN configuration and restrict SSL VPN access to trusted users only.

Recommended fix High confidence

FortiOS 5.6.13 or later; FortiOS 6.0.11 or later

  1. 1. Backup the current FortiOS configuration using 'execute backup config' command
  2. 2. Verify current FortiOS version using 'get system status' command
  3. 3. Download the appropriate firmware upgrade from Fortinet Customer Support portal (https://support.fortinet.com)
  4. 4. Upload the new firmware image to the FortiGate device
  5. 5. Install the firmware upgrade using 'execute upgrade' or through the web UI under System > Firmware
  6. 6. After reboot, verify the new version is running using 'get system status'
  7. 7. Verify the SSL VPN functionality is working correctly
  8. 8. Test that the vulnerability is no longer present
Caveat Major version upgrades (e.g., 5.6 to 6.0 or 6.0 to 7.0) may introduce breaking changes; review Fortinet release notes and test in staging environment before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fortios Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing8.0 h
  • Review / QA4.0 h
20.0 hours of engineering $3,440
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-12820 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data