Vulnerabilities
CVE-2020-12820 — Under non-default configuration, a stack-based buffer overflow in FortiOS version 6.0.10 and below, version 5…
CVE-2020-12820 · FortiOS · CVSS 8.8
In brief
Summary
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.
Weakness: CWE-121, CWE-787
Title, description and the vendor's statements are quoted from the CVE record; a vendor's advisory is summarised in Patcharo's words, with a link.
Affected and fixed versions
Per product and release branch, as the vendor's CVE record and official advisory state them. A fixed release is shown only when a document names it: the vendor's, or CERT-FR's for Check Point, with its source.
2 branches with a published fixed release
Fortinet FortiOS
Source: CVE-2020-12820
- 6.0End of lifeAffected versions6.0.0 – 6.0.10First fixed release6.0.11
- 5.6End of lifeAffected versions5.6.0 – 5.6.12First fixed release5.6.13
Releases the vendor does not list are not affected.
Remediation
Fix available
Vendor fix
- FortiOS 6.06.0.11
- FortiOS 5.65.6.13
Upgrade to a fixed release listed above for your branch, or to a later fixed release the vendor explicitly supports.
Please upgrade to FortiOS versions 5.6.13 or above. Please upgrade to FortiOS versions 6.0.11 or above. FortiOS versions 6.2.0 and above are not impacted. FortiOS versions 6.4.0 and above are not impacted. Workaround: Please ensure that Fortiheartbeat and Endpoint-Compliance are not both enabled on the same interface.
Full text
FortiHeartbeat and Endpoint-Compliance can be disabled on a particular interface by following the below CLI commands: config system interface edit interface set endpoint-compliance disable (<-- Disabled by default) set fortiheartbeat disable next end
SourceFortinet (CVE record)
Vendor advisory
Last verified 1 Oct 2026 at 09:30 UTC
Known exploitation
Not listed in CISA KEV as of the last check.
0.89% probability of exploitation in the next 30 days (percentile 58%).
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CERT-FR
No CERT-FR advisory or alert of the last two years references this CVE, for the vendors Patcharo covers.
Timeline
- Published by the source
- Source record last modified
- EPSS score computed
- Last verified by Patcharo
Official sources
- Fortinet (CVE record)retrieved 1 Oct 2026 at 09:30 UTC
- NVDretrieved 27 Sept 2026 at 23:44 UTC
- FIRST EPSSretrieved 30 Sept 2026 at 00:17 UTC
- FortiGuard FG-IR-20-083retrieved 1 Oct 2026 at 09:30 UTC
Fortinet · NVD · FIRST EPSS · FortiGuard FG-IR-20-083
How Patcharo reads this
Affected and fixed releases come from the vendor's structured statements and are matched against exact versions; CVSS from NVD, exploitation from CISA KEV and the vendor's own advisory, probability from FIRST EPSS. Missing data stays Unknown and never becomes Not affected.
Read the methodologyRead the guide: FortiOS version numbering: branches, patch releases and the Feature and Mature labels
Related vulnerabilities
Other records affecting the same products, newest first.
- CVE-2026-71407A Stack-based Buffer Overflow vulnerability [CWE-121] vulnerability in Fortinet FortiOS 7.6.1 through 7.6.6 m…High
- CVE-2025-53844A out-of-bounds write vulnerability in Fortinet FortiOS 7.6.0 through 7.6.3, FortiOS 7.4.0 through 7.4.8, For…High
- CVE-2025-53847A missing authentication for critical function vulnerability in Fortinet FortiOS 7.6.0 through 7.6.3, FortiOS…High
- CVE-2025-68686Fortinet FortiOS Exposure of Sensitive Information to an Unauthorized Actor VulnerabilityMediumKEV
- CVE-2025-64157A use of externally-controlled format string vulnerability in Fortinet FortiOS 7.6.0 through 7.6.4, FortiOS 7…High