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FortimailApplication · Fortinet

CVE-2021-36166

CRITICAL · 9.8 CVSS v3.1 Published 2022-03-01
Fix available
A fix is available. Upgrade to 6.0.12 / 6.2.8 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges 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
An improper authentication vulnerability in FortiMail before 7.0.1 may allow a remote attacker to efficiently guess one administrative account's authentication token by means of the observation of certain system's properties.

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 · high confidence

FortiMail before 7.0.1 contains an improper authentication vulnerability where a remote attacker can efficiently guess an administrative account's authentication token by observing certain system properties, likely through a timing or side-channel attack, achieving CVSS 9.8 critical severity.

MitigationUpgrade FortiMail to version 7.0.1 or later to receive the vendor patch. Additionally, limit administrative interface exposure to trusted networks and monitor for unusual authentication patterns.

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
FortimailApplication
Affected:<= 5.4.12>= 6.0.0, < 6.0.12>= 6.2.0, < 6.2.8>= 6.4.0, < 6.4.6= 7.0.0

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
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/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. Check FortiMail version via CLI
    Log into the FortiMail CLI and run the command: get system status. Look for the Version field in the output.
    Affected if The displayed version number falls within any of these ranges: <= 5.4.12, >= 6.0.0 and < 6.0.12, >= 6.2.0 and < 6.2.8, >= 6.4.0 and < 6.4.6, or equals 7.0.0
  2. Check FortiMail version via web interface
    Log into the FortiMail web interface as an administrator. Navigate to Dashboard > Status, or go to System > Maintenance > About. Locate the firmware version displayed.
    Affected if The firmware version shown is 7.0.0 or any version earlier than 7.0.1 within the affected branches (5.4.x, 6.0.x before 6.0.12, 6.2.x before 6.2.8, 6.4.x before 6.4.6)
  3. Confirm administrative interface exposure
    Review network exposure of the FortiMail administrative interface (typically ports 443 or 80). Check firewall policies or access control lists to determine if the management interface is reachable from untrusted networks.
    Affected if The administrative interface is accessible from the internet or from untrusted/demilitarized networks, increasing exploitability of the authentication token guess attack

You are affected if your FortiMail version is 7.0.0 or any version earlier than 7.0.1, and the administrative interface is exposed to untrusted networks.

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 6.0.12 / 6.2.8 / 6.4.6 or later
Fixed in 6.0.126.2.86.4.6
Interim mitigation

Upgrade FortiMail to version 7.0.1 or later to receive the vendor patch. Additionally, limit administrative interface exposure to trusted networks and monitor for unusual authentication patterns.

Recommended fix High confidence

7.0.1 or later (or alternatively 6.0.12+, 6.2.8+, or 6.4.6+ if staying on 6.x branches)

  1. 1. Back up the current FortiMail configuration and email data.
  2. 2. Obtain the fixed firmware version 7.0.1 or later from the Fortinet support portal (support.fortinet.com).
  3. 3. Log in to the FortiMail web UI or CLI as administrator.
  4. 4. Navigate to System > Maintenance > Firmware or use the 'execute upgrade' command in CLI.
  5. 5. Upload and install the firmware file (e.g., FMG-v7.0.1-FW-buildXXXXX).
  6. 6. Allow the device to reboot automatically after the upgrade completes.
  7. 7. Log back in and verify the firmware version under Dashboard > Status to confirm 7.0.1 or later is running.
  8. 8. Verify administrative access and functionality are working correctly.

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

Fix this in Fortimail Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
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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-2021-36166 in production — separate from our analysis above.

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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