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FortimailApplication · Fortinet

CVE-2021-24020

CRITICAL · 9.8 CVSS v3.1 Published 2021-07-09
Fix available
A fix is available. Upgrade to 6.4.5 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
A missing cryptographic step in the implementation of the hash digest algorithm in FortiMail 6.4.0 through 6.4.4, and 6.2.0 through 6.2.7 may allow an unauthenticated attacker to tamper with signed URLs by appending further data which allows bypass of signature verification.

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 versions 6.4.0-6.4.4 and 6.2.0-6.2.7 contain a missing cryptographic step in the hash digest algorithm used for signed URL verification, allowing unauthenticated attackers to bypass signature checks by appending arbitrary data to signed URLs.

MitigationUpgrade to FortiMail 6.4.5, 6.2.8, or later versions to apply the cryptographic fix; alternatively, apply vendor-supplied patches if available.

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:>= 6.2.0, <= 6.2.7>= 6.4.0, < 6.4.5

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. Identify FortiMail installation and version
    Log into the FortiMail CLI or access the web UI to view the system firmware version. In CLI, run: get system status. Alternatively, check the FortiMail dashboard or system information page in the web interface.
    Affected if The displayed firmware version falls within 6.2.0 to 6.2.7, or 6.4.0 to 6.4.4.
  2. Confirm signed URL feature is configured
    In the FortiMail web UI, navigate to Profile > URL Pattern or check the signed URL/URL verification settings under the relevant protection or mail settings profile. In CLI, use: get url-protection pattern or show url-protection to see if any signed URL patterns are defined.
    Affected if Any signed URL patterns or URL verification profiles are enabled and configured.
  3. Verify URL signing method
    Examine the configured URL protection profile to determine which hash algorithm is used for signature verification. Look for settings such as 'signature algorithm' or 'hash method' in the URL protection configuration.
    Affected if The profile uses MD5 or SHA1 for URL signature verification (the vulnerable hash digest methods).
  4. Check for URL pattern submissions
    Review FortiMail logs or traffic logs for incoming requests with appended data to signed URLs. Use: execute log filter or grep URL-related logs for patterns containing unusual query parameters after the signature portion.
    Affected if Logs show URL patterns where arbitrary data appears after the signature parameter in inbound requests.

A user is affected if their FortiMail version is 6.2.0-6.2.7 or 6.4.0-6.4.4 AND the signed URL verification feature is enabled using the vulnerable hash algorithm.

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 6.4.5 or later
Fixed in 6.4.5
Interim mitigation

Upgrade to FortiMail 6.4.5, 6.2.8, or later versions to apply the cryptographic fix; alternatively, apply vendor-supplied patches if available.

Recommended fix High confidence

FortiMail 6.4.5 or later (or 6.2.8 or later for the 6.2 branch)

  1. Identify the currently installed FortiMail version using the web UI or CLI command: get system status
  2. If running FortiMail 6.4.0-6.4.4, plan upgrade to version 6.4.5 or later
  3. If running FortiMail 6.2.0-6.2.7, plan upgrade to version 6.2.8 or later
  4. Backup the FortiMail configuration before performing the upgrade
  5. Download the firmware from the Fortinet Customer Support portal for the target fixed version
  6. Upload the firmware through FortiMail web UI (System > Maintenance > Firmware) or via CLI: execute restore firmware
  7. Reboot the device after firmware installation completes
  8. Verify the upgrade was successful and the signature verification bypass is resolved
Caveat Review FortiMail release notes for the target version for any configuration or feature changes that may require adjustment

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

Fix this in Fortimail Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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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-2021-24020 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