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FortimailApplication · Fortinet

CVE-2021-26091

HIGH · 7.5 CVSS v3.1 Published 2025-03-24
Fix available
A fix is available. Upgrade to 6.4.5 or later.
See remediation →
84/100
Remediation priority · High
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 use of a cryptographically weak pseudo-random number generator vulnerability in the authenticator of the Identity Based Encryption service of FortiMail 6.4.0 through 6.4.4, and 6.2.0 through 6.2.7 may allow an unauthenticated attacker to infer parts of users authentication tokens and reset their credentials.

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 use a cryptographically weak PRNG in the Identity Based Encryption service authenticator, allowing unauthenticated attackers to infer portions of user authentication tokens and potentially reset credentials through prediction attacks.

MitigationUpgrade FortiMail to vendor-supported versions beyond 6.4.4 and 6.2.7 that contain the patch for the weak PRNG vulnerability. If immediate patching is not possible, consider restricting network access to the IBE service or implementing additional authentication layers.

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, < 6.4.5

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
None
Availability
None

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

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 installed version
    Log into FortiMail CLI and run: get system status OR check the web UI under Dashboard > Status > Firmware Version
    Affected if The displayed version falls within 6.2.0 through 6.4.4 (any version >= 6.2.0 and < 6.4.5)
  2. Verify IBE service status
    In FortiMail CLI run: get system ibe OR check under Mail Settings > IBE in the web UI to see if Identity Based Encryption is enabled
    Affected if IBE service is enabled and configured on the FortiMail device
  3. Check IBE service network exposure
    Review firewall policies and network accessibility to confirm whether the IBE service ports (typically TCP/443 or IBE-specific ports) are exposed to untrusted networks
    Affected if IBE service is reachable from external or untrusted networks without proper access controls
  4. Review IBE authentication configuration
    Check IBE settings for user authentication token generation: show system ibe OR examine the IBE domain and user configuration
    Affected if IBE is used for password reset or credential management features, as the weak PRNG affects token generation in this service

A FortiMail installation is affected if it runs version 6.2.0 through 6.4.4 AND has the Identity Based Encryption service enabled and accessible, enabling prediction attacks on authentication tokens.

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 FortiMail to vendor-supported versions beyond 6.4.4 and 6.2.7 that contain the patch for the weak PRNG vulnerability. If immediate patching is not possible, consider restricting network access to the IBE service or implementing additional authentication layers.

Recommended fix High confidence

FortiMail 6.4.5 or later

  1. Identify the current FortiMail version using the web UI or CLI command 'get system status'
  2. Backup the current FortiMail configuration using 'execute backup config' or the web UI
  3. Review FortiMail upgrade documentation and release notes for any pre-upgrade requirements
  4. Download FortiMail firmware version 6.4.5 or later from Fortinet support portal
  5. Upgrade the FortiMail device using either the web UI (System > Firmware) or CLI command 'execute firmware upgrade <path-to-firmware>'
  6. Reboot the device if required by the upgrade process
  7. Verify the upgrade was successful by checking the new version with 'get system status'
  8. Confirm the vulnerability is resolved by verifying the system is running version 6.4.5 or later

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
  • Testing6.0 h
  • Review / QA3.0 h
15.0 hours of engineering $2,560
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,096.

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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-2021-26091 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