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FortiwebApplication · Fortinet

CVE-2021-42761

CRITICAL · 9.8 CVSS v3.1 Published 2023-02-16
Fix available
A fix is available. Upgrade to 5.9.2 / 6.0.8 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
A condition for session fixation vulnerability [CWE-384] in the session management of FortiWeb versions 6.4 all versions, 6.3.0 through 6.3.16, 6.2.0 through 6.2.6, 6.1.0 through 6.1.2, 6.0.0 through 6.0.7, 5.9.0 through 5.9.1 may allow a remote, unauthenticated attacker to infer the session identifier of other users and possibly usurp their session.

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

FortiWeb versions 6.4, 6.3.0-6.3.16, 6.2.0-6.2.6, 6.1.0-6.1.2, 6.0.0-6.0.7, and 5.9.0-5.9.1 contain a session fixation vulnerability (CWE-384) in session management that allows remote unauthenticated attackers to infer session identifiers of other users and potentially hijack their sessions.

MitigationUpgrade to a patched FortiWeb version as specified in Fortinet's security advisory. Implement additional session security controls such as session ID regeneration on authentication and secure session handling pending the upgrade.

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
FortiwebApplication
Affected:>= 5.6.0, < 5.9.2>= 6.0.0, < 6.0.8>= 6.1.0, < 6.1.3>= 6.2.0, < 6.2.7>= 6.3.0, < 6.3.17>= 6.4.0, < 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. Identify FortiWeb installed version
    Run the command 'get system status' or 'fwc -v' via CLI, or check the FortiWeb web UI dashboard for the firmware version displayed.
    Affected if The displayed version falls within any of these ranges: 5.6.0-5.9.1, 6.0.0-6.0.7, 6.1.0-6.1.2, 6.2.0-6.2.6, 6.3.0-6.3.16, or 6.4.0-6.4.x (any version below 7.0.0).
  2. Verify session management is active
    Check FortiWeb configuration via CLI with 'show system session' or review the session management settings in the web UI under Server Infrastructure or Session Management settings.
    Affected if Session management is enabled and FortiWeb is handling user sessions, which is the default operational state for FortiWeb as a WAF.
  3. Confirm FortiWeb is accessible from untrusted networks
    Review network exposure by checking interface bindings and firewall policies: use 'show system interface' in CLI and verify if management or WAD interfaces are bound to WAN-facing interfaces.
    Affected if FortiWeb management or proxy interfaces are exposed directly to the internet or untrusted networks, allowing remote unauthenticated attackers to reach the session management component.
  4. Review session ID configuration for fixation protections
    Examine session settings via CLI command 'show system session' or in the web UI under Profile > Session Management to check if session ID regeneration on authentication is configured.
    Affected if Session ID regeneration on login is not enforced, leaving the session identifier static and vulnerable to fixation attacks.

You are affected if your FortiWeb version is below 7.0.0 and the device is accessible to untrusted networks while session management lacks proper ID regeneration controls.

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 5.9.2 / 6.0.8 / 6.1.3 or later
Fixed in 5.9.26.0.86.1.3
Interim mitigation

Upgrade to a patched FortiWeb version as specified in Fortinet's security advisory. Implement additional session security controls such as session ID regeneration on authentication and secure session handling pending the upgrade.

Recommended fix High confidence

Upgrade to FortiWeb 5.9.2, 6.0.8, 6.1.3, 6.2.7 or later (6.3.x and 6.4.x are also fixed per the official description)

  1. 1. Identify the currently installed FortiWeb version via the Web UI (System > Status) or CLI: 'get system status'
  2. 2. Based on your current major version (5.x, 6.0.x, 6.1.x, or 6.2.x), plan upgrade to the minimum fixed version: 5.9.2, 6.0.8, 6.1.3, or 6.2.7 respectively
  3. 3. Backup the FortiWeb configuration via: 'execute backup full-config' to a secure location
  4. 4. Download the appropriate firmware upgrade from Fortinet support portal (fortinet.com/support) for your hardware/model
  5. 5. Upload and install the new firmware via Web UI (System > Firmware) or CLI: 'execute firmware upgrade <image_file>','6. After reboot, verify the upgrade was successful and restore configuration if needed
  6. 7. Confirm the new version is running via: 'get system status'
Caveat Review FortiWeb release notes for your version jump; some upgrades may require sequential version progression or have configuration migration notes

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

Fix this in Fortiweb Scoped from the published advisory
  • Consultation4.0 h
  • Implementation16.0 h
  • Testing8.0 h
  • Review / QA4.0 h
32.0 hours of engineering $5,600
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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-2021-42761 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
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