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FortiwebApplication · Fortinet

CVE-2021-42756

CRITICAL · 9.8 CVSS v3.1 Published 2023-02-16
Fix available
A fix is available. Upgrade to 6.0.8 / 6.1.3 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
Multiple stack-based buffer overflow vulnerabilities [CWE-121] in the proxy daemon of FortiWeb 5.x all versions, 6.0.7 and below, 6.1.2 and below, 6.2.6 and below, 6.3.16 and below, 6.4 all versions may allow an unauthenticated remote attacker to achieve arbitrary code execution via specifically crafted HTTP requests.

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

Multiple stack-based buffer overflow vulnerabilities (CWE-121) in the FortiWeb proxy daemon allow unauthenticated remote attackers to achieve arbitrary code execution via specifically crafted HTTP requests. The vulnerabilities affect FortiWeb versions 5.x, 6.0.7 and below, 6.1.2 and below, 6.2.6 and below, 6.3.16 and below, and 6.4 all versions.

MitigationUpgrade FortiWeb to the latest patched version as specified in Fortinet's security advisory. Given the critical CVSS score (9.8) and remote code execution capability, prioritize this remediation immediately.

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, < 6.0.8>= 6.1.0, < 6.1.3>= 6.2.0, < 6.2.7>= 6.3.0, < 6.3.17>= 6.4.0, <= 6.4.2

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 installation and version
    Run `get system status` via FortiWeb CLI or check the System -> FortiView -> Dashboard in the GUI to view the firmware version
    Affected if The displayed version falls within any of these ranges: 5.6.0 to 6.0.7, 6.1.0 to 6.1.2, 6.2.0 to 6.2.6, 6.3.0 to 6.3.16, or 6.4.0 to 6.4.2
  2. Confirm HTTP/HTTPS management interface is exposed
    Run `show system interface` in CLI or check Network -> Interfaces in GUI to list all configured network interfaces and their HTTP/HTTPS service bindings
    Affected if An interface has HTTP (port 80) or HTTPS (port 443) service enabled and is accessible from untrusted networks

You are affected if your FortiWeb version matches one of the affected ranges AND the proxy daemon is enabled AND HTTP/HTTPS services are exposed to network traffic.

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 6.0.8 / 6.1.3 / 6.2.7 or later
Fixed in 6.0.86.1.36.2.7
Interim mitigation

Upgrade FortiWeb to the latest patched version as specified in Fortinet's security advisory. Given the critical CVSS score (9.8) and remote code execution capability, prioritize this remediation immediately.

Recommended fix High confidence

6.0.8+ for 6.0.x; 6.1.3+ for 6.1.x; 6.2.7+ for 6.2.x; 6.3.17+ for 6.3.x; 6.4.7+ for 6.4.x (or 5.x should migrate to 6.0.8+)

  1. 1. Identify the current FortiWeb version by checking System > Status in the FortiWeb GUI or running 'get system status' in the CLI
  2. 2. Determine which version branch your current installation belongs to (5.x, 6.0.x, 6.1.x, 6.2.x, 6.3.x, or 6.4.x)
  3. 3. Download the appropriate fixed version from Fortinet's support portal: for 5.x branch upgrade to 6.0.8 or later; for 6.0.x upgrade to 6.0.8; for 6.1.x upgrade to 6.1.3; for 6.2.x upgrade to 6.2.7; for 6.3.x upgrade to 6.3.17; for 6.4.x upgrade to 6.4.7 or later
  4. 4. Backup the FortiWeb configuration via System > Maintenance > Backup & Restore in the GUI or 'execute backup full-config' in the CLI
  5. 5. Upload and install the firmware upgrade via System > Firmware in the GUI or 'execute system firmware upgrade' in the CLI
  6. 6. After reboot, verify the upgrade was successful and confirm the new version via 'get system status'
  7. 7. Test that FortiWeb proxy functionality operates normally
Caveat Review FortiWeb release notes for the target version for any configuration or feature changes that may affect your deployment; always test in a staging environment first

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

Fix this in Fortiweb Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing8.0 h
  • Review / QA4.0 h
24.0 hours of engineering $4,160
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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-2021-42756 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