CVE-2023-39240
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 · uneditedIt is identified a format string vulnerability in ASUS RT-AX56U V2’s iperf client function API. This vulnerability is caused by lacking validation for a specific value within its set_iperf3_cli.cgi module. A remote attacker with administrator privilege can exploit this vulnerability to perform remote arbitrary code execution, arbitrary system operation or disrupt service.
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 · moderate confidenceA format string vulnerability exists in the iperf client function API of ASUS RT-AX56U V2 routers, specifically within the set_iperf3_cli.cgi module. The vulnerability stems from insufficient validation of a specific input value, allowing a remote attacker with administrator privileges to achieve remote code execution, perform arbitrary system operations, or cause denial of service.
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= 3.0.0.4.386_50460= 3.0.0.4.386_50460= 3.0.0.4_386_51529CVSS 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
- High
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:H/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 checksWork through these to decide whether this CVE applies to you.
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Identify router modelAccess the router administration interface and navigate to the System Log or Device Info page to confirm the exact model number (RT-AX55, RT-AX56U V2, or RT-AC86U)Affected if Model is not one of RT-AX55, RT-AX56U V2, or RT-AC86U - the vulnerability does not apply to other models
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Check firmware versionIn the router admin interface, go to Administration > Firmware Upgrade or System > Firmware Version to view the installed firmware build numberAffected if Firmware version is exactly 3.0.0.4.386_50460 (for RT-AX55 or RT-AX56U V2) or 3.0.0.4_386_51529 (for RT-AC86U) - other versions are not affected by this specific CVE
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Verify iperf3 client API is accessibleAttempt to access the set_iperf3_cli.cgi endpoint on the router (typically at http://router-ip/cgi-bin/set_iperf3_cli.cgi) or check if the iperf3 client function appears in the router's web interface under network toolsAffected if The iperf3 client feature and its CGI module are present and accessible - the vulnerability only exists when this module is available
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Confirm administrator access existsVerify current administrative credentials are valid by logging into the router web interface or checking for active administrator sessionsAffected if Attacker has valid administrator credentials - the vulnerability requires administrator privileges to be exploited
The environment is affected only if the router model is RT-AX55, RT-AX56U V2, or RT-AC86U AND the firmware version exactly matches the vulnerable builds AND the iperf3 client API module is present AND the attacker has administrator credentials.
Generated from the published advisory. Verify against your own configuration.
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.
From vendor dataApply the vendor-supplied firmware patch when available. Until then, strictly limit administrative access to trusted users only and consider network segmentation to reduce attack surface.
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2023-39240 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2023-39240 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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