CVE-2021-21727
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 · uneditedA ZTE product has a DoS vulnerability. A remote attacker can amplify traffic by sending carefully constructed IPv6 packets to the affected devices, which eventually leads to device denial of service. This affects:<ZXHN F623><All versions up to V6.0.0P3T33>
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 confidenceThe ZTE ZXHN F623 router has a denial-of-service vulnerability where remote attackers can amplify traffic by sending specially crafted IPv6 packets, causing device failure. This affects all firmware versions up to V6.0.0P3T33.
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< 6.0.0p3t34CVSS 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
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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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Confirm device model is ZXHN F623Log into the router web interface (typically http://192.168.1.1) and check the device info page, or access via CLI and run 'show version' or similar command to verify the model number displays as ZXHN F623Affected if Device is not a ZTE ZXHN F623 router
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Check firmware versionIn the router web interface, navigate to Status or Device Info section to view the firmware version; or via CLI run 'sysinfo' or 'show version' command. Compare the version string to the affected range (versions below 6.0.0p3t34)Affected if Firmware version is 6.0.0P3T33 or lower
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Verify IPv6 is enabledIn the router web interface, go to Network settings or WAN settings and check if IPv6 is enabled/configured; or via CLI run 'show ipv6 interface' or similar to list active IPv6 interfacesAffected if IPv6 is enabled on the router
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Check IPv6 network exposureFrom an external IPv6-enabled host, ping the router's public IPv6 address if assigned, or check router firewall/NAT settings to determine if the WAN IPv6 interface is reachable from the internetAffected if Router has a public IPv6 address and allows incoming IPv6 traffic from WAN
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Look for device stability issuesCheck router system logs for repeated crashes, reboots, or high CPU/memory usage; test by sending crafted IPv6 packets (if authorized) to observe device responseAffected if Device exhibits frequent crashes, reboots, or hangs coinciding with IPv6 traffic
The router is affected if it is a ZTE ZXHN F623 running firmware version 6.0.0P3T33 or lower, has IPv6 enabled, and has a publicly accessible IPv6 interface that could receive malicious amplification packets.
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 data6.0.0p3t34
Apply firmware update if available from ZTE; otherwise implement IPv6 traffic filtering or rate limiting at network edge to block malicious amplification packets.
- 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-2021-21727 — 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-2021-21727 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
- No weaponised exploit code, or anything meant to cause harm
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