RouterHardware / appliance · Cisco
CVE-1999-0843
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 · uneditedDenial of service in Cisco routers running NAT via a PORT command from an FTP client to a Telnet port.
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 confidenceDenial of service vulnerability in Cisco routers running Network Address Translation (NAT). The issue is triggered when an FTP client's PORT command is sent to a Telnet port (port 23) instead of the standard FTP port, causing the NAT function to fail or crash the router.
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
RouterHardware / appliance
all versionsCVSS 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
- Authentication
- None
- Confidentiality
- None
- Integrity
- None
- Availability
- Partial
AV:N/AC:L/Au:N/C:N/I:N/A:P
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 is a Cisco routerCheck the device model by running 'show version' or checking the physical device labelingAffected if The device is a Cisco router model (such as 2500, 2600, 3600, 4000, 7000 series or other Cisco router platforms)
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Verify NAT is enabled on the routerRun 'show ip nat statistics' or 'show ip nat translations' in privileged exec mode to see if NAT is activeAffected if NAT is configured and active (entries appear in the NAT translation table or statistics show active translations)
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Check NAT configuration for FTP handlingRun 'show running-config | include nat' or 'show ip nat configuration' to review NAT translation rules and inspect for any FTP-specific NAT mappingsAffected if NAT is configured to handle FTP traffic (either through port-based NAT rules or interface-level NAT)
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Identify exposed NAT-enabled interfacesReview interface configurations with 'show ip interface' and check which interfaces have NAT inside/outside statements configuredAffected if Any interface is configured as 'ip nat inside' or 'ip nat outside', making it potentially reachable by the malformed traffic
The environment is affected if the Cisco router has NAT enabled on any interface, since the vulnerability triggers when PORT commands from FTP clients are sent to port 23 instead of the standard FTP port.
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 dataUpdate Cisco IOS to a version that includes the fix for this vulnerability, or if the affected routers are end-of-life legacy equipment, replace them with modern routers that do not have this vulnerability. Ensure proper firewall rules are in place to prevent malformed FTP traffic from reaching NAT-enabled interfaces.
Fix this in Router
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 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-1999-0843 — 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-1999-0843 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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