iOSOperating system · Cisco
CVE-2020-3408
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 vulnerability in the Split DNS feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition. The vulnerability occurs because the regular expression (regex) engine that is used with the Split DNS feature of affected releases may time out when it processes the DNS name list configuration. An attacker could exploit this vulnerability by trying to resolve an address or hostname that the affected device handles. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.
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 confidenceA vulnerability in the Split DNS feature of Cisco IOS and IOS XE Software allows an unauthenticated remote attacker to cause device reload (DoS). The regex engine used to process DNS name lists in the Split DNS feature can time out when processing specially crafted DNS resolution requests, triggering the reload.
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
iOSOperating system
= 15.8\(3\)m3
Ios XeOperating system
= 15.8\(3\)m3CVSS 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
- Changed
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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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Identify the device OS and versionExecute 'show version' on the device CLI and locate the software version line (e.g., 'Version 15.8(3)m3')Affected if The version is Cisco iOS or IOS XE version 15.8(3)m3 or any earlier 15.8.x version
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Confirm the device runs Cisco IOS or IOS XEExecute 'show version' and verify the software image name contains 'IOS' or 'IOS-XE' in the descriptionAffected if The device runs Cisco IOS or IOS XE software
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Check if Split DNS is configuredExecute 'show running-config | include split-dns' or look for 'ip dns split' commands in the configurationAffected if Split DNS feature is enabled with any DNS name list configuration present
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Verify DNS name list configuration existsExecute 'show running-config | include dns' or search for 'ip dns name-list' commands in the configurationAffected if A DNS name list (regex-based) is configured under the Split DNS feature
The device is affected if it runs Cisco iOS or IOS XE version 15.8(3)m3 or earlier, and has Split DNS enabled with a DNS name list configured, as the vulnerability triggers when the regex engine processes crafted DNS requests against that list.
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 dataUpgrade to a Cisco IOS/IOS XE software version that includes the fix for this vulnerability. As an interim measure, review and minimize the DNS name list configuration in Split DNS to reduce regex processing complexity.
Fix this in iOS
Scoped from the published advisory
- Consultation8.0 h
- Implementation4.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,784.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-3408 — 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-2020-3408 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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- Version or environment caveats, and links to real fixes
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