dbcve.org
iOSOperating system · Cisco

CVE-2021-1377

MEDIUM · 5.8 CVSS v3.1 Published 2021-03-24
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
67/100
Remediation priority · Elevated
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
A vulnerability in Address Resolution Protocol (ARP) management of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to prevent an affected device from resolving ARP entries for legitimate hosts on the connected subnets. This vulnerability exists because ARP entries are mismanaged. An attacker could exploit this vulnerability by continuously sending traffic that results in incomplete ARP entries. A successful exploit could allow the attacker to cause ARP requests on the device to be unsuccessful for legitimate hosts, resulting in a denial of service (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 confidence

A vulnerability in the Address Resolution Protocol (ARP) management of Cisco IOS Software and Cisco IOS XE Software allows an unauthenticated, remote attacker to cause incomplete ARP entries by continuously sending specific traffic. This results in ARP requests failing for legitimate hosts, creating a denial of service condition where the device cannot resolve MAC addresses for valid hosts on connected subnets.

MitigationApply the appropriate Cisco IOS/IOS XE software patch or update as provided by Cisco's security advisory. If immediate patching is not possible, consider network segmentation or ACLs to limit exposure to untrusted sources sending the triggering traffic.

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
Affected:= 12.2\(6\)i1= 15.1\(3\)svr1= 15.1\(3\)svr2= 15.1\(3\)svr3= 15.1\(3\)svs= 15.1\(3\)svs1= 15.2\(2\)e6= 15.2\(2\)e7= 15.2\(2\)e7b= 15.2\(2\)e8= 15.2\(2\)e9= 15.2\(2\)e9a
Ios XeOperating system
Affected:= 3.6.6e= 3.6.7ae= 3.6.7be= 3.6.7e= 3.6.8e= 3.6.9ae= 3.6.9e= 3.6.10e= 3.7.5e= 3.8.4e= 3.8.5ae= 3.8.5e

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
Changed
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L

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 the Cisco IOS or IOS XE version
    Use the command 'show version' on the device CLI to display the installed software version
    Affected if The displayed version matches any of the affected versions listed in the CVE (e.g., 12.2(6)i1, 15.1(3)svr1, 15.2(2)e6 for IOS; 3.6.6e, 3.7.5e, 3.8.4e for IOS XE)
  2. Check for incomplete ARP entries
    Use the command 'show ip arp' or 'show arp' to view the ARP table and look for entries marked as incomplete or showing incomplete MAC address resolution
    Affected if There are multiple ARP entries showing incomplete resolution or missing MAC addresses for valid hosts on connected subnets
  3. Verify ARP table size and completeness
    Compare the number of ARP entries against the number of known hosts on connected subnets using 'show ip arp summary'
    Affected if The ARP table shows significantly fewer complete entries than expected for the known hosts on the network
  4. Check for ARP-related errors in logs
    Use 'show logging' or review syslog buffers for ARP-related error messages, incomplete entry warnings, or MAC resolution failures
    Affected if Logs contain repeated ARP resolution failures or incomplete entry warnings occurring frequently

You are affected if your device runs an affected Cisco IOS or IOS XE version AND you observe incomplete ARP entries or ARP resolution failures for legitimate hosts on connected subnets.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the appropriate Cisco IOS/IOS XE software patch or update as provided by Cisco's security advisory. If immediate patching is not possible, consider network segmentation or ACLs to limit exposure to untrusted sources sending the triggering traffic.

Fix this in iOS Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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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-2021-1377 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