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Ios XrOperating system · Cisco

CVE-2021-1389

MEDIUM · 6.5 CVSS v3.1 Published 2021-02-04
Fix available
A fix is available. Upgrade to 6.6.3 or later.
See remediation →
74/100
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 the IPv6 traffic processing of Cisco IOS XR Software and Cisco NX-OS Software for certain Cisco devices could allow an unauthenticated, remote attacker to bypass an IPv6 access control list (ACL) that is configured for an interface of an affected device. The vulnerability is due to improper processing of IPv6 traffic that is sent through an affected device. An attacker could exploit this vulnerability by sending crafted IPv6 packets that traverse the affected device. A successful exploit could allow the attacker to access resources that would typically be protected by the interface ACL.

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 IPv6 traffic processing in Cisco IOS XR and NX-OS Software allows an unauthenticated remote attacker to bypass IPv6 ACLs configured on device interfaces. The flaw is in how the system processes crafted IPv6 packets traversing the affected device, enabling access to resources that should be protected by the configured ACL.

MitigationApply Cisco security updates for IOS XR and NX-OS when available, and review IPv6 ACL configurations on affected devices to add additional filtering layers as a compensating control.

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
Ios XrOperating system
Affected:< 6.6.3= 7.1.0= 7.2.0
Nx OsOperating system
Affected:all versions

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
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

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

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 device OS and version
    Execute 'show version' on the device CLI to determine if it runs Cisco IOS XR or NX-OS and record the exact software version string
    Affected if Device runs Cisco IOS XR version less than 6.6.3, exactly 7.1.0, exactly 7.2.0, or any version of Cisco NX-OS
  2. Verify IPv6 is enabled on the device
    Execute 'show ipv6 interface brief' or review the running configuration for 'ipv6 enable' or 'ipv6 unicast-routing' statements
    Affected if IPv6 is enabled on any interface or globally on the device
  3. Confirm IPv6 ACLs are configured
    Execute 'show ipv6 access-list' to list all IPv6 access control lists configured on the device, or review configuration mode for 'ipv6 access-list' entries
    Affected if Any IPv6 ACLs are defined on the device (the vulnerability allows these to be bypassed)
  4. Identify interfaces where IPv6 ACLs are applied
    Execute 'show ipv6 interface' to display which interfaces have IPv6 ACLs applied in the inbound or outbound direction
    Affected if IPv6 ACLs are applied to any interface and the device meets version criteria from step 1

The device is affected if it runs Cisco IOS XR (versions listed) or any NX-OS version, has IPv6 enabled, and has IPv6 ACLs applied to interfaces, allowing crafted IPv6 packets to bypass configured access controls.

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
Upgrade available Upgrade to 6.6.3 or later
Fixed in 6.6.3
Interim mitigation

Apply Cisco security updates for IOS XR and NX-OS when available, and review IPv6 ACL configurations on affected devices to add additional filtering layers as a compensating control.

Fix this in Ios Xr Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,330
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Scan for this in your stack

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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-2021-1389 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