dbcve.org
Ios XrOperating system · Cisco

CVE-2021-34713

HIGH · 7.4 CVSS v3.1 Published 2021-09-09
Fix available
A fix is available. Upgrade to 6.6.3 / 6.7.1 or later.
See remediation →
79/100
Remediation priority · High
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 Layer 2 punt code of Cisco IOS XR Software running on Cisco ASR 9000 Series Aggregation Services Routers could allow an unauthenticated, adjacent attacker to cause the affected line card to reboot. This vulnerability is due to incorrect handling of specific Ethernet frames that cause a spin loop that can make the network processors unresponsive. An attacker could exploit this vulnerability by sending specific types of Ethernet frames on the segment where the affected line cards are attached. A successful exploit could allow the attacker to cause the affected line card to reboot.

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 Layer 2 punt code of Cisco IOS XR Software on ASR 9000 Series routers allows an adjacent, unauthenticated attacker to cause line card reboots by sending specific Ethernet frames. The flaw causes a spin loop in network processors due to incorrect handling of these frames, making them unresponsive.

MitigationApply the Cisco IOS XR Software update (refer to Cisco PSIRT advisory) when available. Until then, limit physical access to network segments and implement port-level access controls to prevent unauthorized adjacent attackers from reaching affected line cards.

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:>= 6.4, < 6.6.3>= 6.7, < 6.7.1>= 7.0, < 7.0.2>= 7.1, < 7.1.1

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify hardware platform
    Run 'show platform' or 'show version' on the Cisco IOS XR device to confirm it is an ASR 9000 Series router
    Affected if The device is not an ASR 9000 Series router (the vulnerability only affects this platform)
  2. Check IOS XR software version
    Run 'show version' to display the installed Cisco IOS XR software version number
    Affected if The version falls within these ranges: >= 6.4 and < 6.6.3, OR >= 6.7 and < 6.7.1, OR >= 7.0 and < 7.0.2, OR >= 7.1 and < 7.1.1
  3. Verify Layer 2 punt functionality is active
    Run 'show l2vpn ethernet-segment' or 'show l2vpn forwarding' to check if Layer 2 Ethernet forwarding/punt processing is configured and active on line cards
    Affected if Layer 2 punt code is enabled and processing Ethernet frames on line cards (the vulnerability exists in this code path)
  4. Confirm line card type and status
    Run 'show location' or 'show platform' to list line cards and their operational status
    Affected if Affected line cards are online and processing traffic (vulnerable line cards can be crashed via the crafted frames)

The environment is affected if the device is an ASR 9000 Series router running a Cisco IOS XR version within the affected ranges AND Layer 2 punt/forwarding functionality is active on line cards.

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.

dbcve · scoped
Upgrade available Upgrade to 6.6.3 / 6.7.1 / 7.0.2 or later
Fixed in 6.6.36.7.17.0.2
Interim mitigation

Apply the Cisco IOS XR Software update (refer to Cisco PSIRT advisory) when available. Until then, limit physical access to network segments and implement port-level access controls to prevent unauthorized adjacent attackers from reaching affected line cards.

Recommended fix High confidence

Upgrade to IOS XR 6.6.3, 6.7.1, 7.0.2, 7.1.1 or later (choose the appropriate version in your release train)

  1. 1. Identify the current Cisco IOS XR version running on the ASR 9000 Series Router using 'show version' command
  2. 2. Determine which version branch your current release belongs to (6.4.x, 6.7.x, 7.0.x, or 7.1.x)
  3. 3. Download the corresponding fixed release from Cisco's software download center: 6.6.3 or later for 6.4.x branches, 6.7.1 for 6.7.x, 7.0.2 for 7.0.x, or 7.1.1 for 7.1.x
  4. 4. Plan maintenance window as upgrade will require line card reboot
  5. 5. Follow Cisco IOS XR upgrade procedures using 'install' command or ROMMON bootloader method per Cisco documentation
  6. 6. After upgrade, verify the new version is running with 'show version'
  7. 7. Confirm vulnerability is resolved by checking 'show install active' and verifying fixed package versions
Caveat Standard IOS XR upgrade risks apply - ensure configuration backup, check hardware compatibility, and test in lab environment before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Ios Xr Scoped from the published advisory
  • Consultation6.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,980
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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-2021-34713 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