dbcve.org
Ios XrOperating system · Cisco

CVE-2021-34720

HIGH · 8.6 CVSS v3.1 Published 2021-09-09
Fix available
A fix is available. Upgrade to 6.2.3 / 6.3.2 or later.
See remediation →
95/100
Remediation priority · Urgent
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 IP Service Level Agreements (IP SLA) responder and Two-Way Active Measurement Protocol (TWAMP) features of Cisco IOS XR Software could allow an unauthenticated, remote attacker to cause device packet memory to become exhausted or cause the IP SLA process to crash, resulting in a denial of service (DoS) condition. This vulnerability exists because socket creation failures are mishandled during the IP SLA and TWAMP processes. An attacker could exploit this vulnerability by sending specific IP SLA or TWAMP packets to an affected device. A successful exploit could allow the attacker to exhaust the packet memory, which will impact other processes, such as routing protocols, or crash the IP SLA process.

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 Cisco IOS XR Software's IP SLA and TWAMP features allows an unauthenticated remote attacker to cause packet memory exhaustion or crash the IP SLA process. The root cause is mishandling of socket creation failures during IP SLA and TWAMP packet processing, leading to denial of service that can impact other processes like routing protocols.

MitigationApply the Cisco IOS XR software update addressing this vulnerability. If unable to patch immediately, consider disabling IP SLA and TWAMP features if not required, or implement network-level filtering to restrict unauthorized IP SLA/TWAMP 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
Ios XrOperating system
Affected:< 6.2.3>= 6.3.0, < 6.3.2>= 6.5.0, < 7.2.2

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
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 checks

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

  1. Identify the Cisco IOS XR version
    Run 'show version' or 'admin show version' from the CLI to display the installed software version
    Affected if The displayed version falls into any of these ranges: < 6.2.3, >= 6.3.0 but < 6.3.2, or >= 6.5.0 but < 7.2.2
  2. Determine if IP SLA is configured
    Run 'show ip sla summary' or 'show ip sla configuration' to check for active IP SLA configurations
    Affected if Any IP SLA operations are configured and active on the device
  3. Determine if TWAMP is configured
    Run 'show twamp session' or 'show twamp server session' to check for active TWAMP sessions
    Affected if TWAMP server or reflector sessions are configured and active

The device is affected if it runs a vulnerable Cisco IOS XR version (within the ranges listed) AND has IP SLA or TWAMP features actively configured or enabled.

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 6.2.3 / 6.3.2 / 7.2.2 or later
Fixed in 6.2.36.3.27.2.2
Interim mitigation

Apply the Cisco IOS XR software update addressing this vulnerability. If unable to patch immediately, consider disabling IP SLA and TWAMP features if not required, or implement network-level filtering to restrict unauthorized IP SLA/TWAMP traffic.

Recommended fix High confidence

Minimum fixed versions: 6.2.3 (for 6.1.x and earlier), 6.3.2 (for 6.3.0-6.3.1), 7.2.2 (for 6.5.0-7.2.1); or migrate to latest stable 7.x release

  1. 1. Identify the current Cisco IOS XR version running on the affected device using 'show version' command
  2. 2. Based on the current version branch, plan upgrade to the minimum fixed release: For versions < 6.2.3, upgrade to 6.2.3 or later; For versions >= 6.3.0 and < 6.3.2, upgrade to 6.3.2 or later; For versions >= 6.5.0 and < 7.2.2, upgrade to 7.2.2 or later
  3. 3. Download the appropriate Cisco IOS XR software package from Cisco's software download center (tools.cisco.com) - ensure the image is compatible with your specific hardware platform
  4. 4. Before upgrade, backup the current configuration using 'admin commit backup' or 'copy running-config bootflash:'
  5. 5. Install the upgrade following standard Cisco IOS XR installation procedures, typically using 'install add source' and 'install activate' commands
  6. 6. After upgrade, verify the new version is running with 'show version' and confirm normal operation of IP SLA and TWAMP features
  7. 7. Monitor device memory usage and IP SLA process stability to confirm the vulnerability is remediated
Caveat Cisco IOS XR upgrades may require careful compatibility checking with hardware platform and may introduce command syntax changes; always review release notes before upgrading

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

Fix this in Ios Xr Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2021-34720 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