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Ios XrOperating system · Cisco

CVE-2021-34709

MEDIUM · 6.4 CVSS v3.1 Published 2021-09-09
Fix available
A fix is available. Upgrade to 7.3.2 / 7.4.1 or later.
See remediation →
66/100
Remediation priority · Elevated
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
Multiple vulnerabilities in image verification checks of Cisco Network Convergence System (NCS) 540 Series Routers, only when running Cisco IOS XR NCS540L software images, and Cisco IOS XR Software for Cisco 8000 Series Routers could allow an authenticated, local attacker to execute arbitrary code on the underlying operating system. For more information about these vulnerabilities, see the Details section of this advisory.

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 · moderate confidence

Vulnerability in image verification checks on Cisco NCS 540 Series Routers (NCS540L software) and Cisco 8000 Series Routers running IOS XR allows an authenticated, local attacker to execute arbitrary code on the underlying operating system. The flaw likely involves improper bounds checking or input validation during the image verification process, enabling privilege escalation from authenticated user to code execution with elevated privileges.

MitigationApply Cisco IOS XR software updates addressing CVE-2021-34709 when available. Restrict physical and console access to affected devices to trusted personnel only, and enforce strong authentication controls.

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:< 7.3.2>= 7.4.0, < 7.4.1

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
Local
Complexity
High
Privileges
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/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 device model
    Run 'show inventory' or 'show platform' at the IOS XR CLI to confirm the hardware model. Look for NCS540, NCS540L, or Cisco 8000 Series identifiers.
    Affected if Device is not an NCS 540 Series or Cisco 8000 Series Router - this CVE does not apply to other Cisco platforms.
  2. Check IOS XR software version
    Run 'show version' or 'admin show version' at the IOS XR CLI to display the installed IOS XR software release number.
    Affected if Version cannot be determined or device does not run IOS XR software.
  3. Compare version against affected ranges
    Compare the displayed IOS XR version to the vulnerable ranges: any version below 7.3.2, or any version from 7.4.0 up to but not including 7.4.1. Versions 7.3.2 through 7.3.x and 7.4.1 and later are not affected.
    Affected if Installed version falls within < 7.3.2 OR >= 7.4.0 and < 7.4.1 - the device is running a vulnerable IOS XR release.
  4. Verify image verification is accessible
    Image verification is a built-in feature triggered during software installation or upgrade operations. Check if the device allows authenticated CLI access. Run 'show running-config | include snmp' or review AAA configuration to confirm local authentication is enabled.
    Affected if Attacker requires authenticated local access to the IOS XR CLI to exploit this vulnerability; isolated devices with no local user accounts may have reduced exposure but are still vulnerable if upgraded to a affected version.

A user is affected if they are running IOS XR on an NCS 540 or Cisco 8000 Series device at version 7.3.1 or earlier, or at version 7.4.0, and an authenticated local attacker can access the device CLI.

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 7.3.2 / 7.4.1 or later
Fixed in 7.3.27.4.1
Interim mitigation

Apply Cisco IOS XR software updates addressing CVE-2021-34709 when available. Restrict physical and console access to affected devices to trusted personnel only, and enforce strong authentication controls.

Recommended fix High confidence

Upgrade to IOS XR 7.3.2 or 7.4.1 or later

  1. Confirm current IOS XR version using 'show version' command
  2. Download the fixed IOS XR software image (7.3.2 or 7.4.1 or later) from Cisco.com software download center
  3. Verify software image integrity using Cisco provided MD5/SHA256 checksums
  4. Transfer the new image to the device's boot disk or network location
  5. Execute 'install add' to add the new software package to the device
  6. Execute 'install activate' to activate the new software version
  7. Execute 'install commit' to make the changes persistent across reboots
  8. Reboot the device to complete the upgrade process
Caveat Review Cisco IOS XR 7.3.2 and 7.4.1 release notes for any configuration or command changes before upgrading

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

Fix this in Ios Xr Scoped from the published advisory
  • Consultation8.0 h
  • Implementation12.0 h
  • Testing8.0 h
  • Review / QA4.0 h
32.0 hours of engineering $5,680
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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-2021-34709 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