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Ios XeOperating system · Cisco

CVE-2021-34696

MEDIUM · 5.8 CVSS v3.1 Published 2021-09-23
Fix available
A fix is available. Upgrade to after 17.3.2 or later.
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 the access control list (ACL) programming of Cisco ASR 900 and ASR 920 Series Aggregation Services Routers could allow an unauthenticated, remote attacker to bypass a configured ACL. This vulnerability is due to incorrect programming of hardware when an ACL is configured using a method other than the configuration CLI. An attacker could exploit this vulnerability by attempting to send traffic through an affected device. A successful exploit could allow the attacker to bypass an ACL on the affected device.

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 ACL programming logic of Cisco ASR 900 and ASR 920 Series routers where hardware is incorrectly programmed when ACLs are configured using methods other than the configuration CLI (e.g., REST, NETCONF, or other management interfaces). This allows an unauthenticated remote attacker to bypass configured ACLs by sending traffic through the affected device.

MitigationConfigure ACLs exclusively using the CLI method as specified in Cisco documentation; apply vendor-provided software updates when released. Verify existing ACL configurations were applied via CLI and reapply if necessary.

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 XeOperating system
Affected:<= 17.3.2

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
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/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. Confirm device model
    Execute 'show version' or 'show inventory' command on the router to identify the hardware model
    Affected if Device is ASR 900 Series or ASR 920 Series router
  2. Check IOS XE version
    Execute 'show version' and locate the software version number in the output
    Affected if IOS XE version is 17.3.2 or earlier (any version up to and including 17.3.2)
  3. Identify ACL configuration methods
    Review configuration history, management logs, or use 'show running-config' to examine ACL entries; determine if ACLs were applied via REST API, NETCONF, or other programmatic interfaces rather than the CLI
    Affected if Any ACLs on the device were configured using REST, NETCONF, or non-CLI management interfaces
  4. Verify ACL enforcement
    Test ACL functionality by sending test traffic that should be blocked or permitted according to the configured ACL rules
    Affected if Traffic passes through the device that should be denied by the configured ACLs, indicating the ACL bypass is active

User is affected if the device is an ASR 900/920 running IOS XE version 17.3.2 or earlier and has ACLs configured via REST, NETCONF, or other non-CLI interfaces.

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 a release after 17.3.2
Interim mitigation

Configure ACLs exclusively using the CLI method as specified in Cisco documentation; apply vendor-provided software updates when released. Verify existing ACL configurations were applied via CLI and reapply if necessary.

Recommended fix Moderate confidence

Cisco IOS XE 17.4.1 or later (specific fixed version per Cisco Security Advisory)

  1. 1. Identify the current IOS XE version running on the Cisco ASR 900 or ASR 920 Series device using 'show version' command
  2. 2. Review the Cisco Security Advisory for CVE-2021-34696 on tools.cisco.com to identify the specific fixed release for your platform
  3. 3. Download the fixed IOS XE release from Cisco.com or your Cisco Smart Account
  4. 4. Back up the current device configuration using 'copy running-config startup-config' or 'copy running-config tftp://'
  5. 5. Upload the new IOS XE image to the device's flash storage
  6. 6. Configure the device to boot from the new image using 'boot system flash:<new-image-name>'
  7. 7. Reload the device using 'reload' command
  8. 8. After the device restarts, verify the new IOS XE version is running with 'show version'
Caveat Standard IOS XE upgrade risks apply - review release notes for any command changes or migration requirements

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

Fix this in Ios Xe 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-2021-34696 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