dbcve.org
Ios XrOperating system · Cisco

CVE-2023-20135

HIGH · 7.0 CVSS v3.1 Published 2023-09-13
Fix available
A fix is available. Upgrade to 7.6 / 7.10.1 or later.
See remediation →
72/100
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
A vulnerability in Cisco IOS XR Software image verification checks could allow an authenticated, local attacker to execute arbitrary code on the underlying operating system. This vulnerability is due to a time-of-check, time-of-use (TOCTOU) race condition when an install query regarding an ISO image is performed during an install operation that uses an ISO image. An attacker could exploit this vulnerability by modifying an ISO image and then carrying out install requests in parallel. A successful exploit could allow the attacker to execute arbitrary code on an 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 · high confidence

A TOCTOU race condition in Cisco IOS XR Software's image verification allows an authenticated local attacker to modify an ISO image between verification and use during install operations, enabling arbitrary code execution on the underlying OS.

MitigationApply Cisco's security patches for IOS XR when available; restrict local authenticated access to devices and ensure only trusted, verified ISO images are used for install operations.

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:>= 7.5.2, < 7.6>= 7.7, < 7.10.1

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
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:L/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 IOS XR version
    Run 'show version' or 'admin show version' in the IOS XR CLI to display the installed software version
    Affected if The displayed version falls within 7.5.2 <= version < 7.6 OR 7.7 <= version < 7.10.1
  2. Check for recent install operations
    Review system logs and run 'admin show install request' to identify any recent ISO image install operations performed on the device
    Affected if Install operations using ISO images have been performed on an affected version
  3. Verify ISO image integrity
    Compare the MD5/SHA256 checksums of ISO images used in install operations against officially published Cisco checksums for those images
    Affected if ISO image checksums do not match Cisco's published values or show unexpected modifications
  4. Review authentication logs for local users
    Check system logs for indications of local authenticated user sessions, particularly those with access to install operations. Use 'show logging' and review authpriv messages
    Affected if Local authenticated users with install privileges have accessed the system on affected versions
  5. Audit installed packages and binaries
    Run 'admin show install active' to list currently active software packages and compare against expected baseline for your deployment
    Affected if Unexpected packages or modifications to the software baseline are detected

A system is affected if it runs Cisco IOS XR versions 7.5.2 through 7.5.x or 7.7 through 7.10.0 and has had ISO image install operations performed, with potential for exploitation if local authenticated access is available to an attacker.

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 7.6 / 7.10.1 or later
Fixed in 7.67.10.1
Interim mitigation

Apply Cisco's security patches for IOS XR when available; restrict local authenticated access to devices and ensure only trusted, verified ISO images are used for install operations.

Recommended fix High confidence

Upgrade to IOS XR 7.6.x (for 7.5.x versions) or 7.10.1 and later (for 7.7.x-7.10.x versions)

  1. 1. Identify the current Cisco IOS XR version by running 'show version'
  2. 2. Determine the appropriate upgrade path based on current version (7.5.x goes to 7.6.x; 7.7.x-7.10.x goes to 7.10.1 or later)
  3. 3. Download the appropriate Cisco IOS XR software image from Cisco.com or the Cisco software download portal
  4. 4. Transfer the new ISO image to the device using FTP, SCP, or USB storage
  5. 5. Add the package using 'install add source <path> <package-name>'
  6. 6. Activate the package using 'install activate <package-name>'
  7. 7. Commit the installation using 'install commit'
  8. 8. Reload the device using 'reload' to complete the upgrade
Caveat Review Cisco IOS XR release notes for any behavioral changes, command syntax updates, or compatibility considerations before upgrading

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
Get the upgrade done

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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-2023-20135 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