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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 17 Mar 2022.
iOSOperating system · Cisco

CVE-2018-0173

HIGH · 8.6 CVSS v3.1 Published 2018-03-28
Fix available
A fix is available. Upgrade to after 15.2 or later.
See remediation →
100/100
Remediation priority · Urgent
In the wild 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 Cisco IOS Software and Cisco IOS XE Software function that restores encapsulated option 82 information in DHCP Version 4 (DHCPv4) packets could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a Relay Reply denial of service (DoS) condition. The vulnerability exists because the affected software performs incomplete input validation of encapsulated option 82 information that it receives in DHCPOFFER messages from DHCPv4 servers. An attacker could exploit this vulnerability by sending a crafted DHCPv4 packet to an affected device, which the device would then forward to a DHCPv4 server. When the affected software processes the option 82 information that is encapsulated in the response from the server, an error could occur. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition. Cisco Bug IDs: CSCvg62754.

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/IOS XE DHCPv4 relay function allows an unauthenticated remote attacker to cause device reload by sending a crafted DHCPv4 packet. The device forwards the packet to a DHCPv4 server, which responds with a DHCPOFFER containing encapsulated option 82 information. The affected software performs incomplete input validation on this encapsulated option 82 data, causing an error that triggers a DoS via device reload.

MitigationApply the Cisco software update addressing CVE-2018-0173 to affected Cisco IOS/IOS XE devices. Until patched, monitor for suspicious DHCPv4 traffic targeting the relay function.

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
iOSOperating system
Affected:= denali-16.3.4<= 15.2\(6\)e0a<= 15.2\(4a\)ea5
Ios XeOperating system
Affected:= denali-16.3.4<= 15.2\(6\)e0a<= 15.2\(4a\)ea5

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/IOS XE version
    Run 'show version' or 'show version | include IOS' to display the software version. Look for the release number in the format like 15.2(x) or 16.3(x).
    Affected if The version matches 'denali-16.3.4', or is 15.2(6)e0a or earlier, or is 15.2(4a)ea5 or earlier.
  2. Verify DHCPv4 relay is enabled globally
    Run 'show ip dhep relay' to check if DHCP relay is configured. Alternatively, check the running configuration for 'ip helper-address' under interface configurations.
    Affected if DHCPv4 relay is globally enabled or 'ip helper-address' commands exist on any interface.
  3. Confirm DHCP option 82 handling is active
    Run 'show ip dhcp relay information option' to check if option 82 insertion is enabled. Also check for 'ip dhcp relay information option' in the running configuration.
    Affected if Option 82 insertion or relay-agent information option is enabled in the DHCP relay configuration.
  4. Check for DHCP relay on specific interfaces
    Run 'show ip interface' on relevant interfaces and look for 'DHCP Relay' or verify 'ip helper-address' is configured on VLANs or physical interfaces connected to DHCP clients.
    Affected if The device is acting as a DHCP relay agent for client-facing networks.

The device is affected if it runs a vulnerable Cisco IOS/IOS XE version (as listed in the affected versions) AND has DHCPv4 relay with option 82 enabled, allowing it to process DHCPv4 traffic from untrusted networks.

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 15.2
Interim mitigation

Apply the Cisco software update addressing CVE-2018-0173 to affected Cisco IOS/IOS XE devices. Until patched, monitor for suspicious DHCPv4 traffic targeting the relay function.

Recommended fix Moderate confidence

Cisco IOS/IOS XE fixed releases as specified in the CSCvg62754 security advisory (fixed releases vary by platform; consult tools.cisco.com for your specific hardware)

  1. 1. Identify the exact Cisco IOS or IOS XE version currently running on the device using 'show version'
  2. 2. Access the Cisco Security Advisory for CVE-2018-0173 (Cisco Bug ID CSCvg62754) at tools.cisco.com to obtain the specific fixed releases for your platform
  3. 3. Download the appropriate fixed Cisco IOS or IOS XE software release from the Cisco Software Download Center
  4. 4. Verify the fixed release is supported on your specific hardware platform
  5. 5. Upload the new software image to the device using TFTP, FTP, or SCP
  6. 6. Configure boot system commands to point to the new image
  7. 7. Reload the device with 'reload' command to apply the new image
  8. 8. After reload, verify the new version is running with 'show version' and confirm the DHCP relay functionality is operational
Caveat Review Cisco release notes for the target version for any migration caveats, command syntax changes, or feature modifications

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

Fix this in iOS Exploited in the wild — priority engagement
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
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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-2018-0173 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