# CVE-2021-34737: Cisco IOS XR Software DHCP Version 4 Server Denial of Service Vulnerability

> Remote attackers can crash the DHCPv4 server process via malformed messages, causing temporary service unavailability on Cisco IOS XR devices.

- **Severity:** Medium 5.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 9 Sep 2021
- **Updated:** 7 Nov 2024
- **Also known as:** GHSA-x6m9-rgr7-fm6p

## What is CVE-2021-34737?

CVE-2021-34737 is a medium-severity vulnerability in Cisco Ios Xr, published on 9 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability in the DHCP version 4 (DHCPv4) server feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to trigger a crash of the dhcpd process, resulting in a denial of service (DoS) condition. This vulnerability exists because certain DHCPv4 messages are improperly validated when they are processed by an affected device. An attacker could exploit this vulnerability by sending a malformed DHCPv4 message to an affected device. A successful exploit could allow the attacker to cause a NULL pointer dereference, resulting in a crash of the dhcpd process. While the dhcpd process is restarting, which may take up to approximately two minutes, DHCPv4 server services are unavailable on the affected device. This could temporarily prevent network access to clients that join the network during that time period. Note: Only the dhcpd process crashes and eventually restarts automatically. The router does not reload.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2021-34737?

It affects 1 product.


Products: Cisco Ios Xr.

## How do I fix CVE-2021-34737?

No fix has been published for CVE-2021-34737 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-34737 being exploited?

There is no known exploitation of CVE-2021-34737 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Ios Xr

- [CVE-2020-3118](https://insights.sbomify.com/CVE-2020-3118/): Cisco IOS XR Software Discovery Protocol Format String (high 8.8, exploited in the wild)
- [CVE-2020-3569](https://insights.sbomify.com/CVE-2020-3569/): Cisco IOS XR Software DVMRP Memory Exhaustion (high 8.6, exploited in the wild)
- [CVE-2020-3566](https://insights.sbomify.com/CVE-2020-3566/): Cisco IOS XR Software DVMRP Memory Exhaustion (high 8.6, exploited in the wild)
- [CVE-2010-3035](https://insights.sbomify.com/CVE-2010-3035/): Cisco IOS XR Border Gateway Protocol (BGP) Denial-of-Service (high 7.5, exploited in the wild)
- [CVE-2022-20821](https://insights.sbomify.com/CVE-2022-20821/): Cisco IOS XR Open Port (medium 6.5, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34737 and fix it if it is affected.

What it is: CVE-2021-34737 is a medium-severity vulnerability in Cisco Ios Xr, published on 9 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-34737 in the wild and no public exploit code.

Affected product: Cisco Ios Xr. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-34737/ (Markdown: https://insights.sbomify.com/CVE-2021-34737/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34737
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34737
- OSV: https://osv.dev/vulnerability/CVE-2021-34737

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34737/
Summaries are generated and may contain mistakes; the official description above is the record.