# CVE-2026-20171: Cisco Nexus 3000 and 9000 Series Border Gateway Protocol Denial of Service Vulnerability

> Remote attackers can cause denial of service on Cisco Nexus switches by sending crafted BGP updates when enforce-first-as is enabled.

- **Severity:** Medium 6.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 20 May 2026
- **Updated:** 20 May 2026
- **Also known as:** GHSA-h75f-8f3x-9m8c

## What is CVE-2026-20171?

CVE-2026-20171 is a medium-severity denial of service vulnerability in Cisco NX-OS Software, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability in the Border Gateway Protocol (BGP) enforce-first-as feature of Cisco Nexus 3000 Series Switches and Cisco Nexus 9000 Series Switches in standalone NX-OS mode could allow an unauthenticated, remote attacker to trigger BGP peer flaps, resulting in a denial of service (DoS) condition.

This vulnerability is due to incorrect parsing of a transitive BGP attribute. An attacker could exploit this vulnerability by sending a crafted BGP update through an established BGP peer session. If the update propagates to an affected device, it could cause the device to drop the BGP session and flap with the BGP peer that is forwarding this update, resulting in a DoS condition.

Weakness: CWE-670 (Always-Incorrect Control Flow Implementation)

## Which versions are affected by CVE-2026-20171?


## How do I fix CVE-2026-20171?

No fix has been published for CVE-2026-20171 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-20171 being exploited?

There is no known exploitation of CVE-2026-20171 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-20171 and fix it if it is affected.

What it is: CVE-2026-20171 is a medium-severity denial of service vulnerability in Cisco NX-OS Software, published on 20 May 2026. 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-2026-20171 in the wild and no public exploit code.

Affected product: Cisco NX-OS Software. 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-2026-20171/ (Markdown: https://insights.sbomify.com/CVE-2026-20171/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-20171
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20171
- OSV: https://osv.dev/vulnerability/CVE-2026-20171

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-20171/
Summaries are generated and may contain mistakes; the official description above is the record.