# CVE-2026-92395: @fastify/proxy-addr vulnerable to IP spoofing via IPv4-mapped IPv6 trust subnet

> Attackers can spoof IP addresses by exploiting IPv4-mapped IPv6 trust subnet parsing, bypassing IP-based access controls and rate limiting in Fastify applications.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** GHSA-8cmm-mhw6-v7xq

## What is CVE-2026-92395?

CVE-2026-92395 is a critical vulnerability in @fastify/proxy-addr, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

@fastify/proxy-addr is a Fastify plugin that determines a request's client address behind trusted reverse proxies, and it backs Fastify request.ip and request.ips. In versions 3.0.0 through 5.1.0, a trust subnet written in IPv4-mapped IPv6 notation with an IPv4-sized prefix, such as ::ffff:10.0.0.0/8 instead of the correct ::ffff:10.0.0.0/104, is accepted without error but trusts every IPv4 address on the internet rather than the block it names. Because the socket peer then becomes trusted at hop 0, any unauthenticated client can supply an arbitrary X-Forwarded-For header and control the address the application reads, which defeats IP-based access control, rate limiting, geolocation, and audit logging. The plugin inherited this defect from the upstream proxy-addr module (CVE-2026-90711). The issue is fixed in @fastify/proxy-addr 5.1.1, and users should upgrade to 5.1.1 or later. As a workaround, ensure any IPv4-mapped IPv6 trust subnet uses a prefix length of at least 97, or express the range in plain IPv4 notation.

Weakness: CWE-697 (Incorrect Comparison), CWE-348 (Use of Less Trusted Source), CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2026-92395?


## How do I fix CVE-2026-92395?

No fix has been published for CVE-2026-92395 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-92395 being exploited?

There is no known exploitation of CVE-2026-92395 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-92395 and fix it if it is affected.

What it is: CVE-2026-92395 is a critical vulnerability in @fastify/proxy-addr, published on 16 Sep 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-2026-92395 in the wild and no public exploit code.

Affected product: @fastify/proxy-addr. 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-2026-92395/ (Markdown: https://insights.sbomify.com/CVE-2026-92395/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-92395
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-92395
- OSV: https://osv.dev/vulnerability/CVE-2026-92395

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-92395/
Summaries are generated and may contain mistakes; the official description above is the record.