# CVE-2026-18248: @fastify/aws-lambda vulnerable to Lambda event spoofing via client-controlled x-apigateway-event header

> Attackers can forge Lambda events via headers to bypass authentication and escalate privileges in applications using the affected Fastify adapter.

- **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:** 3 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** GHSA-m93c-jj3f-68ph

## What is CVE-2026-18248?

CVE-2026-18248 is a critical authentication bypass vulnerability in @fastify/aws-lambda, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

@fastify/aws-lambda version 6.4.0 decorates each Fastify request with request.awsLambda.event and request.awsLambda.context, values that applications are documented to use for authorization decisions such as reading API Gateway authorizer claims. In the default configuration, the getter that populates this decoration reads the client-controlled x-apigateway-event and x-apigateway-context HTTP headers before falling back to the trusted internal request token, and those reserved headers are not stripped from the incoming event. An unauthenticated attacker who can set a single HTTP header can therefore forge the entire Lambda proxy event, including the authorizer context, and override the genuine one. This results in a full authentication and authorization bypass and privilege escalation for any application that trusts request.awsLambda.event for identity or access control. Only version 6.4.0 is affected. Patches: upgrade to @fastify/aws-lambda 6.4.1, which resolves the decoration only through the internal per-invocation token and strips the reserved headers before the request is processed.

Weakness: CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-18248?

It affects 1 product.


Products: Fastify Fastify\/Aws-Lambda.

## How do I fix CVE-2026-18248?

No fix has been published for CVE-2026-18248 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-18248 being exploited?

There is no known exploitation of CVE-2026-18248 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18248 and fix it if it is affected.

What it is: CVE-2026-18248 is a critical authentication bypass vulnerability in @fastify/aws-lambda, published on 3 Aug 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-2026-18248 in the wild and no public exploit code.

Affected product: @fastify/aws-lambda. 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-2026-18248/ (Markdown: https://insights.sbomify.com/CVE-2026-18248/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18248
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18248
- OSV: https://osv.dev/vulnerability/CVE-2026-18248

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18248/
Summaries are generated and may contain mistakes; the official description above is the record.