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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
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
Which versions are affected by CVE-2026-18248?
It affects 1 product.
Products include 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.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-18248 being exploited?
There is no known exploitation of CVE-2026-18248 in the wild and no public exploit code.
Sources
Also known as GHSA-m93c-jj3f-68ph.