Fastify aws-lambda 6.4.0 improper authorization

Summaryinfo

A vulnerability has been found in Fastify aws-lambda 6.4.0 and classified as critical. Affected by this vulnerability is an unknown functionality. Performing a manipulation results in improper authorization. This vulnerability is known as CVE-2026-18248. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability classified as critical has been found in Fastify aws-lambda 6.4.0. This affects an unknown part. The manipulation with an unknown input leads to a improper authorization vulnerability. CWE is classifying the issue as CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

@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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-18248 since 07/29/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1548.002 for this issue.

Upgrading to version 6.4.1 eliminates this vulnerability.

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Productinfo

Vendor

• Fastify

Name

• aws-lambda

Version

• 6.4.0

CPE 2.3info

• 🔒

CPE 2.2info

• 🔒

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