CVE-2026-18248
Received Received - Intake

Authentication Bypass in Fastify AWS Lambda Plugin

Publication date: 2026-08-03

Last updated on: 2026-08-03

Assigner: openjs

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.

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Meta Information

Published
2026-08-03
Last Modified
2026-08-03
Generated
2026-08-03
AI Q&A
2026-08-03
EPSS Evaluated
N/A
NVD
CVE-2026-18248
EUVD
EUVD-2026-52329

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
fastify aws_lambda to 6.4.0 (inc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-345 The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

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Executive Summary

This vulnerability affects @fastify/aws-lambda version 6.4.0, where request decorations include client-controlled HTTP headers like x-apigateway-event and x-apigateway-context. An attacker can forge the Lambda proxy event, including authorizer context, by setting a single HTTP header. This bypasses authentication and authorization, allowing privilege escalation.

Impact Analysis

An unauthenticated attacker could gain unauthorized access to your application, escalate privileges, and perform actions as any user. Applications relying on request.awsLambda.event for access control are at risk of full compromise.

Compliance Impact

This vulnerability could lead to unauthorized data access or modification, violating GDPR (data protection) and HIPAA (health data privacy). Compliance may be compromised if sensitive data is exposed or altered due to the authentication bypass.

Mitigation Strategies

Upgrade to @fastify/aws-lambda version 6.4.1 or later to patch the vulnerability. Ensure the application no longer relies on request.awsLambda.event for authorization decisions.

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