External risk intelligence

@fastify/aws-lambda Authentication Bypass via Header Injection CVE-2026-18248

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2026-18248

This vulnerability affects a framework designed specifically to facilitate public-facing web applications and APIs hosted on AWS Lambda via API Gateway. Because these services are commonly exposed to the internet to handle incoming client requests and API traffic, the vulnerable attack surface is frequently reachable in typical deployments.

Privilege Escalation

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

The @fastify/aws-lambda package, in version 6.4.0, contains a flaw where it can be tricked into accepting forged authorization data through specially crafted HTTP headers. This could allow an unauthenticated attacker to bypass authentication and gain unauthorized access to applications that rely on this package for security decisions. The issue is resolved in version 6.4.1.

Forged headers bypass access controls.
Confirms relevance of package to security.
Review affected applications for exposure.

Attack Path

How an attacker could exploit the issue

An unauthenticated attacker can compromise applications by sending a specially crafted HTTP header. This header allows them to impersonate a legitimate user, bypassing authentication and authorization checks. The vulnerability lies in how the `@fastify/aws-lambda` package handles incoming HTTP headers before processing them.

Requires an HTTP header.
Triggers by a crafted header.
Bypasses authentication and authorization.

Live Threat

Current exploitation, exposure, and threat context

An unauthenticated attacker could bypass authentication and authorization controls for applications using @fastify/aws-lambda, when these applications rely on specific HTTP headers for identity or access decisions. This is possible by manipulating HTTP headers to forge the Lambda proxy event, including the authorizer context, leading to an override of legitimate authorization data.

Application authorization decisions.
Forging HTTP headers.
Authentication and authorization bypass.

Operational Fix

Recommended remediation, mitigation, and detection steps

Application owners and platform teams are likely responsible for addressing this critical vulnerability in `@fastify/aws-lambda` as it affects how web applications and APIs hosted on AWS Lambda handle authorization. The immediate first step is to identify all instances of the affected technology, confirm their reachability and business criticality, and then assign ownership for planning remediation based on the identified risk.

Confirm affected technology deployment locations.
Verify reachability and business criticality.
Assign ownership and plan remediation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the @fastify/aws-lambda package?

This software component acts as an adapter that allows developers to run Fastify web applications seamlessly on AWS Lambda. It bridges the gap between the Lambda function's event-based architecture and Fastify's HTTP-focused request handling, enabling the creation of serverless APIs that use standard web frameworks.

What does CVE-2026-18248 mean?

This vulnerability is classified as CWE-345, or Insufficient Verification of Data Authenticity. It means the software mistakenly trusts unverified data provided by a client—specifically via HTTP headers—as if it were legitimate system information, allowing that data to override actual authorization checks.

How is this vulnerability triggered?

An attacker triggers this by sending a request containing specific, forged HTTP headers. The flaw exists because the software reads these client-supplied headers to define the Lambda event. It is important to note that this does not require complex code execution; the vulnerability is only activated if the application logic relies on the compromised headers for identity or access decisions.

Is my application at risk?

According to Halo Surface Signal, this vulnerability is particularly relevant for applications that are internet-facing. Because this package is typically used to build public-facing APIs hosted on AWS Lambda via API Gateway, these services are often directly reachable by external users, making them a primary target for this type of header manipulation.

How do I fix this issue?

The primary resolution is to update the @fastify/aws-lambda package to version 6.4.1 or later. This update changes how the software handles data, ensuring it uses internal, trusted tokens instead of client-controlled headers and actively strips potentially dangerous headers before they can impact your application's security decisions.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.