CVE-2026-27700: Hono is Vulnerable to Authentication Bypass by IP Spoofing in AWS Lambda ALB conninfo

Summary

When using the AWS Lambda adapter (hono/aws-lambda) behind an Application Load Balancer (ALB), the getConnInfo() function incorrectly selected the first value from the X-Forwarded-For header.

Because AWS ALB appends the real client IP address to the end of the X-Forwarded-For header, the first value can be attacker-controlled.

This could allow IP-based access control mechanisms (such as the ipRestriction middleware) to be bypassed.

Details

In ALB environments, AWS appends the actual client IP address to the end of any existing X-Forwarded-For header value. However, the previous implementation of getConnInfo() extracted the leftmost IP address:

ts address = xff.split(',')[0].trim()

If a client sent:

X-Forwarded-For: <spoofed-ip>

ALB would forward:

X-Forwarded-For: <spoofed-ip>, <real-client-ip>

Since the implementation selected the first value, the spoofed IP address was trusted. This affected applications using:

ts ipRestriction(getConnInfo, { allowList: [...] })

or any custom middleware relying on getConnInfo(c).remote.address for authorization decisions.

The issue only affects deployments using the AWS Lambda adapter behind an ALB. API Gateway (v1/v2) and Lambda Function URLs are not affected, as they use AWS-provided source IP values from requestContext.

Impact

An unauthenticated remote attacker could bypass IP-based access restrictions by supplying a crafted X-Forwarded-For header. This may allow access to resources that were intended to be restricted by IP address.

Only applications deployed behind an ALB and relying on getConnInfo() for IP-based authorization are affected.