Compliance Impact

This vulnerability causes repeated request headers such as X-Forwarded-For to be truncated to a single value, which can weaken access control decisions and result in loss of audit trail data.

For compliance with standards like GDPR or HIPAA, which often require accurate access control and thorough auditing, this loss of header information can impair the ability to properly track user requests and enforce security policies.

Therefore, applications relying on the affected Hono Lambda@Edge adapter may face challenges in meeting regulatory requirements related to data integrity, access control, and audit logging until the vulnerability is fixed.

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Executive Summary

The CVE-2026-54289 vulnerability affects the Hono framework's Lambda@Edge adapter. When a request header appears multiple times, such as X-Forwarded-For, Forwarded, or Via, the adapter incorrectly handles these repeated headers by overwriting each value instead of appending them. This means only the last header value is retained and passed to the application.

Because of this, middleware that relies on the full chain of header values—like those used for IP-based access control or auditing—only sees the last value instead of the complete set. This can lead to incorrect or incomplete data being processed.

The root cause is the use of the Headers.set method instead of Headers.append, which causes the truncation of repeated headers. This issue is specific to AWS Lambda@Edge deployments and was fixed in version 4.12.25 of the Hono framework.

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Impact Analysis

This vulnerability can weaken or alter access control decisions in applications that rely on the full chain of repeated headers like X-Forwarded-For for IP restriction. Since only the last header value is seen, the application might incorrectly allow or deny access.

Additionally, auditing and logging tools that depend on the complete header chain lose important hop history information, potentially impacting forensic analysis or compliance monitoring.

Overall, the impact includes reduced confidentiality and integrity of access control decisions, but there is no direct impact on availability.

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Detection Guidance

This vulnerability affects the handling of repeated request headers such as X-Forwarded-For, Forwarded, and Via in AWS Lambda@Edge deployments using the Hono framework. Detection involves verifying whether repeated headers are being truncated to a single value instead of preserving all values.

To detect this issue, you can capture and inspect incoming HTTP request headers at the Lambda@Edge function or CloudFront level to see if repeated headers appear multiple times or only once.

For example, you can use AWS CloudWatch logs to check the headers received by your Lambda@Edge function.

• Use AWS CLI to fetch recent Lambda@Edge logs: `aws logs filter-log-events --log-group-name /aws/lambda/your-lambda-edge-function-name --filter-pattern "X-Forwarded-For"`
• Use curl or similar tools to send requests with repeated headers and observe how they are processed downstream.
• Inspect the headers received by your application middleware to verify if repeated headers are truncated to a single value.

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Mitigation Strategies

The primary mitigation step is to upgrade the Hono framework to version 4.12.25 or later, where this vulnerability is fixed.

Until the upgrade is applied, be aware that repeated headers such as X-Forwarded-For may be truncated, which can weaken access control or auditing based on these headers.

Consider implementing additional validation or logging mechanisms outside of the Lambda@Edge adapter to capture the full header chain if possible.

Review your access control and auditing policies to understand the impact of potentially missing header values.

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