Executive Summary

The vulnerability exists in the Hono web framework's Body Limit Middleware before version 4.12.25. This middleware relies on the Content-Length header of HTTP requests to enforce limits on the size of the request body.

When deployed on AWS Lambda with API Gateway v1/v2, ALB, VPC Lattice, or Lambda@Edge, the actual request body is fully buffered and reconstructed by the adapter. However, the middleware trusts the client-declared Content-Length header without verifying if it matches the actual payload size.

An attacker can exploit this by sending a request with a small Content-Length value but including a much larger payload, effectively bypassing the body size limit enforced by the middleware.

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Impact Analysis

This vulnerability allows an attacker to bypass the request body size limit, causing the server to process larger payloads than intended.

As a result, it can lead to increased CPU and memory usage per request, potentially degrading performance or exhausting resources.

However, the impact is limited by AWS Lambda's platform constraints and isolation, so it does not cause a full denial of service but allows unauthorized resource consumption.

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Detection Guidance

Detection of this vulnerability involves monitoring requests to the Hono web framework deployed on AWS Lambda environments (API Gateway v1/v2, ALB, VPC Lattice, Lambda@Edge) for discrepancies between the declared Content-Length header and the actual payload size.

Since the vulnerability arises from trusting the Content-Length header without verifying the actual body size, you can detect suspicious requests by comparing the Content-Length header value against the actual size of the request body.

Commands or scripts that log or analyze incoming requests to identify cases where the Content-Length header is significantly smaller than the actual payload size can help detect exploitation attempts.

• Use network packet capture tools (e.g., tcpdump, Wireshark) to capture HTTP requests and inspect Content-Length headers versus actual payload sizes.
• Implement logging in your Lambda function or API Gateway to record Content-Length headers and actual body sizes for comparison.
• Example command to capture HTTP traffic on port 443 (if HTTPS is terminated elsewhere): `tcpdump -i any -s 0 -w capture.pcap port 443`
• Use scripts or tools to parse logs or captured traffic to find mismatches between Content-Length and actual body size.

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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 has been fixed.

Until the upgrade can be applied, consider implementing additional validation in your Lambda functions or API Gateway to verify that the actual request body size matches the declared Content-Length header.

Monitoring and alerting on requests with mismatched Content-Length headers and payload sizes can help detect and block potential exploitation attempts.

Review and enforce resource limits on your Lambda functions to reduce the impact of oversized payloads.

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