Detection Guidance

This vulnerability can be detected by inspecting HTTP responses from your AWS Lambda functions using the Hono framework (versions before 4.12.25) when behind an ALB single-header mode or VPC Lattice v2. Specifically, look for multiple Set-Cookie headers being merged into a single comma-separated Set-Cookie header.

You can use network traffic inspection tools like curl or tcpdump to capture and analyze HTTP response headers.

• Use curl to inspect response headers: curl -I https://your-application-endpoint
• Look for the Set-Cookie header in the response and check if multiple cookies are combined into one comma-separated string.
• Use tcpdump or Wireshark to capture HTTP traffic and filter for Set-Cookie headers to verify if cookies are merged improperly.

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Impact Analysis

This vulnerability can cause affected systems to fail to properly apply important cookies such as session, CSRF, or preference cookies.

As a result, users may experience broken sessions or be forced to re-authenticate unexpectedly, which can degrade user experience and potentially disrupt application functionality.

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Mitigation Strategies

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

Alternatively, if upgrading is not immediately possible, consider enabling ALB multi-value headers or avoid using AWS Lambda with ALB single-header mode or VPC Lattice v2, as these configurations are not affected by the vulnerability.

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Compliance Impact

This vulnerability causes improper merging of multiple Set-Cookie headers into a single comma-separated value, which can lead to clients misparsing or dropping cookies. As a result, session, CSRF, or preference cookies may fail to apply correctly, causing broken sessions or forced re-authentication.

While the CVE description and resources do not explicitly mention compliance with standards like GDPR or HIPAA, the failure to properly handle session or CSRF cookies could potentially impact security controls related to user authentication and session management, which are important for protecting personal data under such regulations.

Therefore, this vulnerability may indirectly affect compliance by weakening security mechanisms that help safeguard user data and maintain session integrity, but no direct compliance impact is stated.

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Executive Summary

The vulnerability exists in the Hono web application framework's AWS Lambda adapter before version 4.12.25. When using AWS Lambda with ALB single-header mode or VPC Lattice v2, the adapter improperly merges multiple Set-Cookie headers into a single comma-separated header.

This merging violates RFC 6265, which requires each cookie to be sent in a separate Set-Cookie header. Because commas can appear inside cookie attributes such as Expires dates, clients may misinterpret the combined header, causing them to drop or misparse cookies.

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