The vulnerability CVE-2026-27700 affects the honojs web application framework, specifically versions 4.12.0 and 4.12.1, when deployed with the AWS Lambda adapter (hono/aws-lambda) behind an AWS Application Load Balancer (ALB). The core issue lies in the getConnInfo() function, which is responsible for extracting client connection information, including the client IP address, from HTTP headers. In this scenario, the function incorrectly selects the first IP address from the X-Forwarded-For header. However, AWS ALB appends the true client IP address to the end of this header, meaning the first IP address is actually attacker-controlled and can be spoofed. This incorrect trust in the first IP allows an attacker to bypass IP-based access control mechanisms, such as the ipRestriction middleware, which rely on accurate client IP identification to enforce security policies. The vulnerability is classified under CWE-345 (Insufficient Verification of Data Authenticity) and CWE-290 (Authentication Bypass by Spoofing). Exploitation requires no authentication or user interaction and can be performed remotely by sending crafted requests with manipulated X-Forwarded-For headers. The vulnerability has a CVSS 3.1 score of 8.2 (high severity), reflecting its potential to compromise confidentiality by bypassing IP-based restrictions, though it has limited impact on integrity and no impact on availability. The issue was addressed in hono version 4.12.2 by correcting the logic to select the correct client IP from the X-Forwarded-For header, ensuring the real client IP appended by ALB is used for access control decisions.

This vulnerability can have significant impact on organizations using the honojs framework with AWS Lambda behind an ALB, particularly if they rely on IP-based access controls to restrict access to sensitive resources or administrative interfaces. Attackers can spoof their IP address to appear as a trusted client, bypassing these controls and gaining unauthorized access. This can lead to exposure of confidential data, unauthorized actions, and potential lateral movement within the affected environment. Since the vulnerability requires no authentication and can be exploited remotely, it increases the attack surface considerably. Organizations using ipRestriction middleware or similar IP filtering mechanisms are at high risk. The impact is especially critical for environments where IP filtering is a primary security control, such as internal APIs, management consoles, or services exposed only to specific IP ranges. Although no known exploits are reported in the wild yet, the high CVSS score and ease of exploitation make timely remediation essential to prevent potential attacks.

Organizations should immediately upgrade the honojs framework to version 4.12.2 or later, which contains the patch correcting the IP extraction logic. Until the upgrade can be applied, as a temporary mitigation, organizations can implement additional validation on the X-Forwarded-For header by inspecting the last IP address in the list rather than the first, or by configuring the ALB to use a different header for the real client IP if possible. Additionally, consider implementing defense-in-depth by combining IP-based restrictions with other authentication and authorization mechanisms, such as token-based authentication or mutual TLS, to reduce reliance on IP filtering alone. Monitoring and logging of suspicious IP addresses and unusual access patterns can help detect exploitation attempts. Network-level controls, such as AWS WAF rules, can also be configured to block suspicious requests with malformed or suspicious X-Forwarded-For headers. Finally, educate developers and security teams about the risks of trusting client-supplied headers without proper validation, especially in cloud load balancer environments.