CVE-2026-59897 in Hono
Summary
by MITRE • 07/08/2026
Hono is a Web application framework that provides support for any JavaScript runtime. From 4.3.3 before 4.12.27, the AWS API Gateway v1 adapter can drop a distinct repeated request header value because it de-duplicates values using a substring comparison instead of an exact match, so middleware or application logic that depends on the complete X-Forwarded-For chain, rate limiting, audit logging, or proxy-chain validation can receive incomplete data. This issue is fixed in version 4.12.27.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability in Hono framework versions 4.3.3 through 4.12.26 represents a critical security flaw in the AWS API Gateway v1 adapter component that affects how request headers are processed. This issue stems from improper handling of repeated header values where the system employs substring comparison rather than exact matching during the de-duplication process. The technical implementation error creates a scenario where legitimate header values containing repeated substrings are incorrectly modified, leading to data integrity violations in downstream processing systems.
The core flaw manifests when the framework encounters request headers with multiple values that share common substrings, particularly affecting the X-Forwarded-For header chain which is essential for tracking client request paths through proxy servers. When the system processes these headers, it performs a substring comparison operation that erroneously removes what it perceives as duplicate values based on partial matches rather than exact value matching. This behavior violates fundamental HTTP header processing standards and creates security implications that extend beyond simple data loss.
From an operational perspective, this vulnerability impacts multiple critical security controls including proxy chain validation mechanisms that rely on complete header chains for proper routing decisions. Applications depending on the full X-Forwarded-For chain for audit logging purposes will receive truncated data, potentially compromising security monitoring capabilities and forensic analysis. Rate limiting systems that depend on header values for user identification or request tracking may malfunction due to incomplete data, creating potential bypass opportunities for malicious actors attempting to circumvent access controls.
The vulnerability aligns with CWE-1273 which addresses improper handling of repeated header values in HTTP implementations, and represents a specific instance of the broader ATT&CK technique T1566.002 related to credential access through manipulation of network traffic data. Organizations utilizing Hono framework versions within the affected range face potential security degradation where attackers could exploit this behavior to bypass rate limiting controls or manipulate audit trails. The impact extends to compliance scenarios where complete header chain integrity is required for security auditing and regulatory requirements.
Mitigation strategies should prioritize immediate upgrade to version 4.12.27 or later which implements proper exact matching logic for header de-duplication operations. Organizations should also conduct thorough security reviews of applications relying on X-Forwarded-For chains to identify potential impacts from incomplete data processing. Additional defensive measures include implementing header validation checks at multiple layers of the application architecture and establishing monitoring for anomalous header processing patterns that could indicate exploitation attempts. The fix addresses the root cause by ensuring that header value de-duplication operations maintain exact matching criteria rather than employing problematic substring comparison logic.