CVE-2026-39409 Overview
CVE-2026-39409 is an authorization bypass vulnerability in Hono, a popular Web application framework that provides support for any JavaScript runtime. The vulnerability exists in the ipRestriction() middleware, which fails to properly canonicalize IPv4-mapped IPv6 client addresses (e.g., ::ffff:127.0.0.1) before applying IPv4 allow or deny rules. In environments such as Node.js dual-stack configurations, this can cause IPv4 rules to fail to match, leading to unintended authorization behavior.
Critical Impact
Attackers can bypass IP-based access controls by exploiting the address format mismatch, potentially gaining unauthorized access to protected endpoints in Hono-based applications running on dual-stack environments.
Affected Products
- Hono Web Framework versions prior to 4.12.12
- Applications using ipRestriction() middleware on Node.js dual-stack environments
- Any JavaScript runtime environment where IPv4-mapped IPv6 addresses are used
Discovery Timeline
- 2026-04-08 - CVE CVE-2026-39409 published to NVD
- 2026-04-08 - Last updated in NVD database
Technical Details for CVE-2026-39409
Vulnerability Analysis
This vulnerability falls under CWE-180 (Incorrect Behavior Order: Validate Before Canonicalize). The root issue is that the ipRestriction() middleware in Hono performs IP address matching without first normalizing the address format. When a client connects via IPv6 to a dual-stack server, the client's IPv4 address may be represented in IPv4-mapped IPv6 format (e.g., ::ffff:192.168.1.1 instead of 192.168.1.1). The middleware's allow/deny rules written for IPv4 addresses fail to match against these IPv4-mapped IPv6 representations.
The vulnerability is classified as a medium severity authorization bypass with network-accessible attack vector. While exploitation requires the specific condition of a dual-stack environment, no authentication is required to exploit this flaw.
Root Cause
The ipRestriction() middleware validates client IP addresses against configured rules without first converting IPv4-mapped IPv6 addresses to their canonical IPv4 form. This ordering violation means that legitimate IPv4 rules (both allow and deny) are ineffective against clients whose addresses are presented in the IPv4-mapped IPv6 format.
Attack Vector
An attacker can exploit this vulnerability by connecting to a vulnerable Hono application through an IPv6 interface on a dual-stack Node.js server. When the connection is established, the client's IPv4 address is represented in the ::ffff:x.x.x.x format. If the application has configured ipRestriction() rules using standard IPv4 notation (e.g., 192.168.1.0/24), these rules will not match the incoming request, effectively bypassing the intended access controls.
The exploitation mechanism involves initiating connections through network paths that result in IPv4-mapped IPv6 address representation, which is automatic in many dual-stack configurations. For more technical details on the specific code changes, refer to the GitHub Commit Details.
Detection Methods for CVE-2026-39409
Indicators of Compromise
- Unexpected access to IP-restricted endpoints from addresses that should be blocked
- Log entries showing client addresses in ::ffff:x.x.x.x format accessing restricted resources
- Inconsistent IP restriction behavior between IPv4-only and dual-stack environments
Detection Strategies
- Review application logs for access patterns involving IPv4-mapped IPv6 addresses accessing restricted endpoints
- Audit Hono application configurations for ipRestriction() middleware usage with IPv4-only rules
- Test IP restriction rules by connecting via both IPv4 and IPv6 interfaces to verify consistent behavior
Monitoring Recommendations
- Implement logging that captures both raw and canonicalized client IP addresses
- Monitor for unusual access patterns to endpoints protected by ipRestriction() middleware
- Set up alerts for access from IPv4-mapped IPv6 addresses to sensitive endpoints
How to Mitigate CVE-2026-39409
Immediate Actions Required
- Upgrade Hono to version 4.12.12 or later immediately
- Review all ipRestriction() middleware configurations in deployed applications
- Test IP restriction rules in dual-stack environments after upgrading to verify proper behavior
Patch Information
The vulnerability has been fixed in Hono version 4.12.12. The patch implements proper canonicalization of IPv4-mapped IPv6 addresses before rule matching is performed. Organizations should upgrade to this version or later to remediate the vulnerability. The fix is available via the GitHub Release v4.12.12.
For detailed information about the security fix, review the GitHub Security Advisory GHSA-xpcf-pg52-r92g.
Workarounds
- Configure IP restriction rules to include both IPv4 and IPv4-mapped IPv6 formats (e.g., add rules for both 192.168.1.1 and ::ffff:192.168.1.1)
- Disable IPv6 on the server if dual-stack functionality is not required
- Implement additional network-layer IP restrictions using firewalls or reverse proxies that properly handle address canonicalization
# Upgrade Hono to the patched version
npm update hono@4.12.12
# Or install the specific fixed version
npm install hono@4.12.12
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
