CVE-2026-24398: Hono Auth Bypass Vulnerability

CVE-2026-24398 Overview

CVE-2026-24398 is an Input Validation Error vulnerability affecting the Hono Web application framework. The IP Restriction Middleware contains a flaw in the IPV4_REGEX pattern and convertIPv4ToBinary function within src/utils/ipaddr.ts that fails to properly validate IPv4 octet values are within the valid range of 0-255. This allows attackers to craft malformed IP addresses that bypass IP-based access controls, potentially gaining unauthorized access to protected resources.

Critical Impact

Attackers can bypass IP-based access controls by crafting malformed IPv4 addresses with octet values outside the valid 0-255 range, undermining security controls that depend on IP restriction middleware.

Affected Products

• Hono Web application framework versions prior to 4.11.7
• Applications using Hono IP Restriction Middleware for access control

Discovery Timeline

• 2026-01-27 - CVE CVE-2026-24398 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2026-24398

Vulnerability Analysis

The vulnerability exists in Hono's IP Restriction Middleware, which is designed to control access based on client IP addresses. The root issue lies in the regular expression pattern used to validate IPv4 addresses. The original IPV4_REGEX pattern (/^[0-9]{0,3}\.[0-9]{0,3}\.[0-9]{0,3}\.[0-9]{0,3}$/) only checks that each octet contains up to three digits but does not enforce that the numeric value falls within the valid 0-255 range. This means malformed IP addresses like 999.999.999.999 or 256.1.1.1 would pass validation when they should be rejected.

This weakness is classified under CWE-185 (Incorrect Regular Expression), highlighting the improper regex pattern that fails to properly constrain input values.

Root Cause

The vulnerability stems from insufficient input validation in the IPv4 address parsing logic. The IPV4_REGEX pattern was designed to match the general format of an IPv4 address but lacked the semantic validation needed to ensure each octet represents a valid 8-bit value. The regex merely checked for digit patterns without enforcing the mathematical constraints of IPv4 addressing.

Attack Vector

An attacker can exploit this vulnerability by providing a crafted IP address with octet values exceeding 255 when making requests to an application protected by Hono's IP Restriction Middleware. If the middleware is configured to allow or deny access based on IP addresses or ranges, the flawed validation could cause the middleware to misinterpret the attacker's address, potentially allowing access that should be denied or vice versa. This attack is network-based and requires no authentication or user interaction.

// Security patch in src/utils/ipaddr.ts
   return sections.join(':')
 }
 
-const IPV4_REGEX = /^[0-9]{0,3}\.[0-9]{0,3}\.[0-9]{0,3}\.[0-9]{0,3}$/
+const IPV4_OCTET_PART = '(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9]?[0-9])'
+const IPV4_REGEX = new RegExp(`^(?:${IPV4_OCTET_PART}\\.){3}${IPV4_OCTET_PART}$`)
 
 /**
  * Distinct Remote Addr

Source: GitHub Commit

The patch replaces the permissive regex with a strict pattern that only matches octets in the valid 0-255 range using alternation groups for different value ranges.

Detection Methods for CVE-2026-24398

Indicators of Compromise

• Unusual IP addresses in access logs containing octet values greater than 255 (e.g., 300.1.1.1, 256.256.256.256)
• Unexpected access to IP-restricted endpoints from sources that should be blocked
• Anomalous authentication or authorization patterns on protected resources
• Log entries showing malformed IP address formats being processed

Detection Strategies

• Review application logs for IP addresses with invalid octet values exceeding 255
• Implement additional validation layers to detect malformed IP addresses before they reach the middleware
• Monitor for access patterns that suggest IP restriction bypass attempts
• Audit configurations of IP Restriction Middleware for sensitive endpoints

Monitoring Recommendations

• Enable detailed logging for the IP Restriction Middleware to capture all validation decisions
• Set up alerts for requests containing IP addresses with unusual numeric patterns
• Monitor for unexpected successful authentications from IP ranges that should be blocked
• Periodically review access logs for anomalies in IP address formatting

How to Mitigate CVE-2026-24398

Immediate Actions Required

• Upgrade Hono to version 4.11.7 or later immediately
• Review access logs for any evidence of exploitation attempts using malformed IP addresses
• Audit all endpoints protected by IP Restriction Middleware for unauthorized access
• Consider implementing additional network-level IP filtering as defense in depth

Patch Information

The vulnerability has been addressed in Hono version 4.11.7. The fix updates the IPV4_REGEX pattern in src/utils/ipaddr.ts to properly validate that each IPv4 octet falls within the valid 0-255 range. Organizations should upgrade to the patched version as soon as possible. Detailed patch information is available in the GitHub Release v4.11.7 and the GitHub Security Advisory GHSA-r354-f388-2fhh.

Workarounds

• Implement additional IP validation at the network or reverse proxy level before requests reach the Hono application
• Deploy a web application firewall (WAF) rule to reject requests with malformed IP address headers
• Add custom middleware to validate IP address format before the IP Restriction Middleware processes requests
• Consider using network-level access controls (firewall rules, security groups) as primary IP restrictions while the application is being patched

# Update Hono to patched version
npm update hono@4.11.7
# Or install specific version
npm install hono@4.11.7