CVE-2026-27700: Hono AWS Lambda Auth Bypass Vulnerability

CVE-2026-27700 Overview

CVE-2026-27700 is an Authentication Bypass vulnerability affecting the Hono Web application framework, specifically within the AWS Lambda adapter (hono/aws-lambda). When deployed behind an AWS Application Load Balancer (ALB), the getConnInfo() function incorrectly parses the X-Forwarded-For header, selecting the first value instead of the last. Since AWS ALB appends the real client IP address to the end of this header, an attacker can inject arbitrary IP addresses at the beginning of the header, effectively bypassing IP-based access control mechanisms such as the ipRestriction middleware.

Critical Impact

Attackers can bypass IP-based access controls by manipulating the X-Forwarded-For header, potentially gaining unauthorized access to restricted resources or endpoints protected by IP allowlisting.

Affected Products

• Hono v4.12.0
• Hono v4.12.1
• Applications using hono/aws-lambda adapter behind AWS Application Load Balancer

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-27700 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27700

Vulnerability Analysis

This vulnerability stems from an incorrect assumption about the X-Forwarded-For header format when operating behind AWS Application Load Balancer. The getConnInfo() function in the AWS Lambda adapter was designed to extract the client IP address from the X-Forwarded-For header but used flawed parsing logic that selected the first IP address in the comma-separated list.

AWS ALB follows a specific behavior: it appends the actual client IP address to the end of any existing X-Forwarded-For header value. This means if an attacker sends a request with a pre-populated X-Forwarded-For header containing a spoofed IP address, the ALB will append the real IP to the end, but the vulnerable code would still read the attacker-controlled first value.

This behavior enables attackers to bypass IP-based security controls, including the ipRestriction middleware, by simply injecting a trusted or allowlisted IP address at the beginning of the header.

Root Cause

The vulnerability is classified under CWE-290 (Authentication Bypass by Spoofing). The root cause lies in the incorrect parsing order of the X-Forwarded-For header values in src/adapter/aws-lambda/conninfo.ts. The code assumed the first IP in the header chain represented the original client, which contradicts AWS ALB's behavior of appending the real client IP to the end of the header.

Attack Vector

An attacker can exploit this vulnerability by including a crafted X-Forwarded-For header in their HTTP request. When the request passes through the AWS ALB, the load balancer appends the attacker's real IP to the end of the header. However, because the vulnerable Hono adapter reads the first value, the application sees the attacker-controlled spoofed IP instead of the actual client IP. This allows attackers to:

1. Bypass IP allowlist restrictions
2. Circumvent geographic or network-based access controls
3. Evade IP-based rate limiting
4. Potentially access admin panels or internal resources protected by IP filtering

   else {
     const xff = c.req.header('x-forwarded-for')
     if (xff) {
-      // First IP is the client
-      address = xff.split(',')[0].trim()
+      const ips = xff.split(',')
+      // ALB appends the real client IP to the end of the header
+      address = ips[ips.length - 1].trim()
     }
   }
 

Source: GitHub Commit Update

Detection Methods for CVE-2026-27700

Indicators of Compromise

• Requests containing multiple IP addresses in the X-Forwarded-For header where the first IP matches internal or trusted ranges
• Access log entries showing unexpected source IPs bypassing IP-restricted endpoints
• Sudden access from "trusted" IPs that don't correlate with legitimate traffic patterns
• Failed or successful authentication attempts from spoofed IP addresses

Detection Strategies

• Implement log analysis to identify requests with unusually long or suspicious X-Forwarded-For header chains
• Monitor for access to restricted endpoints from IP addresses that should be blocked
• Deploy Web Application Firewall (WAF) rules to detect and alert on multi-value X-Forwarded-For headers
• Review application access logs for IP address inconsistencies between client-provided and ALB-appended values

Monitoring Recommendations

• Enable detailed logging of all incoming request headers, particularly X-Forwarded-For
• Configure alerts for access to IP-restricted resources from unexpected network ranges
• Implement anomaly detection for sudden changes in IP-based access patterns
• Review audit logs for any access control violations or bypasses

How to Mitigate CVE-2026-27700

Immediate Actions Required

• Upgrade Hono to version 4.12.2 or later immediately
• Audit access logs for any evidence of IP spoofing attempts during the vulnerable period
• Review any access control decisions made based on client IP addresses since deploying versions 4.12.0 or 4.12.1
• Consider implementing additional authentication mechanisms beyond IP-based controls

Patch Information

The vulnerability has been patched in Hono version 4.12.2. The fix modifies the getConnInfo() function to correctly read the last IP address from the X-Forwarded-For header, which represents the actual client IP when operating behind AWS ALB. Users should upgrade immediately by updating their package dependencies.

For detailed information about the security patch, refer to the GitHub Security Advisory GHSA-xh87-mx6m-69f3 and the GitHub Release v4.12.2.

Workarounds

• Implement additional authentication layers beyond IP-based access control
• Use AWS WAF rules to validate and sanitize X-Forwarded-For headers before they reach the application
• Deploy custom middleware to correctly parse the last IP from the X-Forwarded-For header until the patch can be applied
• Consider using alternative client identification methods such as mTLS or API keys for critical access controls

# Configuration example
# Upgrade Hono to the patched version
npm update hono@4.12.2

# Or specify exact version in package.json
npm install hono@4.12.2 --save-exact