CVE-2026-39410: Hono Framework Auth Bypass Vulnerability

CVE-2026-39410 Overview

CVE-2026-39410 is an Input Validation Error vulnerability in Hono, a Web application framework that provides support for any JavaScript runtime. Prior to version 4.12.12, a discrepancy between browser cookie parsing and the parse() function handling allows cookie prefix protections to be bypassed. Cookie names that are treated as distinct by the browser may be normalized to the same key by parse(), allowing attacker-controlled cookies to override legitimate ones.

Critical Impact

Attackers can bypass cookie prefix security protections by exploiting parsing inconsistencies, potentially enabling session hijacking or authentication bypass through cookie override attacks.

Affected Products

• Hono versions prior to 4.12.12

Discovery Timeline

• 2026-04-08 - CVE-2026-39410 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39410

Vulnerability Analysis

This vulnerability stems from inconsistent handling of whitespace characters in cookie names between browser implementations and Hono's parse() function. When a browser receives cookies, it may treat cookie names with leading or trailing whitespace as distinct from those without. However, Hono's cookie parsing logic did not properly normalize these values, creating a mismatch that attackers could exploit.

Cookie prefix security mechanisms (such as __Secure- and __Host- prefixes) are designed to provide additional protection for sensitive cookies. By crafting malicious cookies with whitespace-padded names that normalize to the same key as legitimate protected cookies, an attacker could override these security-critical values.

Root Cause

The root cause is classified as CWE-20 (Improper Input Validation). The parse() function in src/utils/cookie.ts failed to properly trim whitespace characters (specifically space 0x20 and tab 0x09) from cookie names before processing. This allowed cookies with names like " session" or "session " to be treated differently by the browser but resolve to the same key (session) during server-side parsing.

Attack Vector

The attack exploits the network-accessible cookie parsing mechanism. An attacker who can set cookies (through subdomain control, XSS on a related domain, or other cookie injection vectors) could craft cookies with whitespace-padded names designed to collide with legitimate cookies after normalization. This could lead to:

1. Bypassing cookie prefix protections (__Secure-, __Host-)
2. Overriding session identifiers
3. Manipulating application state stored in cookies

// Security patch in src/utils/cookie.ts - Merge commit from fork
 // (see: https://github.com/golang/go/issues/7243)
 const validCookieValueRegEx = /^[ !#-:<-[\]-~]*$/
 
+const trimCookieWhitespace = (value: string): string => {
+  let start = 0
+  let end = value.length
+
+  while (start < end) {
+    const charCode = value.charCodeAt(start)
+    if (charCode !== 0x20 && charCode !== 0x09) {
+      break
+    }
+    start++
+  }
+
+  while (end > start) {
+    const charCode = value.charCodeAt(end - 1)
+    if (charCode !== 0x20 && charCode !== 0x09) {
+      break
+    }
+    end--
+  }
+
+  return start === 0 && end === value.length ? value : value.slice(start, end)
+}
+
 export const parse = (cookie: string, name?: string): Cookie => {
   if (name && cookie.indexOf(name) === -1) {
     // Fast-path: return immediately if the demanded-key is not in the cookie string
     return {}

Source: GitHub Commit

Detection Methods for CVE-2026-39410

Indicators of Compromise

• Cookies with unusual whitespace characters in their names (leading/trailing spaces or tabs)
• Multiple cookies resolving to the same key after server-side parsing
• Unexpected changes to session or authentication cookies

Detection Strategies

• Monitor application logs for cookie parsing anomalies where duplicate cookie keys appear
• Implement server-side logging to track cookie name/value pairs before and after parsing
• Review web application firewall (WAF) logs for requests containing malformed cookie headers

Monitoring Recommendations

• Enable detailed request logging for cookie headers in your Hono applications
• Set up alerts for authentication failures that may indicate cookie manipulation attempts
• Monitor for unusual patterns in cookie-dependent authentication flows

How to Mitigate CVE-2026-39410

Immediate Actions Required

• Upgrade Hono to version 4.12.12 or later immediately
• Audit applications for reliance on cookie prefix security mechanisms
• Review recent authentication logs for signs of exploitation

Patch Information

The vulnerability is fixed in Hono version 4.12.12. The patch introduces a trimCookieWhitespace() function that properly normalizes cookie names by removing leading and trailing space (0x20) and tab (0x09) characters before processing. This ensures consistent behavior between browser cookie handling and server-side parsing.

For detailed patch information, see the GitHub Security Advisory GHSA-r5rp-j6wh-rvv4 and GitHub Release v4.12.12.

Workarounds

• Implement additional server-side validation to reject cookies with whitespace in names
• Use secondary validation mechanisms alongside cookie-based authentication
• Consider implementing defense-in-depth with additional session binding techniques

# Configuration example
# Upgrade Hono to the patched version
npm install hono@4.12.12

# Or update package.json and run install
npm update hono