CVE-2026-39408: Hono Framework Path Traversal Vulnerability

CVE-2026-39408 Overview

CVE-2026-39408 is a path traversal vulnerability in the Hono web application framework that affects the toSSG() function used for static site generation. The vulnerability allows files to be written outside the configured output directory when using dynamic route parameters via ssgParams. Specially crafted parameter values can cause generated file paths to escape the intended output directory, potentially enabling an attacker to overwrite arbitrary files on the system.

Critical Impact

Attackers can write arbitrary files outside the intended output directory during static site generation, potentially overwriting system files or injecting malicious content.

Affected Products

• Hono versions prior to 4.12.12
• Applications using toSSG() with dynamic route parameters via ssgParams
• Any JavaScript runtime environments running vulnerable Hono versions

Discovery Timeline

• 2026-04-08 - CVE CVE-2026-39408 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39408

Vulnerability Analysis

This path traversal vulnerability (CWE-22) exists in the static site generation functionality of the Hono framework. The toSSG() function processes dynamic route parameters without properly validating that the resulting file paths remain within the configured output directory. When an application uses ssgParams to generate static pages with user-controlled or dynamic values, an attacker can craft parameter values containing path traversal sequences (such as ../) that cause files to be written to locations outside the intended directory structure.

The vulnerability requires local access and specific conditions to exploit, as it manifests during the build/generation process rather than runtime. However, if an attacker can influence the parameters passed to ssgParams, they could potentially overwrite configuration files, inject malicious scripts, or corrupt application data.

Root Cause

The root cause of this vulnerability is insufficient path validation in the static site generation utilities. Prior to the fix, the toSSG() function in src/helper/ssg/ssg.ts did not verify that generated file paths remained within the configured output directory. The file path construction logic allowed path traversal sequences in dynamic route parameters to escape the output directory boundary.

Attack Vector

The attack requires local access to influence the ssgParams values used during static site generation. An attacker could exploit this vulnerability by:

1. Providing malicious values to dynamic route parameters that contain path traversal sequences
2. Triggering the static site generation process
3. Causing files to be written to arbitrary locations outside the configured output directory

// Security patch in src/helper/ssg/ssg.ts - Merge commit from fork
 import type { AddedSSGDataRequest, SSGParams } from './middleware'
 import { SSG_CONTEXT, X_HONO_DISABLE_SSG_HEADER_KEY } from './middleware'
 import { defaultPlugin } from './plugins'
-import { dirname, filterStaticGenerateRoutes, isDynamicRoute, joinPaths } from './utils'
+import {
+  dirname,
+  ensureWithinOutDir,
+  filterStaticGenerateRoutes,
+  isDynamicRoute,
+  joinPaths,
+} from './utils'
 
 const DEFAULT_CONCURRENCY = 2 // default concurrency for ssg

Source: GitHub Commit Update

// Security patch in src/helper/ssg/utils.ts - Merge commit from fork
 export const isDynamicRoute = (path: string): boolean => {
   return path.split('/').some((segment) => segment.startsWith(':') || segment.includes('*'))
 }
+
+export const ensureWithinOutDir = (outDir: string, filePath: string): void => {
+  const normalizedOutDir = joinPaths('/', outDir)
+  const normalizedFilePath = joinPaths('/', filePath)
+
+  if (
+    normalizedFilePath !== normalizedOutDir &&
+    !normalizedFilePath.startsWith(`${normalizedOutDir}/`)
+  ) {
+    throw new Error(`Path traversal detected: "${filePath}" is outside the output directory`)
+  }
+}

Source: GitHub Commit Update

Detection Methods for CVE-2026-39408

Indicators of Compromise

• Unexpected files appearing outside the configured SSG output directory
• Build logs showing file writes to parent directories or system paths
• Unusual ../ sequences in route parameter values during static site generation
• Modified configuration files or unexpected script files in application directories

Detection Strategies

• Audit ssgParams implementations to identify any user-controlled or external data sources influencing parameter values
• Review build logs and file system activity during static site generation processes
• Implement file integrity monitoring on directories outside the SSG output path
• Search codebase for toSSG() usage patterns with dynamic route parameters

Monitoring Recommendations

• Monitor file system writes during build processes to detect unexpected write locations
• Implement alerts for file modifications outside designated output directories
• Track Hono package versions in dependency manifests to identify vulnerable installations
• Enable verbose logging during SSG builds to capture full file path information

How to Mitigate CVE-2026-39408

Immediate Actions Required

• Upgrade Hono to version 4.12.12 or later immediately
• Audit all applications using toSSG() with dynamic route parameters
• Review ssgParams implementations for any untrusted or user-controlled input sources
• Verify file system integrity after any recent builds using vulnerable versions

Patch Information

The vulnerability has been fixed in Hono version 4.12.12. The patch introduces a new ensureWithinOutDir() utility function that validates generated file paths remain within the configured output directory. The fix normalizes both the output directory and target file path, then performs a prefix check to ensure the file path does not escape the intended directory boundary.

For detailed information, refer to the GitHub Security Advisory GHSA-xf4j-xp2r-rqqx and the GitHub Release v4.12.12.

Workarounds

• Validate and sanitize all ssgParams values before passing them to toSSG() to remove path traversal sequences
• Implement strict allowlists for acceptable parameter values in dynamic routes
• Run static site generation in isolated environments with restricted file system permissions
• Use containerization to limit the impact of potential file writes outside the output directory

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

# Or specify the exact version in package.json
npm install hono@4.12.12 --save-exact

# Verify the installed version
npm list hono