CVE-2026-39408: Hono has a path traversal in toSSG() allows writing files outside the output directory

Summary

A path traversal issue in toSSG() allows files to be written outside the configured output directory during static site generation. When using dynamic route parameters via ssgParams, specially crafted values can cause generated file paths to escape the intended output directory.

Details

The static site generation process creates output files based on route paths derived from application routes and parameters. When ssgParams is used to provide values for dynamic routes, those values are used to construct output file paths. If these values contain traversal sequences (e.g. ..), the resulting output path may resolve outside the configured output directory. As a result, files may be written to unintended locations instead of being confined within the specified output directory.

For example: ts import { Hono } from 'hono' import { toSSG, ssgParams } from 'hono/ssg'

const app = new Hono()

app.get('/:id', ssgParams([{ id: '../pwned' }]), (c) => { return c.text('pwned') })

toSSG(app, fs, { dir: './static' })

In this case, the generated output path may resolve outside ./static, resulting in a file being written outside the intended output directory.

Impact

An attacker who can influence values passed to ssgParams during the build process may be able to write files outside the intended output directory.

Depending on the build and deployment environment, this may:

overwrite unintended files affect generated artifacts impact deployment outputs or downstream tooling

This issue is limited to build-time static site generation and does not affect request-time routing.