CVE-2026-84365: Hono: Incomplete fix for CVE-2026-39408: `toSSG()` still writes files outside the output directory
Summary
The fix released for CVE-2026-39408 does not cover every traversal sequence. toSSG() can still write files outside the configured output directory when a route parameter contains consecutive parent-directory segments.
Details
Static site generation builds each output path from the route path and the values supplied through ssgParams, then verifies that the result stays inside the output directory. That check normalizes the path with the same routine that built it, and the routine did not fully collapse runs of consecutive parent-directory segments. A value carrying enough of them produces a path the check accepts, but the filesystem resolves outside the output directory.
The earlier fix handled a single parent-directory segment, so it blocks the sequence reported at the time while leaving longer runs unhandled. The check also treated output directories that differ only in how they are rooted as equivalent.
This arises when an application generates a static site from route parameter values it does not fully control — slugs coming from a CMS, an API, or user submissions.
Impact
A value reaching ssgParams from an untrusted source can cause build output to be written outside the intended output directory, carrying whatever content the route handler produced.
This may lead to:
- files being created or overwritten elsewhere in the build environment - generated artifacts or deployment output being altered
This affects build-time static site generation only; request-time routing is not affected. Applications whose ssgParams values are entirely developer-controlled are not affected.
Other sources
Hono is a Web application framework that provides support for any JavaScript runtime. From 4.12.12 until 4.13.5, the fix released for CVE-2026-39408 does not cover every traversal sequence, and toSSG() can still write files outside the configured output directory when a route parameter contains consecutive parent-directory segments. Static site generation builds each output path from the route path and values supplied through ssgParams, then verifies that the result stays inside the output directory using the same normalization routine that built the path. That routine does not fully collapse runs of consecutive parent-directory segments, allowing a path that the check accepts to resolve outside the output directory, and the check also treats output directories that differ in how they are rooted as equivalent. This arises when an application generates a static site from route parameter values it does not fully control, such as slugs from a CMS, API, or user submission. An untrusted ssgParams value can create or overwrite files elsewhere in the build environment and alter generated artifacts or deployment output. The vulnerability affects build-time static site generation only; request-time routing and applications with entirely developer-controlled ssgParams values are not affected. This issue is fixed in version 4.13.5.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/honoto a version that resolves this vulnerability.Fixed in 4.13.5 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.13.5 - Compensating control
Ensure any route parameter values used as `ssgParams` for build-time static site generation are fully developer-controlled (i.e., do not derive them from a CMS, API, or user submissions) to avoid writing files outside the configured output directory via traversal sequences.
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Only build-time static site generation using toSSG() is affected. Exposure exists where route parameters are populated through ssgParams values that are not fully controlled, such as slugs from a CMS, API, or user submissions.
What does an attacker need to exploit it?
An attacker needs a way to influence a value supplied through ssgParams for a route parameter. Consecutive parent-directory segments in that value can cause toSSG() to write outside the configured output directory.
What is the potential impact on the build environment?
A crafted parameter can create or overwrite files elsewhere in the build environment. This can alter generated artifacts or deployment output.
How can teams identify whether they are affected?
Review Hono versions from 4.12.12 until 4.13.5 and identify builds that call toSSG(). Prioritize builds where ssgParams values originate from CMS content, APIs, or user-controlled data.