CVE-2026-84365 in Hono
Summary
by MITRE • 09/02/2026
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.
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Analysis
by VulDB Data Team • 09/02/2026
The Hono web application framework, designed to support various JavaScript runtimes, contains a critical path traversal vulnerability within its static site generation (SSG) functionality. Specifically affecting versions from 4.12.12 through 4.13.5, this flaw stems from an incomplete implementation of the fix for CVE-2026-39408. The core issue lies in how Hono constructs and validates file paths during the build process when generating static sites. When a route parameter contains consecutive parent-directory segments, such as double dots representing directory traversal sequences, the framework fails to properly sanitize these inputs before writing files to disk. This oversight allows an attacker or malicious input source to manipulate the output path structure, potentially leading to unauthorized file system access and modification of critical build artifacts.
The technical mechanism behind this vulnerability involves a discrepancy between how paths are normalized for construction versus how they are validated against the configured output directory. During static site generation, Hono builds each output path by combining the route path with values supplied through ssgParams. It then employs a normalization routine to verify that the resulting path remains within the designated output directory. However, this specific normalization logic does not fully collapse runs of consecutive parent-directory segments. Consequently, it is possible for a crafted input sequence to pass the validation check while still resolving to a location outside the intended sandboxed directory when interpreted by the underlying file system. Additionally, the vulnerability exploits edge cases where output directories that differ in their root representation are treated as equivalent, further widening the attack surface for path manipulation attacks during the build phase.
The operational impact of this flaw is significant for applications that generate static sites from dynamic or untrusted sources. Since ssgParams often derive values from external inputs such as content management system slugs, API responses, or user submissions, developers may inadvertently expose their build environments to malicious actors. An attacker who controls these input parameters can create new files in arbitrary locations within the build environment or overwrite existing critical artifacts. This capability not only compromises the integrity of the generated static site but also poses a risk to the broader server infrastructure if the build process runs with elevated privileges or accesses sensitive directories beyond the project root. The vulnerability is strictly limited to the build-time phase, meaning that runtime request handling and routing mechanisms remain unaffected, provided the application does not rely on untrusted data for its static generation parameters.
To mitigate this risk, developers must upgrade their Hono dependencies to version 4.13.5 or later, where the path normalization logic has been corrected to properly handle all traversal sequences. In environments where upgrading is not immediately feasible, strict input validation should be implemented on any values passed into ssgParams before they are used in route generation. This includes sanitizing inputs to remove or escape special characters and ensuring that only expected alphanumeric slugs or identifiers are accepted from external sources like CMS platforms or user forms. By enforcing these controls, developers can prevent malicious path construction during the static site build process.
From a classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, as it involves an insufficient restriction on file system access based on external input. It also relates to CWE-73: External Control of File Name or Path, highlighting the failure to neutralize special elements in filenames that could cause unintended directory traversal. In terms of offensive security frameworks, this flaw can be leveraged within the ATT&CK technique T1083: File and Directory Discovery, allowing an attacker to explore and manipulate the file system structure during the build phase. Understanding these classifications helps security teams prioritize remediation efforts and apply appropriate controls to protect static site generation pipelines from similar path traversal exploits in other frameworks.