CVE-2026-100690 in Hugo
Summary
by MITRE • 09/26/2026
Hugo versions from v0.161.0 through v0.165.0 run Node.js tools (css.PostCSS, css.TailwindCSS, js.Babel) under the Node.js permission model to restrict file system reads to the project directory and configured mounts. Because the Node.js permission model validates only the lexical path and follows symbolic links that point outside the allowed set, Hugo did not detect symlinks escaping the sandbox. An attacker who can contribute content to a Hugo project (for example via a pull request) can commit a symlink such as assets/css/x.css -> /etc/passwd together with a PostCSS plugin that reads it, allowing any file readable by the Hugo build process to be disclosed and potentially embedded in the published site. This affects builds using the default security configuration; projects that do not invoke Node.js tools are unaffected. Fixed in v0.166.0, which scans allowed paths and fails the build when a symbolic link resolves outside them.
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Analysis
by VulDB Data Team • 09/26/2026
The vulnerability identified in Hugo versions ranging from v0.161.0 to v0.165.0 represents a critical path traversal flaw within the static site generator's integration with Node.js-based tooling, specifically affecting PostCSS, TailwindCSS, and Babel processing pipelines. This issue stems from an insufficient validation mechanism in how Hugo enforces sandbox boundaries for external processes. While Hugo attempts to restrict file system access by validating paths against a configured set of allowed directories, the underlying logic relies on lexical path comparison rather than resolving symbolic links prior to evaluation. Consequently, if a malicious actor can introduce content into the project repository, they can exploit this gap by creating symbolic links that point outside the designated sandbox boundaries, such as pointing to sensitive system files like /etc/passwd or other configuration data accessible to the build process user account.
From a technical perspective, the core flaw lies in the failure of the Node.js permission model integration within Hugo's execution context. When Hugo invokes these Node.js tools, it passes file paths that are supposed to be confined to the project directory and configured mounts. However, because the validation step does not resolve symbolic links before checking against the allowlist, a symlink created by an attacker can bypass this check entirely. For instance, if an attacker commits a CSS asset with a path like assets/css/x.css which is actually a symbolic link pointing to /etc/passwd, Hugo's security layer will perceive it as being within the allowed directory structure based on its lexical name alone. When PostCSS or similar tools process this file, they follow the symlink and read the target location, effectively breaking out of the intended sandbox environment.
The operational impact of this vulnerability is significant for any project utilizing default security configurations that rely on these Node.js integrations during the build phase. An attacker with write access to the repository, such as through a pull request or direct commit permission, can craft malicious assets containing symlinks combined with specific plugins or configuration files designed to read and output external file contents. This allows for arbitrary file disclosure from any location readable by the user account executing the Hugo build process. The disclosed data is not merely stored but potentially embedded directly into the published static site, leading to a severe information leakage that could expose sensitive server configurations, environment variables, or other internal system details depending on the permissions of the build service.
This vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, as it involves traversing outside restricted directories via symbolic link manipulation. In terms of attack patterns, this behavior is consistent with ATT&CK technique T1083: File and Directory Discovery, where an adversary explores the file system to gather information for further exploitation. The vulnerability highlights the risks associated with trusting lexical path validation in security-critical contexts without resolving actual filesystem targets first.
The issue was resolved in Hugo version v0.166.0 by implementing a more robust validation mechanism that actively scans allowed paths and resolves symbolic links before permitting file access. If a symbolic link is found to resolve outside the set of allowed directories, the build process now fails immediately rather than proceeding with the operation. To mitigate this risk for users who cannot upgrade immediately or are maintaining legacy systems, it is recommended to disable Node.js-based processing tools if they are not strictly necessary for the project's styling and scripting needs. Additionally, enforcing strict code review processes that check for symbolic links in pull requests can prevent malicious symlinks from being merged into the main branch. Ensuring that build environments run with minimal privileges further reduces the potential impact of any successful file disclosure attempt by limiting which files are readable even if a symlink escape occurs.