CVE-2026-7120 in fastify-static
Summary
by MITRE • 07/23/2026
@fastify/static evaluates the allowedPath callback before normalizing dot segments and duplicate path separators in the pathname used for file resolution. Versions up to and including 10.1.1 are affected. An unauthenticated attacker can bypass allowedPath restrictions by requesting equivalent non-canonical pathnames, causing files that were intended to be denied to be served anyway. The bypass does not allow access outside the configured static root by itself, it defeats path-based filtering only. The issue is patched in @fastify/static 10.1.2.
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Analysis
by VulDB Data Team • 07/23/2026
The vulnerability in @fastify/static represents a classic path traversal weakness that exploits the order of operations during path validation. This flaw resides in the security model where the allowedPath callback is invoked before canonical path normalization occurs, creating a window where attackers can manipulate file paths through non-canonical representations. The issue specifically affects versions up to and including 10.1.1, where the middleware processes incoming requests without first normalizing dot segments or duplicate path separators in the pathname before applying access controls.
The technical implementation flaw stems from the sequence of operations within the static file serving logic. When a request arrives, the system evaluates the allowedPath callback against the raw, unnormalized pathname before performing standard path normalization that would typically resolve .. and // sequences. This ordering allows attackers to craft requests using equivalent non-canonical pathnames such as /foo/./bar or /foo//bar that pass the access control checks but resolve to different locations than intended. The vulnerability operates at the intersection of path manipulation and access control enforcement, where the security boundary is bypassed through syntactic rather than semantic path representation.
The operational impact of this vulnerability is significant within environments where file access controls are critical for security. While the bypass does not enable complete directory traversal outside the configured static root, it effectively defeats path-based filtering mechanisms that rely on allowedPath callbacks for access control decisions. Attackers can exploit this to access files that should be restricted based on their path patterns, potentially exposing sensitive configuration files, source code, or other protected resources within the intended serving scope. The vulnerability essentially allows privilege escalation within the confines of the static file server's configured root directory.
This issue aligns with CWE-22 Path Traversal and follows patterns commonly associated with ATT&CK technique T1059.007 Command and Scripting Interpreter: Python, where path manipulation techniques are used to bypass security controls. The vulnerability demonstrates how seemingly benign path normalization order can create significant security implications in web application frameworks. Organizations using fastify static file serving should immediately update to version 10.1.2 or later, as this patch corrects the order of operations to normalize paths before applying access control checks. The fix ensures that all path validation occurs against normalized path representations, preventing attackers from exploiting the temporal gap between access control evaluation and path normalization.
The broader implications extend beyond this specific package to highlight the importance of consistent path handling in security-critical code paths. This vulnerability serves as a reminder that even minor implementation details in path processing can create substantial security gaps, particularly when dealing with access control enforcement. Security teams should review their own applications for similar patterns where path validation occurs before normalization, especially in frameworks or libraries that handle file system operations. The fix implemented in version 10.1.2 represents a fundamental correction to the security model, ensuring that all path-based access controls are applied against properly normalized paths rather than potentially manipulated syntactic representations.