CVE-2026-103088 in handlebars.java
Summary
by MITRE • 09/30/2026
Handlebars.java before 4.5.5 allows directory traversal. In handlebars-springmvc 4.5.3 and 4.5.4, the path-containment fix for CVE-2026-63490 validates template locations as raw percent-encoded strings, whereas the template file is opened through a URL handler that percent-decodes the path. In a Spring MVC application with a file: template prefix and a request-derived view name, a percent-encoded traversal such as %2e%2e/ bypasses both the view-resolver check and the loader-side containment and reads files outside the configured template base directory.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in Handlebars.java versions prior to 4.5.5 represents a critical path-traversal flaw within the handlebars-springmvc integration, specifically affecting version 4.5.3 and 4.5.4. This issue stems from an inconsistency in how template paths are validated versus how they are ultimately resolved by the underlying file system handlers. The core of the problem lies in the disparity between the security checks performed during view resolution and the actual mechanism used to load the template files. When a Spring MVC application is configured with a file-based template prefix, it relies on a specific loader to retrieve templates from the local filesystem. In this context, an attacker can exploit the lack of normalization in the validation logic to bypass containment controls entirely.
The technical flaw originates from the way path-containment checks are implemented for CVE-2026-63490 mitigation efforts within these specific versions. The view resolver validates template locations by treating them as raw percent-encoded strings, meaning it does not decode URL encoding before performing its security assessment. However, when the system proceeds to open the actual template file, it utilizes a URL handler that automatically percent-decodes the path string. This discrepancy creates a window of exploitation where an attacker can craft a request containing percent-encoded directory traversal sequences, such as %2e%2e/, which represent dot-dot-slash patterns used to move up directories in a filesystem hierarchy. Because the initial validation sees these characters as literal alphanumeric or special symbols rather than navigation commands, it incorrectly deems the path safe and within bounds.
In an operational context involving Spring MVC applications that derive view names directly from user-supplied request parameters, this vulnerability allows for unauthorized file access outside of the configured template base directory. An attacker can send a malicious HTTP request with a crafted URL parameter that includes these encoded traversal sequences. The application accepts this input as valid due to the flawed validation logic and subsequently passes it to the loader. Once decoded by the URL handler during file opening, the path resolves to locations on the server's filesystem that are not intended for public or template access. This can lead to the disclosure of sensitive configuration files, source code, credentials, or other critical system resources depending on the permissions under which the application runs and the structure of the underlying operating system.
This vulnerability is classified as a Path Traversal issue, aligning with CWE-22: Improper Limitation of a Pathname to a Restricted Directory. The attack vector involves manipulating input parameters that are not properly sanitized before being used in file operations, which also relates to CWE-73: External Control of File Name or Path. From an offensive security perspective, this behavior maps directly to the ATT&CK technique T1083: File and Directory Discovery, where adversaries seek to enumerate files on a compromised system to gather intelligence for further exploitation. The specific mechanism of bypassing validation through encoding manipulation highlights the importance of canonicalization in security checks, as failing to normalize input before comparison is a common source of such logic errors.
To mitigate this vulnerability, organizations must upgrade Handlebars.java and its Spring MVC integration to version 4.5.5 or later, where the path containment fix has been properly implemented to handle percent-encoded paths correctly. In environments where an immediate upgrade is not feasible, administrators should implement strict input validation at the application layer to reject any request parameters containing encoded traversal sequences before they reach the view resolver. Additionally, configuring the template loader with a restricted base directory and ensuring that file access operations are performed using secure APIs that canonicalize paths prior to evaluation can provide defense in depth. It is also advisable to review Spring MVC configurations to ensure that user-supplied data used for resource location undergoes rigorous sanitization and normalization processes consistent with industry best practices for preventing path traversal attacks.