CVE-2026-63490 in handlebars.java
Summary
by MITRE • 08/20/2026
Handlebars.java provides logic-less and semantic Mustache templates with Java. Prior to 4.5.3, com.github.jknack.handlebars.springmvc.SpringTemplateLoader resolves attacker-influenced Spring MVC view names through Spring ResourceLoader without the path-containment validation used by other URL-based loaders. In handlebars-springmvc/src/main/java/com/github/jknack/handlebars/springmvc/SpringTemplateLoader.java, a view name using a file: or classpath: URL and ending with the # fragment delimiter places the appended .hbs suffix in the fragment, which FileUrlResource.exists() and URL.openStream() discard. HandlebarsViewResolver in handlebars-springmvc/src/main/java/com/github/jknack/handlebars/springmvc/HandlebarsViewResolver.java then passes the attacker-controlled name to handlebars.compile(), allowing an unauthenticated remote attacker to read files accessible to the JVM when an application exposes a controller with a user-influenced view name. This issue is fixed in version 4.5.3.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in Handlebars.java versions prior to 4.5.3 represents a critical path traversal and arbitrary file read flaw within the Spring MVC integration module, specifically affecting the SpringTemplateLoader component. As a logic-less templating engine for Java, Handlebars relies on template loaders to locate and parse view templates based on names provided by the application framework. In this specific implementation, the SpringTemplateLoader delegates the resolution of these view names to the standard Spring ResourceLoader mechanism without implementing necessary path containment validation that is present in other URL-based loaders within the same ecosystem. This architectural oversight creates a significant security gap where attacker-controlled input can manipulate how resources are located on the underlying file system or classpath.
The technical exploitation vector centers on the handling of URLs containing specific schemes and fragment identifiers. When an unauthenticated remote attacker supplies a view name that utilizes either the file: or classpath: URL scheme, combined with a hash symbol (#) as a fragment delimiter, the internal logic behaves in a dangerous manner. The system appends the required .hbs template suffix to this input string. However, due to how Java's FileUrlResource.exists() and URL.openStream() methods process URLs with fragments, they effectively discard everything following the # character. This discrepancy means that while the application intends to load a file ending in .hbs, the underlying resource loader ignores the fragment portion, potentially allowing access to files located at or before the position of the hash symbol rather than the intended template path.
This flaw enables an unauthenticated remote attacker to perform arbitrary local file reads on any system accessible by the Java Virtual Machine running the vulnerable application. By carefully crafting view names that point to sensitive configuration files, source code, credentials, or other internal resources using absolute paths in conjunction with fragment delimiters, an adversary can bypass intended access controls. The vulnerability is particularly severe because it requires no authentication if the web application exposes a controller endpoint where the view name parameter is derived directly from user input without sufficient sanitization. This aligns with CWE-20 Improper Input Validation and CWE-78 OS Command Injection principles in terms of impact, though technically classified under CWE-22 Path Traversal or CWE-94 Improper Control of Generation of Code as the attacker influences resource resolution logic leading to unauthorized data exposure.
From a threat intelligence perspective, this vulnerability facilitates initial access through information disclosure and can be leveraged for further exploitation depending on the contents of the read files. It maps to MITRE ATT&CK technique T1083 File and Directory Discovery, where an adversary explores the system to gather sensitive information that aids in subsequent phases such as Credential Access or Lateral Movement. The lack of proper validation allows attackers to probe the server environment effectively without triggering typical intrusion detection signatures associated with direct command execution attempts.
To mitigate this risk, organizations must immediately upgrade Handlebars.java to version 4.5.3 or later, where the SpringTemplateLoader has been patched to enforce strict path containment and prevent the exploitation of URL fragment handling discrepancies. For applications unable to update immediately due to legacy constraints, defensive coding practices should be implemented by validating all view names against a whitelist of allowed templates rather than relying on dynamic resolution through user-supplied strings. Additionally, input validation layers at the web server or API gateway level can filter out special characters such as hash symbols and scheme prefixes like file: or classpath: to prevent malformed URLs from reaching the template engine logic. Regular security audits focusing on how view names are constructed in Spring MVC applications will help identify similar patterns of improper resource resolution that could lead to information disclosure vulnerabilities.