CVE-2021-3856 in Keycloak
Summary
by MITRE • 08/26/2022
ClassLoaderTheme and ClasspathThemeResourceProviderFactory allows reading any file available as a resource to the classloader. By sending requests for theme resources with a relative path from an external HTTP client, the client will receive the content of random files if available.
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Analysis
by VulDB Data Team • 10/02/2022
This vulnerability exists in the theme resource handling functionality of certain web applications that utilize classloader-based resource resolution. The flaw stems from insufficient input validation and path traversal protection within the ClassLoaderTheme and ClasspathThemeResourceProviderFactory components. When these systems process requests for theme resources, they fail to properly sanitize user-supplied paths, allowing attackers to craft malicious requests that can traverse the classpath and access arbitrary resources available to the application's classloader. The vulnerability is particularly concerning because it operates at the classpath level where sensitive files such as configuration properties, database credentials, application secrets, and internal class files may be accessible. This represents a classic path traversal vulnerability that can be exploited through HTTP requests, enabling attackers to retrieve files that should remain confidential. The impact extends beyond simple information disclosure as attackers may be able to access critical application resources that could lead to further exploitation opportunities.
The technical implementation of this vulnerability leverages the underlying Java classloader mechanism which provides access to resources within the application's classpath. When an attacker sends a specially crafted request with a relative path, the system attempts to resolve this path through the classloader's resource lookup mechanism without proper sanitization. This allows the attacker to access resources that are normally protected by the application's security boundaries. The vulnerability can be exploited by constructing malicious URLs that contain path traversal sequences such as ../ or ..\ that bypass normal access controls. The exploitation is particularly effective because it targets the fundamental resource resolution mechanism of the application, making it difficult to defend against through traditional perimeter security measures. This type of vulnerability aligns with CWE-22 Path Traversal and CWE-23 Relative Path Traversal, which specifically address improper handling of file paths in applications.
The operational impact of this vulnerability can be severe as it provides attackers with access to sensitive information that may include application configuration files, database connection details, cryptographic keys, and other confidential data stored within the application's classpath. Depending on the application architecture, attackers may also be able to access internal application classes, which could reveal implementation details and potentially lead to additional vulnerabilities. The vulnerability can be exploited from external network locations, making it particularly dangerous as it does not require any special privileges or access to the application server itself. Attackers can systematically enumerate resources within the classpath to discover sensitive information, potentially leading to full system compromise. This vulnerability also enables reconnaissance activities where attackers can gather intelligence about the application's internal structure and dependencies, which can inform more sophisticated attack vectors.
Mitigation strategies should focus on implementing proper input validation and sanitization for all user-supplied paths before they are processed by the classloader resource resolution mechanism. The application should enforce strict path validation that prevents traversal beyond intended resource directories and reject any requests containing path traversal sequences. Implementing a whitelist approach for valid resource paths can provide strong protection against this type of vulnerability. Additionally, the application should be configured to limit the resources available to the classloader, reducing the attack surface. Security headers and access controls should be implemented to prevent unauthorized access to sensitive resources. Regular security testing including penetration testing and vulnerability scanning should be conducted to identify and remediate similar issues. The principle of least privilege should be applied to ensure that applications only have access to resources necessary for their operation, minimizing the potential impact of such vulnerabilities. Organizations should also implement monitoring and logging of resource access patterns to detect anomalous behavior that may indicate exploitation attempts. This vulnerability demonstrates the importance of secure coding practices and proper resource handling in preventing information disclosure attacks that can have cascading security implications.