CVE-2021-3374 in Shiny Server
Summary
by MITRE • 04/03/2021
Directory traversal in RStudio Shiny Server before 1.5.16 allows attackers to read the application source code, involving an encoded slash.
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Analysis
by VulDB Data Team • 04/10/2021
The vulnerability identified as CVE-2021-3374 represents a directory traversal flaw within RStudio Shiny Server versions prior to 1.5.16, which exposes critical security risks through improper handling of encoded slash characters in file paths. This weakness enables unauthorized access to application source code and sensitive files through carefully crafted requests that exploit the server's insufficient input validation mechanisms. The vulnerability specifically targets the path traversal logic that processes user-supplied data, allowing attackers to bypass normal access controls by leveraging encoded slash sequences that are not properly sanitized or decoded before file system operations are executed.
The technical exploitation of this vulnerability occurs when Shiny Server processes requests containing encoded slash characters such as %2F or other URL-encoded sequences that represent forward slashes. These encoded characters are typically used to represent special characters in web applications but in this case are not properly handled by the server's path resolution logic. When an attacker submits a request containing such encoded sequences, the server fails to properly decode or validate these inputs, allowing the traversal to occur through the file system and potentially access files outside the intended application directory structure. This behavior aligns with CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as path traversal or directory traversal attacks.
The operational impact of this vulnerability extends beyond simple code disclosure, as attackers can potentially access sensitive configuration files, database credentials, application secrets, and other confidential information stored within the server's file system. The ability to read application source code provides attackers with valuable insights into the application's architecture, business logic, and potential additional vulnerabilities that may exist within the codebase. This information can be leveraged to plan more sophisticated attacks or to identify other weaknesses in the application's security posture. The vulnerability particularly affects environments where Shiny Server is used to host applications that contain sensitive data processing capabilities or where the server operates with elevated privileges that could allow access to additional system resources.
Organizations using affected versions of RStudio Shiny Server should prioritize immediate patching to address this vulnerability, as the impact can be severe given the potential for unauthorized access to application source code and associated sensitive data. The recommended mitigation involves upgrading to Shiny Server version 1.5.16 or later, which includes proper input validation and sanitization of path traversal sequences. Additionally, implementing proper access controls, restricting file system permissions, and monitoring for unusual file access patterns can help reduce the risk of exploitation. This vulnerability demonstrates the importance of proper input validation and the potential consequences of insufficient sanitization of user-supplied data in web applications, aligning with ATT&CK technique T1059.007 for execution through web shells and T1566 for initial access through web application attacks. Organizations should also consider implementing web application firewalls and security monitoring solutions to detect and prevent exploitation attempts targeting similar path traversal vulnerabilities.