CVE-2021-28966 in Ruby
Summary
by MITRE • 07/30/2021
In Ruby through 3.0 on Windows, a remote attacker can submit a crafted path when a Web application handles a parameter with TmpDir.
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Analysis
by VulDB Data Team • 05/09/2026
The vulnerability identified as CVE-2021-28966 represents a critical security flaw in Ruby versions 3.0 and earlier when operating on Windows platforms. This issue stems from improper handling of temporary directory paths within web applications, creating a potential attack vector for remote adversaries. The vulnerability specifically affects applications that process user-supplied parameters through the TmpDir functionality, which is commonly used for temporary file operations in Ruby applications. Attackers can exploit this weakness by crafting malicious paths that manipulate how temporary files are created or accessed, potentially leading to unauthorized file operations or information disclosure.
The technical root cause of this vulnerability lies in the insufficient validation and sanitization of path inputs when Ruby's temporary directory handling mechanisms are invoked. On Windows systems, where file path semantics differ significantly from Unix-like systems, the vulnerability becomes particularly pronounced. When a web application accepts user input and passes it through TmpDir operations, the application fails to properly sanitize or validate the path structure, allowing attackers to inject malicious path components. This flaw can be exploited to manipulate temporary file creation processes, potentially leading to directory traversal attacks or arbitrary file access. The vulnerability aligns with CWE-22, which addresses directory traversal issues, and CWE-73, which covers external control of file name or path. The attack surface is particularly concerning because it leverages common web application patterns where user input is processed through standard Ruby temporary file handling routines.
The operational impact of CVE-2021-28966 extends beyond simple privilege escalation or information disclosure, as it can enable more sophisticated attacks depending on the application context. Remote attackers could potentially leverage this vulnerability to execute arbitrary code by manipulating temporary file creation processes, especially when applications lack proper sandboxing or file access controls. The vulnerability is particularly dangerous in web applications where user input is directly processed through temporary directory operations, as it could allow attackers to overwrite critical system files, create malicious temporary files that are later executed, or gain unauthorized access to sensitive data stored in temporary locations. Applications using Ruby's built-in temporary directory handling functions, such as those involving file uploads, session management, or temporary data processing, are at risk. The Windows-specific nature of this vulnerability means that organizations running Ruby applications on Windows servers face increased risk compared to Unix-based deployments, though similar issues could theoretically exist in other platforms with improper path validation.
Mitigation strategies for CVE-2021-28966 should focus on both immediate patching and defensive programming practices. The primary recommendation is to upgrade to Ruby versions 3.1 or later, where this vulnerability has been addressed through improved path validation mechanisms. Organizations should implement comprehensive input validation for all parameters that may be passed to temporary directory functions, ensuring that path inputs are properly sanitized and normalized before processing. The use of secure temporary file creation methods, such as those that avoid user-supplied paths in temporary directory operations, should be enforced. Additionally, implementing proper file access controls and sandboxing mechanisms can limit the potential impact of exploitation attempts. Security controls should include monitoring for unusual temporary file creation patterns and implementing least privilege principles for temporary directory access. Organizations should also consider implementing web application firewalls that can detect and block suspicious path manipulation attempts. This vulnerability demonstrates the importance of secure coding practices and proper input validation in preventing path-based attacks, aligning with ATT&CK technique T1059.007 for command and scripting interpreter, and T1078.004 for valid accounts, when attackers attempt to leverage temporary file operations for persistence or privilege escalation.