CVE-2021-29425 Medium Severity Vulnerability In Apache Commons IO 2.6

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This article delves into the CVE-2021-29425 vulnerability, a medium-severity security flaw detected in the widely used Apache Commons IO library, specifically version 2.6. This vulnerability highlights the importance of maintaining up-to-date software libraries and understanding the potential risks associated with path traversal issues. We will explore the details of the vulnerability, its impact, technical specifics, and recommended steps to mitigate it.

Understanding the Vulnerability

At its core, CVE-2021-29425 is a path traversal vulnerability found within the FileNameUtils.normalize method of the Apache Commons IO library. Path traversal vulnerabilities occur when an application improperly handles user-supplied input that can influence file paths. This can allow attackers to access files and directories outside of the intended scope, potentially leading to sensitive data exposure or even system compromise. In this particular case, the FileNameUtils.normalize method, designed to clean and normalize file paths, failed to correctly handle specific input strings. When provided with an improper input string, such as "//../foo" or "\\..\foo", the method would return the input string unchanged. This seemingly minor flaw could be exploited if the calling code used the result of this method to construct a file path, potentially granting access to files in the parent directory.

Technical Deep Dive into CVE-2021-29425

To fully grasp the implications of this vulnerability, it's crucial to understand the inner workings of the FileNameUtils.normalize method and how path traversal attacks function. The normalize method is intended to remove redundancies and simplify file paths, such as converting "/foo/./bar/.." to "/foo". However, the vulnerability arises from its failure to properly sanitize specific malformed inputs. The use of double slashes (//) or mixed slashes (\) followed by the ../ sequence (which signifies moving one directory up) bypasses the intended normalization logic. This is a classic example of a limited path traversal vulnerability, as it only allows access to files within the parent directory and not further up the directory tree. The impact of this vulnerability is categorized as medium severity because while it doesn't grant complete file system access, it still poses a significant risk. An attacker could potentially access configuration files, logs, or other sensitive data stored in the parent directory, depending on the application's file structure and permissions. It's important to note that the severity can vary based on the context in which the vulnerable library is used.

The Impact of the Vulnerability

The potential impact of CVE-2021-29425 can vary depending on the specific application and its configuration. However, the core risk lies in the potential for unauthorized access to files and directories. In a web application context, for example, an attacker might be able to access configuration files containing database credentials or other sensitive information. They might also be able to read log files that could reveal information about system behavior or user activity. The vulnerability's medium severity rating reflects the limited scope of the path traversal – it only allows access to the parent directory. However, this is still a significant risk, as the parent directory often contains important files and directories. The impact is further exacerbated if the application is running with elevated privileges, as this could allow the attacker to access more sensitive data. Moreover, in some cases, the vulnerability could be chained with other vulnerabilities to achieve a more significant impact, such as remote code execution. Therefore, even though it's classified as medium severity, it's crucial to address this vulnerability promptly to prevent potential exploitation.

Technical Details of the Vulnerability

Let's delve deeper into the technical specifics of CVE-2021-29425. The vulnerability lies within the FileNameUtils.normalize method in Apache Commons IO versions prior to 2.7. This method aims to normalize file paths by resolving relative paths, removing redundant separators, and handling special characters. However, it fails to properly sanitize specific input strings containing sequences like "//../" or "\\../". These sequences are intended to traverse up the directory structure, but the normalize method's flawed logic allows them to bypass the sanitization process. As a result, the method returns the input string without proper normalization, potentially leading to path traversal vulnerabilities when the output is used to construct file paths. The vulnerability arises from an incomplete or incorrect implementation of path normalization, making it susceptible to specific forms of malicious input. The Common Vulnerability Scoring System (CVSS) v3 score for CVE-2021-29425 is 4.8 (Medium), with the following breakdown:

  • Attack Vector (AV): Network (N)
  • Attack Complexity (AC): High (H)
  • Privileges Required (PR): None (N)
  • User Interaction (UI): None (N)
  • Scope (S): Unchanged (U)
  • Confidentiality Impact (C): Low (L)
  • Integrity Impact (I): Low (L)
  • Availability Impact (A): None (N)

The CVSS score reflects the moderate risk posed by this vulnerability. The high attack complexity suggests that exploiting this vulnerability requires some level of technical expertise. However, the fact that no privileges or user interaction are required makes it a potentially exploitable issue. The low impact on confidentiality and integrity indicates that the vulnerability may allow an attacker to read or modify some data, but is unlikely to lead to complete system compromise.

Remediation and Mitigation Strategies

The suggested fix for CVE-2021-29425 is to upgrade to Apache Commons IO version 2.7 or later. This version includes a fix for the path traversal vulnerability in the FileNameUtils.normalize method. Upgrading is the most effective way to address the vulnerability and prevent potential exploitation. The fix implemented in version 2.7 includes improved input validation and sanitization to properly handle potentially malicious path sequences. This ensures that the normalize method correctly processes file paths, preventing attackers from using path traversal techniques to access unauthorized files or directories.

Steps to Remediate the Vulnerability

  1. Identify Affected Applications: The first step is to identify all applications within your environment that use Apache Commons IO version 2.6 or earlier. This can be done through dependency scanning tools or by manually reviewing project dependencies.
  2. Upgrade to Version 2.7 or Later: Once you've identified the affected applications, upgrade the Apache Commons IO library to version 2.7 or later in each application. This can typically be done by updating the project's dependency management configuration (e.g., pom.xml in Maven projects, build.gradle in Gradle projects).
  3. Test the Application: After upgrading the library, thoroughly test the application to ensure that the upgrade has not introduced any compatibility issues or regressions. Pay close attention to any functionality that involves file path manipulation or access.
  4. Monitor for New Vulnerabilities: It's essential to continuously monitor your applications for new vulnerabilities, including those in third-party libraries like Apache Commons IO. This can be done through vulnerability scanning tools and by staying informed about security advisories and updates.

Additional Mitigation Measures

In addition to upgrading the library, consider implementing other security best practices to mitigate the risk of path traversal vulnerabilities:

  • Input Validation: Implement robust input validation to ensure that user-supplied input is properly sanitized and validated before being used to construct file paths. This can help prevent attackers from injecting malicious path sequences.
  • Principle of Least Privilege: Ensure that applications run with the minimum necessary privileges. This can limit the potential damage if a vulnerability is exploited.
  • Web Application Firewalls (WAFs): Deploy a WAF to detect and block path traversal attacks. WAFs can provide an additional layer of security by inspecting HTTP requests and filtering out malicious traffic.

Conclusion

CVE-2021-29425 in Apache Commons IO 2.6 is a medium-severity path traversal vulnerability that highlights the importance of keeping software libraries up to date and implementing secure coding practices. By understanding the technical details of the vulnerability, its potential impact, and the recommended remediation steps, developers and system administrators can effectively mitigate the risk and protect their applications from exploitation. Upgrading to version 2.7 or later is the primary solution, but implementing additional security measures like input validation and using a WAF can further enhance the security posture of your applications. Regular security audits and vulnerability scanning are crucial for identifying and addressing potential security flaws proactively.