CVE-2025-27553: Apache Commons VFS Path Traversal Flaw

CVE-2025-27553 Overview

A Relative Path Traversal vulnerability exists in Apache Commons VFS before version 2.10.0. The FileObject API in Commons VFS has a resolveFile method that takes a scope parameter. When specifying NameScope.DESCENDENT, the API promises that an exception will be thrown if the resolved file is not a descendant of the base file. However, when the path contains URL-encoded ".." characters (for example, %2E%2E/bar.txt), the method may return file objects that are not descendants of the base file without throwing an exception. This allows attackers to bypass intended access restrictions and potentially read sensitive files outside the expected directory scope.

Critical Impact

Attackers can bypass path traversal protections by using URL-encoded directory traversal sequences to access files outside of restricted directories, potentially exposing sensitive configuration files, credentials, or other confidential data.

Affected Products

• Apache Commons VFS versions before 2.10.0
• Applications using the FileObject.resolveFile() API with NameScope.DESCENDENT scope
• Java applications that rely on Commons VFS for secure file access controls

Discovery Timeline

• 2025-03-23 - CVE-2025-27553 published to NVD
• 2025-04-02 - Last updated in NVD database

Technical Details for CVE-2025-27553

Vulnerability Analysis

This path traversal vulnerability arises from insufficient input validation in the Apache Commons VFS library. The core issue lies in how the resolveFile method handles URL-encoded path components when enforcing the NameScope.DESCENDENT security constraint.

When developers use NameScope.DESCENDENT to restrict file access to descendants of a base directory, they expect the library to prevent any path that would escape that directory boundary. However, the validation logic fails to properly decode and normalize URL-encoded sequences before checking path constraints.

An attacker can exploit this by providing paths containing %2E%2E (URL-encoded "..") which bypass the descendant check while still being interpreted as parent directory references when the actual file system operation occurs. This creates a disconnect between the security validation and the file resolution, enabling unauthorized access to files outside the intended scope.

Root Cause

The root cause is improper input validation (CWE-23: Relative Path Traversal) in the path normalization process. The resolveFile method does not consistently decode URL-encoded characters before validating whether the resolved path remains within the descendant scope. This allows encoded traversal sequences to slip through validation checks that would otherwise catch literal ".." sequences.

Attack Vector

The vulnerability is exploitable over the network by any unauthenticated attacker who can influence the file path parameter passed to the resolveFile method. This commonly occurs in web applications where user-supplied input is used to construct file paths. For example, an API endpoint that allows users to download files from a specific directory could be exploited by providing a path like %2E%2E/%2E%2E/etc/passwd to traverse outside the intended directory and access sensitive system files.

The attack requires no authentication or user interaction, and the complexity is low since the attacker simply needs to URL-encode the traversal sequence. Successful exploitation results in unauthorized read access to files on the server, potentially exposing sensitive configuration data, credentials, or other confidential information.

Detection Methods for CVE-2025-27553

Indicators of Compromise

• Unusual file access patterns in application logs showing encoded path sequences like %2E%2E or %252E%252E
• Web server access logs containing URL-encoded traversal attempts targeting file download or retrieval endpoints
• Application errors or exceptions related to file access attempts outside expected directories
• Unexpected access to sensitive files such as /etc/passwd, configuration files, or application secrets

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block URL-encoded path traversal sequences (%2E%2E, %252E%252E, etc.)
• Deploy application-level logging to capture and alert on file path parameters containing encoded special characters
• Use static application security testing (SAST) tools to identify usage of vulnerable resolveFile calls with NameScope.DESCENDENT
• Monitor file system access patterns for anomalous reads outside expected application directories

Monitoring Recommendations

• Enable verbose logging for file operations in applications using Apache Commons VFS to capture attempted path traversal attacks
• Set up alerts for any file access attempts that would normally trigger the descendant scope exception but proceed without error
• Review application logs for patterns of requests with URL-encoded characters targeting file retrieval functionality
• Implement runtime application self-protection (RASP) to monitor and block path traversal attempts in real-time

How to Mitigate CVE-2025-27553

Immediate Actions Required

• Upgrade Apache Commons VFS to version 2.10.0 or later immediately
• Audit all application code for usage of resolveFile with NameScope.DESCENDENT to identify potentially affected components
• Implement additional input validation to decode and normalize file paths before passing them to Commons VFS APIs
• Deploy WAF rules to block URL-encoded path traversal attempts as a defense-in-depth measure

Patch Information

Apache has released version 2.10.0 of Commons VFS which addresses this vulnerability. Users should upgrade to this version or later to receive the fix. For detailed information about the security update, refer to the Apache Mailing List Discussion. Debian users can find additional guidance in the Debian LTS Announcement.

Workarounds

• Implement application-level URL decoding and path normalization before calling resolveFile to ensure encoded traversal sequences are detected
• Add explicit validation to reject any file paths containing ".." sequences after decoding, regardless of encoding format
• Use allowlist-based file access controls that explicitly enumerate permitted files rather than relying solely on directory scope restrictions
• Consider sandboxing file operations using operating system-level access controls as an additional security layer

# Configuration example - Maven dependency update
# Update your pom.xml to use the patched version:
# <dependency>
#     <groupId>org.apache.commons</groupId>
#     <artifactId>commons-vfs2</artifactId>
#     <version>2.10.0</version>
# </dependency>

# For Gradle, update build.gradle:
# implementation 'org.apache.commons:commons-vfs2:2.10.0'