CVE-2024-27456: Rack CORS Middleware File Permissions Issue

CVE-2024-27456 Overview

CVE-2024-27456 is an insecure permissions vulnerability affecting rack-cors (Rack CORS Middleware) version 2.0.1. The vulnerability stems from the Ruby (.rb) files being installed with overly permissive 0666 file permissions, which allows any user on the system to read and write to these files. This configuration flaw could enable local attackers to modify the middleware's source code, potentially leading to code execution, information disclosure, or complete compromise of applications using this library.

Critical Impact

Insecure file permissions (0666) on Ruby source files allow any local user to modify the rack-cors middleware, potentially enabling code injection, data theft, or complete application compromise.

Affected Products

• Rylabs Rack CORS Middleware version 2.0.1
• Applications using rack-cors gem version 2.0.1
• Ruby on Rails applications with vulnerable rack-cors dependency

Discovery Timeline

• 2024-02-26 - CVE-2024-27456 published to NVD
• 2025-09-18 - Last updated in NVD database

Technical Details for CVE-2024-27456

Vulnerability Analysis

This vulnerability represents a classic insecure permissions configuration issue (CWE-200: Information Exposure). When rack-cors version 2.0.1 is installed, the Ruby source files are created with 0666 permissions, meaning the owner, group, and all other users have read and write access to these files. In Unix-like systems, proper file permissions for executable or library code should typically be 0644 (read/write for owner, read-only for group and others) or more restrictive.

The vulnerability allows any authenticated user on the system to modify the middleware's behavior by editing the source files directly. Since rack-cors is middleware that handles Cross-Origin Resource Sharing (CORS) policies, an attacker could manipulate CORS headers to bypass security controls, inject malicious code that executes within the application context, or exfiltrate sensitive data processed by the application.

Root Cause

The root cause of this vulnerability is improper file permission handling during the gem installation or packaging process. The .rb files in the rack-cors gem version 2.0.1 are distributed or installed with world-writable permissions (0666), violating the principle of least privilege. This configuration error allows any local user to tamper with the library's source code without requiring elevated privileges.

Attack Vector

The attack vector is network-accessible but requires local system access for exploitation. An attacker with any level of user access to the system hosting the vulnerable rack-cors installation could:

1. Locate the rack-cors gem installation directory
2. Modify the Ruby source files to inject malicious code
3. Wait for the application to reload or restart
4. Execute arbitrary code within the application's context

The attack could be chained with other vulnerabilities or used by malicious insiders to escalate privileges or maintain persistence on compromised systems. Since rack-cors handles CORS policies, modifications could also weaken the application's cross-origin security controls, enabling browser-based attacks against users.

Detection Methods for CVE-2024-27456

Indicators of Compromise

• Unexpected modifications to .rb files in the rack-cors gem directory
• File modification timestamps on rack-cors source files that don't align with legitimate updates
• Unusual CORS headers being returned by the application
• Unexpected outbound connections from the application server

Detection Strategies

• Implement file integrity monitoring (FIM) on gem installation directories to detect unauthorized modifications
• Regularly audit file permissions on installed gems using find /path/to/gems -perm -0002 -type f to identify world-writable files
• Monitor application logs for unexpected CORS policy changes or errors
• Use runtime application self-protection (RASP) solutions to detect code injection attempts

Monitoring Recommendations

• Configure security information and event management (SIEM) alerts for file permission anomalies in application directories
• Implement regular automated scans of gem dependencies using tools like bundler-audit to identify vulnerable versions
• Monitor system calls and file access patterns for the Ruby process to detect tampering attempts

How to Mitigate CVE-2024-27456

Immediate Actions Required

• Audit the file permissions of the rack-cors gem installation and correct any world-writable files
• Update to a patched version of rack-cors if available (check the GitHub Issue Discussion for updates)
• Implement file integrity monitoring on the gem installation directory
• Review application logs for any signs of exploitation or unauthorized CORS policy changes

Patch Information

Organizations should monitor the official GitHub Issue Discussion for patch availability and updates from the maintainers. Until an official patch is released, manual remediation of file permissions is recommended.

Workarounds

• Manually fix file permissions on all rack-cors .rb files by running chmod 644 on each file
• Implement mandatory access controls (MAC) using SELinux or AppArmor to restrict write access to gem directories
• Run the application under a dedicated service account with minimal privileges to limit the impact of potential exploitation
• Consider using containerization to isolate the application environment and limit local attack surface

# Configuration example
# Fix insecure file permissions on rack-cors gem files
find $(bundle info rack-cors --path) -name "*.rb" -exec chmod 644 {} \;

# Verify permissions are corrected
find $(bundle info rack-cors --path) -name "*.rb" -exec ls -la {} \;

# Set restrictive permissions on the entire gems directory
chmod -R o-w $(gem environment gemdir)/gems/