Linux Temporary Directory Security: Best Practices and Common Pitfalls

The proper use of temporary directories in Linux systems has long been a source of confusion and potential security vulnerabilities. Recent discussions in the developer community have highlighted the importance of understanding the distinctions between /tmp/ and /var/tmp/, as well as the security implications of their usage.

Key Security Considerations

RAM vs. Persistent Storage

One of the most critical distinctions that developers need to understand is that /tmp/ is typically RAM-backed using tmpfs, while /var/tmp/ uses persistent storage. This difference has important implications for both performance and data persistence:

• /tmp/ is cleared on reboot and should only be used for smaller, temporary files
• /var/tmp/ persists across reboots and is better suited for larger temporary files

Namespace Vulnerabilities

A significant security concern that has emerged from community discussions is the shared namespace problem. Using predictable filenames in these directories can lead to:

• Denial of Service (DoS) attacks
• Race conditions
• Potential security breaches through file manipulation

Best Practices for Modern Development

Recommended Approaches

1. Use Modern APIs
   • memfd_create() for memory-based temporary files
   • O_TMPFILE for secure file creation
   • mkstemp() and related POSIX functions for compatibility

Service Isolation

The community particularly emphasizes the importance of using systemd's PrivateTmp= feature for system services, which provides:

• Isolated temporary directories per service
• Automatic cleanup on service shutdown
• Additional protection against cross-service tampering

Automatic Cleanup Considerations

A notable point of discussion among developers is the automatic cleanup mechanism:

• Files in /tmp/ are removed after 10 days of inactivity
• Files in /var/tmp/ are cleaned up after 30 days
• BSD file locks (flock) can prevent premature cleanup of important temporary files

Early Boot and Resource Management

Developers working on early boot processes should be particularly careful:

• Neither /tmp/ nor /var/tmp/ may be available during early boot
• Using /dev/shm/ as an alternative is discouraged
• Package-specific directories in /run/ are recommended for early boot operations

The community consensus emphasizes that while temporary directories are essential tools, their proper usage requires careful consideration of security, resource management, and system state to avoid common pitfalls and vulnerabilities.