Real-time Script Injection Prevention (Tetragon LSM)

Goal: Prevent script injection by blocking unauthorized writes to mission-critical .sh, .py, and .js files in real time—before changes hit disk.

Problem

• Traditional File Integrity Monitoring (FIM) is reactive—it alerts after modification.
• Script injection attacks compromise trusted environments by modifying legitimate scripts.
• Legacy hook points (e.g., kprobes) can be vulnerable to TOCTOU (Time-of-Check to Time-of-Use) race conditions.

Solution

Implemented a high-performance Cilium Tetragon TracingPolicy using the Linux Security Modules (LSM) framework to shift from detection to active enforcement.

How it works

• LSM-native enforcement: Hooks at file_permission to inspect write attempts at an authoritative kernel stage.
• Fast scoping: Uses Prefix path matching to protect only critical directories (e.g., /var/www/html/scripts/, /home/user/my_scripts/) with minimal overhead.
• Non-disruptive blocking: Uses Override to inject -13 (EACCES) into the syscall result, denying unauthorized writes without killing the process.

Technical Specs

• Framework: Cilium Tetragon (eBPF-driven)
• Security layer: LSM
• Kernel hook: file_permission
• Matching strategy: Prefix operator
• Intercepted flag: MAY_WRITE (mask: 2)
• Enforcement action: Synchronous error injection (Override: -13)

Outcomes / Impact

• Active prevention: Blocks unauthorized modifications before any data is written.
• Operational stability: Returns “Permission denied” instead of terminating processes—reducing downtime and crash loops.
• Race-condition resilience: LSM enforcement is atomic and final, eliminating common TOCTOU bypass windows.
• Production suitability: Prefix-based filtering keeps latency low for high-traffic environments.