WebAssembly Security Sandbox¶

Why WebAssembly for Security?¶

WebAssembly (Wasm) provides a sandboxed execution environment with:

• Memory Safety: Linear memory with bounds checking
• Capability-Based Security: Explicit imports for system access
• Deterministic Execution: Same inputs = same outputs
• Platform Independence: Runs anywhere with a Wasm runtime

Security Model¶

Memory Isolation¶

┌─────────────────────────────┐
│     Host Environment        │
├─────────────────────────────┤
│  Wasm Runtime (Wasmtime)    │
├─────────────────────────────┤
│  ┌───────────────────────┐  │
│  │   Linear Memory       │  │
│  │   (Bounded, Isolated) │  │
│  └───────────────────────┘  │
│           Wasm Module       │
└─────────────────────────────┘

Import/Export System¶

Wasm modules can only access what is explicitly provided:

// Host provides these functions
#[link(wasm_import_module = "host")]
extern "C" {
    fn log_message(ptr: *const u8, len: usize);
    fn get_timestamp() -> u64;
}

Implementing Secure Rules (Rust to Wasm)¶

Step 1: Write Detection Logic¶

#[no_mangle]
pub extern "C" fn check_file_access(path_ptr: *const u8, path_len: usize) -> i32 {
    let path = unsafe {
        std::str::from_utf8_unchecked(
            std::slice::from_raw_parts(path_ptr, path_len)
        )
    };

    // Check for sensitive paths
    if path.contains("/etc/passwd") || path.contains("/etc/shadow") {
        return 1; // Alert
    }
    0 // Normal
}

Step 2: Compile to Wasm¶

cargo build --target wasm32-wasi --release

Step 3: Load in Host (Go)¶

func loadRule(wasmPath string) (*wasmtime.Instance, error) {
    engine := wasmtime.NewEngine()
    store := wasmtime.NewStore(engine)
    module, _ := wasmtime.NewModuleFromFile(engine, wasmPath)

    return wasmtime.NewInstance(store, module, nil)
}

Best Practices¶

1. Minimal Imports: Only expose necessary host functions
2. Memory Limits: Set maximum memory pages
3. Execution Timeouts: Prevent infinite loops
4. Input Validation: Sanitize data before passing to Wasm
5. Capability Tokens: Use WASI capabilities sparingly

Performance Considerations¶

WasmSentinel Architecture¶

Events (eBPF) → Collector → Wasm Rules → Alert Handler
                              ↑
                        Rule Hot-Reload