Security risks of WebAssembly

WebAssembly (abbreviated as Wasm) is a low-level binary instruction format designed to provide near-native execution performance for web applications. Although it excels in performance optimization, the security risks it introduces cannot be ignored, especially in areas such as sandbox escape, memory safety, and malicious code injection.

Basic Security Model of WebAssembly

WebAssembly was designed with a security sandbox mechanism in mind, preventing direct access to the host environment's (e.g., browser) system resources by default. It ensures foundational security through the following mechanisms:

1. Linear Memory Model: Wasm uses contiguous memory blocks, and memory access must undergo explicit bounds checking.
2. Isolated Execution: Direct system API calls are impossible; interaction must occur through JavaScript glue code.
3. Type Safety: A strict static type system prevents type confusion attacks.

// Example: Bounds checking for Wasm memory access
const memory = new WebAssembly.Memory({ initial: 1 });
const buffer = new Uint8Array(memory.buffer);
// Legal access
buffer[0] = 42;
// Illegal out-of-bounds access (will throw an exception)
// buffer[memory.buffer.byteLength] = 0;

Memory Safety Risks

1. Heap Overflow and Out-of-Bounds Access

Despite Wasm's theoretical memory safety guarantees, the following scenarios can still cause issues:

• Compiler Vulnerabilities: For example, Emscripten-generated helper code may have missing bounds checks.
• Manual Memory Management Errors: Developers may introduce vulnerabilities when customizing memory allocators.

// Example of potential risks in C code compiled to Wasm
void unsafe_copy(char* dest, char* src, size_t len) {
    for (size_t i = 0; i <= len; i++) {  // Deliberate out-of-bounds error
        dest[i] = src[i];
    }
}

2. Memory Leak Attacks

Malicious modules may exhaust memory through:

• Actively allocating oversized memory blocks
• Creating memory fragmentation to prevent recycling
• Exploiting GC interaction vulnerabilities (e.g., maintaining references through JavaScript callbacks)

Sandbox Escape Techniques

1. Breaking Isolation via Import/Export Tables

Wasm modules obtain host functionality through import tables, which may expose dangerous APIs if misconfigured:

// Dangerous import table configuration
const imports = {
    env: {
        system: () => { /* Simulate system calls */ },
        fopen: (filename) => { /* File operations */ }
    }
};

2. Spectre-like Side-Channel Attacks

Wasm's deterministic execution environment can be exploited for timing attacks:

• Using branch prediction bias to read sensitive data
• Inferring encryption keys through memory access patterns

// Pseudocode: Detecting cache hit timing differences
function probeMemory(address) {
    const start = performance.now();
    // Force read target memory
    const dummy = loadMemory(address);
    return performance.now() - start;
}

Malicious Code Injection Pathways

1. Module Tampering Attacks

Attackers may:

• Modify .wasm files during CDN transmission
• Inject malicious code using build toolchains (e.g., malicious loader plugins)

# Hypothetical build process attack example
npm install malicious-wasm-loader --save-dev

2. Dynamic Code Generation Risks

Even though Wasm prohibits direct dynamic code execution, it can still be bypassed via:

• Combining exported functions from multiple modules
• Using JIT Spraying techniques

// Dynamically combining function calls
const funcPtrs = [module1.exports.func1, module2.exports.func2];
const maliciousFunc = funcPtrs[userInput];  // Potentially manipulated index

Supply Chain Attack Surfaces

1. Third-Party Module Risks

Wasm modules imported from repositories like npm may:

• Contain undeclared backdoor functionality
• Deliberately trigger vulnerabilities in specific environments

// Potentially hidden malicious dependencies in package.json
{
    "dependencies": {
        "optimized-lib": "^1.0.0"  // Actually contains malicious Wasm code
    }
}

2. Development Toolchain Pollution

• Compiler backdoors (e.g., modifying binaryen output)
• Debugging tools injecting sampling code

Defense Best Practices

1. Strict Resource Limits

// Secure configuration when creating a Wasm instance
const instance = await WebAssembly.instantiate(module, {
    env: {
        memory: new WebAssembly.Memory({
            initial: 10,
            maximum: 100  // Hard memory limit
        })
    }
});

2. Integrity Verification

• Use Subresource Integrity (SRI) to protect .wasm files
• Validate exported function signatures at runtime

<script type="application/wasm"
        src="module.wasm"
        integrity="sha384-..."></script>

3. Defense-in-Depth Strategies

• Combine WASI to restrict system access
• Enable Chromium's Wasm trap handler
• Regularly audit compiler output

# Use wasm-opt for security optimization
wasm-opt --detect-features --safe-stack module.wasm -o secured.wasm

Emerging Attack Vectors

1. Shared Memory Attacks in Multithreading

When using SharedArrayBuffer, issues may arise such as:

• Race conditions in atomic operations
• Inconsistent memory views

const sharedMemory = new WebAssembly.Memory({
    initial: 1,
    maximum: 1,
    shared: true
});

2. WASI Extension Risks

Server-side Wasm accessing system resources via WASI may lead to:

• File descriptor leaks
• Unrestricted network access permissions

// Example of potential risks in Rust compiled to WASI
#[no_mangle]
pub unsafe extern "C" fn read_file() {
    let fd = libc::open(b"/etc/passwd\0".as_ptr() as _, 0);
    // Potentially abused system calls
}