89 releases (24 breaking)

0.208.1 May 20, 2024
0.207.0 May 7, 2024
0.202.0 Mar 26, 2024
0.13.1 Nov 29, 2023
0.1.11 Nov 23, 2020

#678 in WebAssembly

Download history 5322/week @ 2024-02-05 4990/week @ 2024-02-12 6164/week @ 2024-02-19 6866/week @ 2024-02-26 7311/week @ 2024-03-04 8560/week @ 2024-03-11 9042/week @ 2024-03-18 6336/week @ 2024-03-25 8248/week @ 2024-04-01 7826/week @ 2024-04-08 8629/week @ 2024-04-15 10515/week @ 2024-04-22 8149/week @ 2024-04-29 9959/week @ 2024-05-06 13785/week @ 2024-05-13 11731/week @ 2024-05-20

44,258 downloads per month
Used in 4 crates

Apache-2.0 WITH LLVM-exception

25K SLoC


A WebAssembly test case generator.


  • Always valid: All generated Wasm modules pass validation. wasm-smith gets past your wasm parser and validator, exercising the guts of your Wasm compiler, runtime, or tool.

  • Supports the full WebAssembly language: Doesn't have blind spots or unimplemented instructions.

  • Implements the Arbitrary trait: Easy to use with cargo fuzz and libfuzzer-sys!

  • Deterministic: Given the same input seed, always generates the same output Wasm module, so you can always reproduce test failures.

  • Plays nice with mutation-based fuzzers: Small changes to the input tend to produce small changes to the output Wasm module. Larger inputs tend to generate larger Wasm modules.


With cargo fuzz and libfuzzer-sys

First, use cargo fuzz to define a new fuzz target:

$ cargo fuzz add my_wasm_smith_fuzz_target

Next, add wasm-smith to your dependencies:

$ cargo add wasm-smith

Then, define your fuzz target so that it takes arbitrary wasm_smith::Modules as an argument, convert the module into serialized Wasm bytes via the to_bytes method, and then feed it into your system:

// fuzz/fuzz_targets/my_wasm_smith_fuzz_target.rs


use libfuzzer_sys::fuzz_target;
use wasm_smith::Module;

fuzz_target!(|module: Module| {
    let wasm_bytes = module.to_bytes();

    // Your code here...

Finally, start fuzzing:

$ cargo fuzz run my_wasm_smith_fuzz_target

Note: Also check out the validate fuzz target defined in this repository. Using the wasmparser crate, it checks that every module generated by wasm-smith validates successfully.

As a Command Line Tool

Install the CLI tool via cargo:

$ cargo install wasm-tools

Convert some arbitrary input into a valid Wasm module:

$ head -c 100 /dev/urandom | wasm-tools smith -o test.wasm

Finally, run your tool on the generated Wasm module:

$ my-wasm-tool test.wasm


~48K SLoC