created_at2023-12-16 14:17:31.318073
updated_at2024-05-24 18:51:27.009797
descriptionLibrary to parse context-free grammars and create grammar-based fuzzing tools
z2 (z2-2z)



~~~ fuzzing with grammar-based mutations ~~~

This project is a reimplementation of Gramatron that is

  • performant: 4x higher throughput than Gramatron or LibAFL's Gramatron implementation
  • versatile: usable with LibAFL, libfuzzer, in a custom AFL++ mutator or standalone
  • easy to use: no more orchestration of different scripts to get the fuzzing campaign running, everything is batteries-included
  • extendable: at its core, peacock is a library that you can use at your leisure to customize every step of the grammar fuzzing process
  • backwards compatible: it works with grammars that you have already written for other tools

How it works

Peacock implements so-called "grammar-based mutations". This means that it will mutate its inputs in such a way that they will always adhere to a given grammar.

The way mutations work is the same as in Gramatron. A grammar is converted to a PDA such that an input can be represented as a walk through the automaton. Then, a mutation of an input is simply a modification of an automaton walk. We cut off the walk at a random point and let it find a new random path through the automaton from there.

While Gramatron and LibAFL realize the automaton as an adjacency matrix, peacock generates C code that encodes the automaton in its control flow. This saves us a lot of memory accesses and makes the mutation procedure faster.

The generated C code exposes a certain API that can be used by any application, e.g. a libfuzzer harness, an AFL++ custom mutator or even Rust code.

But peacock also ships a ready to use fuzzer that can fuzz any binary that has been compiled with AFL++'s compilers or implements an AFL-style forkserver.

How to use it

Clone the repo and execute

cargo build --release

This creates 5 ready-to-use tools:

  1. peacock-fuzz: A coverage-guided fuzzer that can fuzz any binary compiled with AFL++'s compilers or anything that speaks AFL's forkserver protocol
  2. peacock-dump: peacock-fuzz saves crashes and queue items in a raw, binary format to disk. Use this tool to get a human readable output from any such file. All these binary files have the prefix peacock-raw-
  3. peacock-compile: Takes a grammar and compiles it to C code
  4. peacock-merge: Merge multiple grammar files into one or convert a grammar file from one format into another
  5. peacock-gen: Generate individual inputs from a grammar

If you want more fine-grained control you can use the crate peacock_fuzz, which is the backbone of all the tools from above. See the documentation at in order to get started with peacock as a library.

How to write grammars

Peacock accepts its context-free grammars in JSON format. A context-free grammar has production rules of the form:

A -> X Y Z ...

where A must be a non-terminal and X,Y,Z can be non-terminals or terminals. The right-hand-side must contain at least one symbol.

Non-terminals are enclosed in <>, so the non-terminal A would be represented as <A>. Terminals are enclosed in ''.

The set of rules

A -> a B
A -> a
B -> b B
B -> Ɛ

would be written as

    // Comments are also possible :)
    "<A>": [
        ["'a'", "<B>"],
    "<B>": [
        ["'b'", "<B>"],
        ["''"] // Ɛ = ''

and corresponds to the regular expression a(b*).

Peacock also supports the Gramatron format, which is a bit different and does not allow for comments.

The non-terminal <ENTRYPOINT> is the entrypoint of the grammar.

C API Documentation

  • void seed_generator (size_t new_seed)
    Supply a seed for the RNG of the mutator.

  • size_t unparse_sequence (size_t* seq_buf, size_t seq_capacity, unsigned char* input, size_t input_len)
    Given an input that adheres to the grammar, find the corresponding automaton walk. This function may be slow, use outside of hot loop.

    • seq_buf: Automaton walk will be written into this buffer
    • seq_capacity: Maximum number of elements that seq_buf can hold (not number of bytes)
    • input: User input adhering to grammar
    • input_len: Length of input

    Returns the number of elements written to seq_buf or 0 if input does not adhere to grammar.

  • size_t mutate_sequence (size_t* buf, size_t len, size_t capacity)
    Given an automaton walk, create a random mutant of the walk.

    • buf: Pointer to array that holds automaton walk
    • len: Number of items in buf (not number of bytes)
    • capacity: Maximum number of items that buf can hold (not number of bytes)

    Returns the length of the new walk.

  • size_t serialize_sequence (size_t* seq, size_t seq_len, unsigned char* out, size_t out_len)
    Given an automaton walk, create the corresponding output.

    • seq: Pointer to automaton walk
    • seq_len: Number of items in seq (not number of bytes)
    • out: Output will be written into that buffer
    • out_len: Number of bytes in out

    Returns how many bytes have been written to out.


  • MAKE_THREAD_SAFE: Define this to make the mutator completely thread-safe
  • MAKE_VISIBLE: Define this to explicitly set the visibility of the functions from above to "default"
  • STATIC_SEED: Compile-time seed for the RNG
  • DISABLE_rand: Don't include the internal rand function and use an external one with the signature size_t rand (void)
  • DISABLE_seed_generator: Don't include the function seed_generator
Commit count: 174

cargo fmt