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Blazing Fast Hybrid Fuzzer for Smart Contracts

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ItyFuzz 🍦

Fast hybrid fuzzer for EVM & MoveVM (WIP) smart contracts.

You can generate exploits instantly by just providing the contract address:

Research Paper / Fuzzing EVM Contracts / Fuzzing Move Contracts

Run ItyFuzz with UI

Install Docker and run docker image suitable for your system architecture:

docker pull fuzzland/ityfuzz:stable
docker run -p 8000:8000 fuzzland/ityfuzz:stable

Then, you can visit the interface at http://localhost:8000

Note: The container uses public ETH RPC, may time out / be slow

Statistics & Comparison

Time taken for finding vulnerabilities / generating exploits:

Project Name Vulnerability Mythril SMARTIAN Slither ItyFuzz
AES Business Logic Inf Unsupported No 4hrs
Carrot Arbitrary External Call 17s 11s Yes 1s
Olympus Access Control 36s Inf Yes 1s
MUMUG Price Manipulation Inf Unsupported No 18hrs
Omni Reentrancy Inf Unsupported Yes* 22hrs
Verilog CTF-2 Reentrancy Inf Unsupported Yes* 3s

* Slither only finds the reentrancy location, but not how to leverage reentrancy to trigger final buggy code. The output also contains significant amount of false positives.

Test Coverage:

Dataset SMARTIAN Echidna ItyFuzz
B1 97.1% 47.1% 99.2%
B2 86.2% 82.9% 95.4%
Tests Unsupported 52.9% 100%

* B1 and B2 contain 72 single-contract projects from SMARTIAN artifacts. Tests are the projects in tests directory. The coverage is calculated as (instruction covered) / (total instruction - dead code).

Building (EVM)

You first need to install Rust through https://rustup.rs/

You need to have libssl-dev (OpenSSL) and libz3-dev (refer to Z3 Installation section for instruction) installed.

git clone https://github.com/fuzzland/ityfuzz.git && cd ityfuzz && git checkout stable
git submodule update --recursive --init
cd cli/
cargo build --release

You can enable certain debug gates in Cargo.toml

solc is needed for compiling smart contracts. You can use solc-select tool to manage the version of solc.

Run (EVM)

Compile Smart Contracts:

cd ./tests/evm/multi-contract/
# include the library from ./solidity_utils for example
solc *.sol -o . --bin --abi --overwrite --base-path ../../../

Run Fuzzer:

# after building, there should be a binary in ./cli/target/release/cli
cd ./cli/
./target/release/cli evm -t '../tests/evm/multi-contract/*'

Demo

Verilog CTF Challenge 2 tests/verilog-2/

Flashloan attack + Reentrancy. The target is to reach line 34 in Bounty.sol.

Exact Exploit:

0. Borrow k MATIC such that k > balance() / 10
1. depositMATIC() with k MATIC
2. redeem(k * 1e18) -- reentrancy contract --> getBounty()
3. Return k MATIC

Use fuzzer to detect the vulnerability and generate the exploit (takes 0 - 200s):

# build contracts in tests/evm/verilog-2/
solc *.sol -o . --bin --abi --overwrite --base-path ../../
# run fuzzer
./target/release/cli evm -f -t "./tests/evm/verilog-2/*"

-f flag enables automated flashloan, which hooks all ERC20 external calls and make any users to have infinite balance.

Fuzz a Project (Offline)

You can fuzz a project by providing a path to the project directory.

./target/release/cli evm -t '[DIR_PATH]/*'

ItyFuzz would attempt to deploy all artifacts in the directory to a blockchain with no other smart contracts.

Specifically, the project directory should contain a few [X].abi and [X].bin files. For example, to fuzz a contract named main.sol, you should ensure main.abi and main.bin exist in the project directory. The fuzzer will automatically detect the contracts in directory, the correlation between them (see tests/evm/multi-contract), and fuzz them.

Optionally, if ItyFuzz fails to infer the correlation between contracts, you can add a [X].address, where [X] is the contract name, to specify the address of the contract.

Caveats:

  • Keep in mind that ItyFuzz is fuzzing on a clean blockchain, so you should ensure all related contracts (e.g., ERC20 token, Uniswap, etc.) are deployed to the blockchain before fuzzing.
  • If your smart contract requires constructor arguments, please refer to below Constructor Arguments section.

Fuzz a Project (Online)

Rebuild with flashloan_v2 (only supported in onchain) enabled to get better result.

python3 -c 'content=open("Cargo.toml").read().replace("default = [", "default = [\"flashloan_v2\",");open("Cargo.toml","w").write(content);'
cd ./cli/
cargo build --release

You can fuzz a project by providing an address, a block, and a chain type.

./target/release/cli evm -o -t [TARGET_ADDR] --onchain-block-number [BLOCK] -c [CHAIN_TYPE] --onchain-etherscan-api-key [Etherscan API Key]

Example: Fuzzing WETH contract (0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2) on Ethereum mainnet at latest block.

./target/release/cli evm -o -t 0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2 --onchain-block-number 0 -c ETH --onchain-etherscan-api-key PXUUKVEQ7Y4VCQYPQC2CEK4CAKF8SG7MVF

Fuzzing with flashloan and oracles enabled:

./target/release/cli evm -o -t 0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2 --onchain-block-number 0 -c ETH -f -i -p --onchain-etherscan-api-key PXUUKVEQ7Y4VCQYPQC2CEK4CAKF8SG7MVF

ItyFuzz would pull the ABI of the contract from Etherscan and fuzz it. If ItyFuzz encounters an unknown slot in the memory, it would pull the slot from chain RPC. If ItyFuzz encounters calls to external unknown contract, it would pull the bytecode and ABI of that contract. If its ABI is not available, ItyFuzz would not send any transaction to that contract.

Constructor Arguments

ItyFuzz provides two methods to pass in constructor arguments. These arguments are necessary for initializing the state of the contract when deployed.

Method 1: CLI Arguments

The first method is to pass in the constructor arguments directly as CLI arguments.

When you run ItyFuzz using the CLI, you can include the --constructor-args flag followed by a string that specifies the arguments for each constructor.

The format is as follows:

./target/release/cli evm -t 'tests/evm/multi-contract/*' --constructor-args "ContractName:arg1,arg2,...;AnotherContract:arg1,arg2,..;"

For example, if you have two contracts, main and main2, both having a bytes32 and a uint256 as constructor arguments, you would pass them in like this:

./target/release/cli evm -t 'tests/evm/multi-contract/*' --constructor-args "main:1,0x6100000000000000000000000000000000000000000000000000000000000000;main2:2,0x6200000000000000000000000000000000000000000000000000000000000000;"

Method 2: Server Forwarding

The second method is to use our server to forward requests to a user-specified RPC, and cli will fetch the constructor arguments from the transactions sent to the RPC.

Firstly, go to the /server directory, and install the necessary packages:

cd /server
npm install

Then, start the server using the following command:

node app.js

By default, the server will forward requests to http://localhost:8545, which is the default address for Ganache, if you do not have a local blockchain running, you can use Ganache to start one. If you wish to forward requests to another location, you can specify the address as a command-line argument like so:

node app.js http://localhost:8546

Once the server is running, you can deploy your contract to localhost:5001 using a tool of your choice.

For example, you can use Foundry to deploy your contract through the server:

forge create src/flashloan.sol:main2 --rpc-url http://127.0.0.1:5001 --private-key 0x0000000000000000000000000000000000000000000000000000000000000000 --constructor-args "1" "0x6100000000000000000000000000000000000000000000000000000000000000"

Finally, you can fetch the constructor arguments using the --fetch-tx-data flag:

./target/release/cli evm -t 'tests/evm/multi-contract/*' --fetch-tx-data

ItyFuzz will fetch the constructor arguments from the transactions forwarded to the RPC through the server.

Finding Custom Bugs (EVM)

You can simply insert bug() or typed_bug(string message) in your contract to report a condition when bug is found.

For instance, a simple case can be written as follows:

function buy_token() public {
    if (msg.sender != owner) {
        bug();
    }
}

The implementation of bug() is as follows:

library FuzzLand {
    event AssertionFailed(string message);
  
    function bug() internal {
        emit AssertionFailed("Bug");
    }
  
    function typed_bug(string memory data) internal {
        emit AssertionFailed(data);
    }

}

function bug()  {
    FuzzLand.bug();
}

function typed_bug(string memory data)  {
    FuzzLand.typed_bug(data);
}

You can either paste the code above into your contract or import it from solidity_utils/lib.sol, if you are using bug or typed_bug.

Echidna Support

Any contracts bearing functions starting with echidna_ will be treated as invariants and will be tested by ItyFuzz. If it returns false, the fuzzer will report a bug.

function echidna_test() public {
    assert(false);
}

Scribble Support

Scribble is a tool for writing specifications for Solidity contracts. ItyFuzz supports Scribble annotations after it is compiled by scribble.

For example, the following contract has a Scribble annotation that specifies the return value of inc:

contract Foo {
    /// #if_succeeds {:msg "P1"} y == x + 2;
    function inc(uint x) public pure returns (uint y) {
        return x+1;
    }
}

You need to compile the contract using scribble and pass the compiled contract to ItyFuzz

Note that you must add --no-assert to the scribble command. Otherwise, ItyFuzz will not detect any bugs.

scribble test.sol --output-mode flat --output compiled.sol --no-assert

Then compile with solc and run ItyFuzz:

solc compiled.sol --bin --abi --overwrite -o build
./target/release/cli evm -t "build/*" [More Arguments]

Test Coverage

ItyFuzz can collect instruction and branch coverage information for all the contracts it fuzzes. You simply need to append --replay-file [WORKDIR]/corpus/*_replayable to collect all these information.

./target/release/cli evm -t [Targets] [Options Used During Fuzzing] --replay-file '[WORKDIR]/corpus/*_replayable'

Example:

./target/release/cli evm -t 'tests/evm/multi-contract/*' --replay-file 'work_dir/corpus/*_replayable'

You may add source map information to the targets to get more accurate coverage information and uncovered source code. To get source map information, you simply need to append --combined-json bin-runtime,srcmap-runtime to the solc command when building the targets.

# run in your target building directory (where you run solc)
solc [Options Used During Building] --combined-json bin-runtime,srcmap-runtime

Example:

# build contracts in tests/evm/verilog-2/
solc *.sol -o . --bin --abi --overwrite --base-path ../../ --combined-json bin-runtime,srcmap-runtime

Rarely, ItyFuzz has trouble to figure out the source code location. You may supply the absolute path to the base location (what you passed to solc's --base-path or if you didn't pass anything, it is the building directory) to ItyFuzz.

./target/release/cli evm -t [Targets] [Options Used During Fuzzing] --replay-file '[WORKDIR]/corpus/*_replayable' --base-path [ABSOLUTE PATH TO BASE LOCATION]

Example:

# note that we used --base-path ../../ when building the targets so it is /home/user/ityfuzz/tests/evm/verilog-2/../../
./target/release/cli evm -t 'tests/evm/multi-contract/*' --replay-file 'work_dir/corpus/*_replayable' --base-path /home/user/ityfuzz

We do not track coverage of static calls (view, pure functions) by default!

Building With Move (Sui) Support

Build with feature sui_support in ./Cargo.toml to enable Move support.

# add sui_support feature to Cargo.toml
python3 -c 'content=open("Cargo.toml").read().replace("default = [", "default = [\"sui_support\",");open("Cargo.toml","w").write(content);'

# build ItyFuzz with sui_support feature
cd cli/
cargo build --release

You may also want to install sui-cli to build Move contracts.

Run (Move)

Compile the contracts with sui move build and run ItyFuzz:

# build example contract that contains a bug
cd ./tests/move/share_object
sui move build

# get back to ItyFuzz CLI and run fuzzing on the built contract
cd ../../../cli/
./target/release/cli move -t "./tests/move/share_object/build"

Reporting Bugs (Move)

You can emit a event of `` in your contract to report a condition when bug is found.

// define the event struct
use sui::event;

struct AAAA__fuzzland_move_bug has drop, copy, store {
    info: u64
}

... 
    // inside function
    event::emit(AAAA__fuzzland_move_bug { info: 1 });
...

An example contract that report a bug can be found in tests/move/share_object/sources/test.move.

Troubleshooting

Z3 Installation

macOS

git clone https://github.com/Z3Prover/z3 && cd z3
python scripts/mk_make.py --prefix=/usr/local
cd build && make -j64 && sudo make install

If the build command still fails for not finding z3.h, do export Z3_SYS_Z3_HEADER=/usr/local/include/z3.h

Ubuntu

apt install libz3-dev

Onchain Fetching

ItyFuzz attempts to fetch storage from blockchain nodes when SLOAD is encountered and the target is uninitialized. There are three ways of fetching:

  • OneByOne: fetch one slot at a time. This is the default mode. It is slow but never fails.
  • All: fetch all slots at once using custom API eth_getStorageAll on our nodes. This is the fastest mode, but it may fail if the contract is too large.
  • Dump: dump storage using debug API debug_storageRangeAt. This only works for ETH (for now) and fails most of the time.

Telemetry

ItyFuzz collects telemetry data to help us improve the fuzzer. The data is collected anonymously and is not used for any commercial purpose. You can disable telemetry by setting NO_TELEMETRY=1 in your environment variable.

Citation

@inproceedings{10.1145/3597926.3598059,
  author = {Shou, Chaofan and Tan, Shangyin and Sen, Koushik},
  title = {ItyFuzz: Snapshot-Based Fuzzer for Smart Contract},
  year = {2023},
  isbn = {9798400702211},
  publisher = {Association for Computing Machinery},
  address = {New York, NY, USA},
  url = {https://doi.org/10.1145/3597926.3598059},
  doi = {10.1145/3597926.3598059},
  booktitle = {Proceedings of the 32nd ACM SIGSOFT International Symposium on Software Testing and Analysis},
  pages = {322–333},
  numpages = {12},
  location = {Seattle, WA, USA},
  series = {ISSTA 2023}
}

Acknowledgement

This work was supported in part by NSF grants CCF-1900968, CCF1908870, and CNS1817122 and SKY Lab industrial sponsors and affiliates Astronomer, Google, IBM, Intel, Lacework, Microsoft, Mohamed Bin Zayed University of Artificial Intelligence, Nexla, Samsung SDS, Uber, and VMware. Any opinions, findings, conclusions, or recommendations in this repo do not necessarily reflect the position or the policy of the sponsors.

Grants:

Grants Description
Grants from Sui Foundation for building Move and chain-specfic support
Grants from Web3 Foundation for building Substrate pallets and Ink! support

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