Echidna: A Fast Smart Contract Fuzzer

Echidna is a weird creature that eats bugs and is highly electrosensitive (with apologies to Jacob Stanley)

More seriously, Echidna is a Haskell program designed for fuzzing/property-based testing of Ethereum smarts contracts. It uses sophisticated grammar-based fuzzing campaigns based on a contract ABI to falsify user-defined predicates or Solidity assertions. We designed Echidna with modularity in mind, so it can be easily extended to include new mutations or test specific contracts in specific cases.

Features

• Generates inputs tailored to your actual code
• Optional corpus collection, mutation and coverage guidance to find deeper bugs
• Powered by Slither to extract useful information before the fuzzing campaign
• Source code integration to identify which lines are covered after the fuzzing campaign
• Curses-based retro UI, text-only or JSON output
• Automatic testcase minimization for quick triage
• Seamless integration into the development workflow
• Maximum gas usage reporting of the fuzzing campaign
• Support for a complex contract initialization with Etheno and Truffle

.. and a beautiful high-resolution handcrafted logo.

Usage

Executing the test runner

The core Echidna functionality is an executable called echidna-test. echidna-test takes a contract and a list of invariants (properties that should always remain true) as input. For each invariant, it generates random sequences of calls to the contract and checks if the invariant holds. If it can find some way to falsify the invariant, it prints the call sequence that does so. If it can't, you have some assurance the contract is safe.

Writing invariants

Invariants are expressed as Solidity functions with names that begin with echidna_, have no arguments, and return a boolean. For example, if you have some balance variable that should never go below 20, you can write an extra function in your contract like this one:

function echidna_check_balance() public returns (bool) {
    return(balance >= 20);
}

To check these invariants, run:

$ echidna-test myContract.sol

An example contract with tests can be found tests/solidity/basic/flags.sol. To run it, you should execute:

$ echidna-test tests/solidity/basic/flags.sol

Echidna should find a a call sequence that falsifies echidna_sometimesfalse and should be unable to find a falsifying input for echidna_alwaystrue.

Collecting and visualizing coverage

After finishing a campaign, Echidna can save a coverage maximizing corpus in a special directory specified with the corpusDir config option. This directory will contain two entries: (1) a directory named coverage with JSON files that can be replayed by Echidna and (2) a plain-text file named covered.txt, a copy of the source code with coverage annotations.

If you run tests/solidity/basic/flags.sol example, Echidna will save a few files serialized transactions in the coverage directory and a covered.$(date +%s).txt file with the following lines:

*r  |  function set0(int val) public returns (bool){
*   |    if (val % 100 == 0)
*   |      flag0 = false;
  }

*r  |  function set1(int val) public returns (bool){
*   |    if (val % 10 == 0 && !flag0)
*   |      flag1 = false;
  }

Our tool signals each execution trace in the corpus with the following "line marker":

• * if an execution ended with a STOP
• r if an execution ended with a REVERT
• o if an execution ended with an out-of-gas error
• e if an execution ended with any other error (zero division, assertion failure, etc)

Support for smart contract build systems

Echidna can test contracts compiled with different smart contract build systems, including Truffle or hardhat using crytic-compile. To invoke echidna with the current compilation framework, use echidna-test ..

On top of that, Echidna supports two modes of testing complex contracts. Firstly, one can describe an initialization procedure with Truffle and Etheno and use that as the base state for Echidna. Secondly, echidna can call into any contract with a known ABI by passing in the corresponding solidity source in the CLI. Use multi-abi: true in your config to turn this on.

Crash course on Echidna

Our Building Secure Smart Contracts repository contains a crash course on Echidna, including examples, lessons and exercises.

Using Echidna in a GitHub Actions workflow

There is an Echidna action which can be used to run echidna-test as part of a GitHub Actions workflow. Please refer to the crytic/echidna-action repository for usage instructions and examples.

Configuration options

Echidna's CLI can be used to choose the contract to test and load a configuration file.

$ echidna-test contract.sol --contract TEST --config config.yaml

The configuration file allows users to choose EVM and test generation parameters. An example of a complete and annotated config file with the default options can be found at tests/solidity/basic/default.yaml. More detailed documentation on the configuration options is available in our wiki.

Echidna supports three different output drivers. There is the default text driver, a json driver, and a none driver, which should suppress all stdout output. The JSON driver reports the overall campaign as follows.

Campaign = {
  "success"      : bool,
  "error"        : string?,
  "tests"        : [Test],
  "seed"         : number,
  "coverage"     : Coverage,
  "gas_info"     : [GasInfo]
}
Test = {
  "contract"     : string,
  "name"         : string,
  "status"       : string,
  "error"        : string?,
  "testType"     : string,
  "transactions" : [Transaction]?
}
Transaction = {
  "contract"     : string,
  "function"     : string,
  "arguments"    : [string]?,
  "gas"          : number,
  "gasprice"     : number
}

Coverage is a dict describing certain coverage increasing calls. Each GasInfo entry is a tuple that describes how maximal gas usage was achieved, and also not too important. These interfaces are subject to change to be slightly more user friendly at a later date. testType will either be property or assertion, and status always takes on either fuzzing, shrinking, solved, passed, or error.

Debugging Performance Problems

The best way to deal with an Echidna performance issue is to run echidna-test with profiling on. This creates a text file, echidna-test.prof, which shows which functions take up the most CPU and memory usage.

To build a version of echidna-test that supports profiling, either Stack or Nix should be used. With Stack, adding the flag --profile will make the build support profiling. With Nix, running nix-build --arg profiling true will make the build support profiling.

To run with profiling on, add the flags +RTS -p to your original echidna-test command.

Performance issues in the past have been because of functions getting called repeatedly when they could be memoized, and memory leaks related to Haskell's lazy evaluation; checking for these would be a good place to start.

Limitations and known issues

EVM emulation and testing is hard. Echidna has a number of limitations in the latest release. Some of these are inherited from hevm while some are results from design/performance decisions or simply bugs in our code. We list them here including their corresponding issue and the status ("wont fix", "in review", "fixed"). Issues that are "fixed" are expected to be included in the next Echidna release.

Description	Issue	Status
Vyper support is limited	#652	wont fix
Limited library support for testing	#651	wont fix

Installation

Precompiled binaries

Before starting, make sure Slither is installed (pip3 install slither-analyzer --user). If you want to quickly test Echidna in Linux or MacOS, we provide statically linked Linux binaries built on Ubuntu and mostly static MacOS binaries on our releases page. You can also grab the same type of binaries from our CI pipeline, just click the commit to find binaries for Linux or MacOS.

Docker container

If you prefer to use a pre-built Docker container, log into Github on your local docker client and check out our docker package, which are also auto-built via Github Actions. Otherwise, if you want to install the latest released version of Echidna, we recommend using docker:

$ docker build -t echidna .

Then, run it via:

$ docker run -it -v `pwd`:/src echidna echidna-test /src/tests/solidity/basic/flags.sol

Building using Stack

If you'd prefer to build from source, use Stack. stack install should build and compile echidna-test in ~/.local/bin. You will need to link against libreadline and libsecp256k1 (built with recovery enabled), which should be installed with the package manager of your choosing. You also need to install the latest release of libff. Refer to our CI tests for guidance.

Some Linux distributions do not ship static libraries for certain things that Haskell needs, e.g. Arch Linux, which will cause stack build to fail with linking errors because we use the -static flag. Removing these from package.yaml should get everything to build if you are not looking for a static build.

If you're getting errors building related to linking, try tinkering with --extra-include-dirs and --extra-lib-dirs.

Building using Nix (works natively on Apple M1 systems)

Nix users can install the lastest Echidna with:

$ nix-env -i -f https://github.com/crytic/echidna/tarball/master

To build a standalone release for non-Nix macOS systems, the following will bundle Echidna and all linked dylibs in a tarball:

$ nix-build macos-release.nix
$ ll result/
bin    echidna-1.7.3-aarch64-darwin.tar.gz

It is possible to develop Echidna with Cabal inside nix-shell. Nix will automatically install all the dependencies required for development including crytic-compile and solc. A quick way to get GHCi with Echidna ready for work:

$ git clone https://github.com/crytic/echidna
$ cd echidna
$ nix-shell
[nix-shell]$ cabal new-repl

Running the test suite:

nix-shell --run 'cabal test'

Public use of Echidna

Property testing suites

This is a partial list of smart contracts projects that use Echidna for testing:

• Uniswap-v3
• Balancer
• MakerDAO vest
• Optimism DAI Bridge
• WETH10
• Yield
• Convexity Protocol
• Aragon Staking
• Centre Token
• Tokencard
• Minimalist USD Stablecoin

Trophies

The following security vulnerabilities were found by Echidna. If you found a security vulnerability using our tool, please submit a PR with the relevant information.

Project	Vulnerability	Date
0x Protocol	If an order cannot be filled, then it cannot be canceled	Oct 2019
0x Protocol	If an order can be partially filled with zero, then it can be partially filled with one token	Oct 2019
0x Protocol	The cobbdouglas function does not revert when valid input parameters are used	Oct 2019
Balancer Core	An attacker cannot steal assets from a public pool	Jan 2020
Balancer Core	An attacker cannot generate free pool tokens with joinPool	Jan 2020
Balancer Core	Calling joinPool-exitPool does not lead to free pool tokens	Jan 2020
Balancer Core	Calling exitswapExternAmountOut does not lead to free assets	Jan 2020
Liquity Dollar	Closing troves require to hold the full amount of LUSD minted	Dec 2020
Liquity Dollar	Troves can be improperly removed	Dec 2020
Liquity Dollar	Initial redeem can revert unexpectedly	Dec 2020
Liquity Dollar	Redeem without redemptions might still return success	Dec 2020
Origin Dollar	Users are allowed to transfer more tokens that they have	Nov 2020
Origin Dollar	User balances can be larger than total supply	Nov 2020
Yield Protocol	Arithmetic computation for buying and selling tokens is imprecise	Aug 2020

Research

We can also use Echidna to reproduce research examples from smart contract fuzzing papers to show how quickly it can find the solution. All these can be solved, from a few seconds to one or two minutes on a laptop computer.

Source	Code
Using automatic analysis tools with MakerDAO contracts	SimpleDSChief
Integer precision bug in Sigma Prime	VerifyFunWithNumbers
Learning to Fuzz from Symbolic Execution with Application to Smart Contracts	Crowdsale
Harvey: A Greybox Fuzzer for Smart Contracts	Foo, Baz

Academic Publications

Paper Title	Venue	Publication Date
echidna-parade: Diverse multicore smart contract fuzzing	ISSTA 2021	July 2021
Echidna: Effective, usable, and fast fuzzing for smart contracts	ISSTA 2020	July 2020
Echidna: A Practical Smart Contract Fuzzer	FC 2020	Feb 2020

If you are using Echidna for academic work, consider applying to the Crytic $10k Research Prize.

Getting help

Feel free to stop by our #ethereum slack channel in Empire Hacking for help using or extending Echidna.

• Get started by reviewing these simple Echidna invariants

• Considering emailing the Echidna development team directly for more detailed questions

License

Echidna is licensed and distributed under the AGPLv3 license.