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EF/CF - the Extremely Fast (ethereum smart) Contract Fuzzer

EF/CF is a new approach to smart contract fuzzing: instead of using a new custom built fuzzer, it repurposes existing fuzzing infrastructure of C/C++ code to smart contracts. Currently, AFL++ is the primarily supported fuzzer, although there is some very rudimentary support for libfuzzer and honggfuzz as well.

Why use existing fuzzing infrastructure?

  • Speed. We can fuzz faster. We regularly get around 20k execs/sec/core.
  • Native code fuzzers are well engineered and optimized.
  • Proper coverage-guidance, queue-management, deterministic test case replay, etc.

What are some problems that we encounter along the way?

  • We need to teach the fuzzer about structure: namely what is a transaction and what is the smart contract's ABI. We use a custom mutator for this: ./src/ethmutator/
  • To increase speed and get useful coverage feedback, we translate EVM bytecode to C++ using a custom transpiler ./src/evm2cpp/

This repository the primary entry point for the EF/CF project. It contains all the relevant code as sub-projects in ./src/ and several convenience scripts for installation, scripts for launching a fuzzing campaigns and various datasets to test the fuzzer (and compare against other tools).

  • ./src/ - contains all source necessary to build and run EF/CF; for reproducibility all direct dependencies are added as git submodules.
  • ./data/ - contains the datasets used during evaluation
  • ./scripts - contains scripts to run experiments, installation, etc.
  • ./docker - Dockerfile for container-based workflow
    • Standard is Ubuntu, but you can also have a Fedora or Arch Linux based container if you like.
    • ./docker/tools/ contains dockerfiles for tools that we evaluated EF/CF against. We tried our best to use fix the versions we evaluated in our paper in the dockerfiles.
  • ./EXPERIMENTS.md - contains a guide to reproduce the experiments from our paper.
  • ./examples - contains example outputs produced by EF/CF

The EF/CF Paper

We describe EF/CF's architecture, implementation, and summarize our evaluation results in our paper: arxiv.org preprint

Citation in Academic Work

When referring to EF/CF in academic work please use the following bibtex entry for citation:

@InProceedings{efcf2023,
  author       = "Michael Rodler and David Paaßen and Wenting Li and Lukas Bernhard and Thorsten Holz and Ghassan Karame and Lucas Davi",
  title        = "EF/CF: High Performance Smart Contract Fuzzing for Exploit Generation",
  booktitle    = "{IEEE} European Symposium on Security and Privacy ({EuroS\&P})",
  publisher    = "{IEEE}",
  year         = "2023",
}

Quickstart

The recommended way is to run EF/CF as an interactive docker container.

  1. Enter the container with a shell 
    docker run --rm -it ghcr.io/uni-due-syssec/efcf-framework
    or build the container from the cloned repository 
    make gitmodules  # to fetch the git submodules
make container-enter
  2. Compile and then fuzz a solidity contract until the first crash/bug is discovered: 
    efcfuzz --until-crash --out ./baby_bank_results/ --source ./data/examples/baby_bank.sol
  3. Inspect the identified crash 
    cd /tmp/baby_bank_results/
./r.sh crashes_min/default_id:000000*

Installation / Setup

Git Submodules

No git? if you use a tarball/docker release, ignore this.

Run git submodule update --init to fetch the latest submodule commits on already cloned repositories. Make sure to run this also in ./src/eEVM.

git submodule update --init; cd src/eEVM/; git submodule update --init; cd ../../

Warning: Running git clone --recursive $repo or passing the --recursive argument to git sumbodule (update|init) will make git recursive into submodules of the AFL++ repository, which are not needed for this project. So to save some space it is better to avoid the recusive submodule checkouts.

Container

We provide the following convenience make targets for container-based workflows:

make container-build  # build default efcf container
make container-enter  # enter default efcf container in current working dir

If you want to ensure a clean build, you can use the following command

make container-build CLEAN_CHECKOUT=1

Alternatively the container can be built with the following docker command:

docker build \
    -f docker/ubuntu.Dockerfile \
    -t efcf:latest \
    .

Note that there is also an Archlinux and Fedora based Dockerfile. They should work as well, but are not as well tested.

For manually distributing a docker image (e.g., if including some local changes), use:

make container-release
docker load -i ./efcf*.tar

We recommend the following docker options for launching:

  • --security-opt seccomp=unconfined - better fuzzing perf
  • --net=host - for easy access to a local ethereum node
  • --tmpfs "/tmp/efcf/":exec,size=6g - put EF/CF's temporary files onto a ramdisk if possible (less disk wear)
  • --privileged - to run afl-system-config or efcfuzz --configure-system
  • -v - to persist the output data of EF/CF

VM / Bare-Metal

For VM or bare-metal-based workflows:

make system-install   # install efcf to current system (requires root or sudo rights)

Note that a lot of the scripts work on the relative directory layout anyway, so this mostly installs dependencies and some tools that are handy to have in your PATH. We have tested running EF/CF on the following Linux distributions:

  • Ubuntu Jammy (or later)
  • Fedora ($ > 35 $)
  • Archlinux

(Distro does not matter that much, we tested LLVM 13 and 14 with 14 being the preferred choice. LLVM 11 or 12 might also still work, but as always - the newer the better. The important part is that there is a LLVM that is compatible with our fork of AFL++.)

On Mac OS / M1

We have not tested EF/CF on Mac OS natively. Likely things won't work (e.g., afl-clang-lto on Mac OS seems to not work). The best option is to utilize docker.

# make sure that the submodules are initialized
make gitmodules
# pull the linux/amd64 base image
docker pull ubuntu:jammy --platform linux/amd64
# build the ef/cf image
docker build -t efcf:latest -f docker/ubuntu.Dockerfile --platform linux/amd64 .
# launch the EF/CF container
docker run --tmpfs "/tmp/efcf/":exec,size=8g --platform linux/amd64 --rm -it -v $(pwd):$(pwd) -w $(pwd) efcf:latest

We tested using docker desktop v4.21.1 and basic EF/CF usage works. However, consider the following:

  • If you see segfaults while building: try increasing the memory limit of the VM docker uses on Mac OS.
  • Try enabling acceleration using rosetta in docker - hopefully this is a bit faster.

Development Setup

The tools generally do not need to be installed. Install the required dependencies as in the system-install.sh script or as in the Dockerfiles.

For convenience we have some scripts to update your PATH:

# POSIX-like shells (i.e., bash, ...)
source ./scripts/env.sh

# for the fish shell
source ./scripts/env.fish

Etherscan API Key

Some of the scripts require a API key for fetching metadata (e.g., ABI) from the Etherscan service. If you have an API key you have to set the ETHERSCAN_API_KEY environment variable to pass this to scripts. For a docker-based workflow you can either launch the docker container with the --env flag or put your API key into the .etherscan_api_key file, which will bake the API key into the docker container.

Starting EF/CF with the launcher

For convenience, we utilize a wrapper script that takes cares of all the details for you, when launching the EF/CF fuzzer: efcfuzz

You can set many command line options to configure the fuzzer's behavior with respect to the build and fuzzing process. Take a look at efcfuzz --help for a list of options.

Examples

Compile solidity source code to EF/CF native code and start fuzzing for 5 minutes (aka 300 seconds).

efcfuzz --timeout 300 --source ./data/examples/baby_bank.sol

Alternatively, launch with reduced fuzzing output (--quiet suppresses the base fuzzer's output, while --print-progress will print a short summary of the fuzzing progress), and launching the fuzzer on 4 cores.

efcfuzz --quiet --print-progress --cores 4 --timeout 300 --source ./data/examples/baby_bank.sol

Use already compiled bytecode and compile the bytecode to EF/CF native code and start fuzzing.

# efcfuzz can handle the combined.json output of the solidity compiler
pushd ./data/examples/; make baby_bank.combined.json; popd
efcfuzz --timeout 300 --bin-runtime ./data/examples/baby_bank.combined.json

# but you can also explicitely pass the runtime and deploy bytecode and the ABI
# definition. This is useful if you want to fuzz contracts using other
# compilers (e.g., vyper).
pushd ./data/examples/; make baby_bank; popd
efcfuzz --timeout 300 \
    --bin-runtime ./data/examples/baby_bank.bin-runtime \
    --bin-deploy ./data/examples/baby_bank.bin \
    --abi ./data/examples/baby_bank.abi

The wrapper can export a contract's state from a go-ethereum/erigon node and start fuzzing from there.

$ efcfuzz --timeout 300 --live-state 0xfffF8D17CB019E0825c478c666B251A7099df3FD

Additionally, you can pass --include-address-deps=y to recursively search for addresses of other accounts in the exported contract's storage and also include those into the state export. However, this does not include other contracts stored in Solidity mapping types. To really export the whole state recursively pass also the --include-mapping-deps=y flag.

But beware, this recursive lookup can lead to long compilation times and poor fuzzing performance. Especially, frequently used contracts can have a lot of internal state and using their exported state can make the fuzzing slow. Check whether the fuzzer can achieve more than 1k execs/sec. If not you should rather try to craft an artificial and smaller state. Try running a local go-ethereum node in the --dev mode and deploy your contracts there. Then export the live state from there.

The wrapper caches builds, so a second fuzzing run should launch much faster, because the initial compilation time is not needed anymore. If you want to only build and place it into the cache then you can pass the --build-only argument.

Example: Fuzzing with properties

EF/CF also supports property-based fuzzing using the same property definition as the echidna fuzzer. Properties (or invariants) are expressed as solidity functions that act as a bug oracle to the fuzzer. For example, you can add a solidity function:

function test_property_balance() public view returns (bool) {
    return total_balance < 1000;
}

Which represents the property that the total_balance should always be below 1000. EF/CF will then report a bug if it manages to violate this property using some transaction sequence, i.e., the oracle returns false.

To tell EF/CF that this is a property you need to specify a list of function signatures in a file, which will be picked up by EF/CF as a list of properties to check during fuzzing.

The easiest way is to obtain the relevant signatures using the --hashes flag to the solidity compiler, e.g.,

solc --hashes ./path/to/your.sol | grep test_property > property_list

Now you can launch the fuzzer with:

efcfuzz --source ./path/to/your.sol --properties ./property_list -C

You can also add --disable-detectors to disable the built-in ether-based bug oracles.

You can try the following example for property based fuzzing:

efcfuzz \
    --properties ./data/examples/harvey_baz_properties.signatures
    --disable-detectors \
    --until-crash --timeout 120 \
    --source ./data/examples/harvey_baz.sol \

Example: Fuzzing for Events

EF/CF supports fuzzing for assertion violations that are expressed through events. Actually, we also support using arbitrary custom events as a bug oracle. By default, EF/CF will identify a bug if the target contract logged one of the following events: AssertionFailed(), AssertionFailed(uint256), AssertionFailed(string), and Panic(uint256).

efcfuzz --event-assertions \
    --timeout 120 --until-crash \
    --source ./data/properties-assertions-tests/verifyfunwithnumbers.sol

You can also specify additional custom event topics/hashes to look out for in a file with --event-assertions-list ./path/to/eventslist.txt. As with the property list before, you can obtain the format by using solc --hashes and copying the event hashes and names to the event list file.

By default, EF/CF will ignore events that were not issued by the target contract. If you want to change that use --event-assertions-target-only=n.

(Note: you can use --assertions to enable both event and solidity assertion checking)

Example: Fuzzing for Solidity ^0.8 Assertions

Currently, we do not support fuzzing for arbitrary assertions in solidity code for solidity version below 0.8. Previously solidity assertions would simply trigger a invalid opcode, resulting in a quite forceful revert. Solidity version 0.8 changed the behavior to instead of using the invalid opcode to revert transactions, they now utilize the revert mechanism and signal errors back to the caller. We can utilize this type of error propagation as a bug oracle in EF/CF. Currently, EF/CF supports checking for the Solidity error type Panic(uint256). More information on Solidity errors.

efcfuzz --sol-assertions \
    --timeout 120 --until-crash \
    --source ./data/assertions-tests/overflow.sol

(Note: you can use --assertions to enable both event and solidity assertion checking)

System Specs and Configuration

We recommend allocating 4 to 16 cores and roughly 1 GB of memory per core. You can utilize the --configure-system flag to configure your system for high speed fuzzing, or configure it yourself. In docker containers you also need to configure the host for best performance. If it is an uncritical host you can launch the container as --privileged and use /usr/local/bin/afl-system-config to configure the system for high speed fuzzing (note that this essentially runs the container as root).

# configure system
docker run --rm -it --privileged efcf afl-system-config
# run fuzzer (somewhat sandboxed and using a tmpfs for less SSD wear)
docker run --rm -it \
    --security-opt seccomp=unconfined \
    --tmpfs "/tmp/efcf/":exec,size=6g \
    efcf

Running a Fuzzing Experiment

To run the experiment on the data/tests/ dataset you can use the following command to build the contracts and their fuzzing harness and then run the fuzzer with different settings, and multiple repetitions, etc. Because this would take quite a while we can run those experiments in parallel. We split the fuzzing experiments into a build step and a fuzz step. The build steps will build all smart contracts sequentially (the building itself uses multiple cores though). Then we launch 8 fuzzer instances in the background, which will take the build artifacts from the build step and start fuzzing runs. The Makefile will automatically attempt to launch everything in the proper container if either docker or podman is available.

make build-tests
make fuzz-tests CONTAINER_BACKGROUND=1 FUZZER_INSTANCES=8

We disable seccomp and network sandboxing when launching the containers in background. Disabling seccomp sandboxing, improves fuzzing performance. Using the host network allows EF/CF to access Ethereum nodes in the local network without further configuration.

We used the script ./scripts/run-tools-on-dataset.py to run the other tools inside docker containers on these datasets, e.g. with these commands for the multi dataset:

python3 ./scripts/run-tools-on-dataset.py ./data/multi/
cd ./results/tools-multi/
python3 ../../scripts/get-tools-on-dataset-stats.py
head stats.csv

You need to adapt the script to configure the tools and number of runs.

Setting up a Fuzzing Experiment

Here, we use the tests experiment as an example. Simply replace the string tests with the name of the experiment in the following steps:

  1. Gather your dataset in ./data/, e.g., the ./data/tests dataset with test contracts. For solidity contracts we have a generic Makefile to build the contracts: sol.Makefile. You can reuse this if you wish, see ./data/tests/Makefile for an example.
  2. Create a script to build the build artifacts including any pre-processing/scraping step necessary. For example for the tests dataset we have the ./scripts/build-tests.sh script. The build artifacts should be stored in ./builds/tests/${contract}.build.tar.xz.
  3. Create a script to launch the fuzzing campaign, e.g. for the tests dataset create a script called ./scripts/fuzz-tests.sh. Typically you can use the common fuzzing campaign function from ./scripts/common.sh. Take a look at the fuzz-tests.sh for a template.
  4. The results of fuzz-tests.sh will be stored in ./results/run-fuzz-tests/.
  5. To summarize the results we provide ./scripts/summarize.py for the bash based launcher scripts and ./scripts/summarize_l.py for python launcher scripts (the efcfuzz tool).
    You might need to adapt these scripts depending on your step 3.

Existing Fuzzing Experiments

Benchmarks

  • ./data/multi contains the scalability benchmark we used to assess how well an analysis tool scales to longer transaction sequences. It consists of three types of contracts:
    • multi_gen_*.sol - automatically synthesized contracts, which do a bunch of require(input <= MAGIC) and then set an internal state variable. If all state variables are set, then the selfdestruct (or echidna oracle) can be triggered.
    • multi_man_complex_*.sol - manually created variants that work similarly to the multi_gen type of contracts, but features a bit more tricky constraints (e.g., other things than equality and inequality with a magic value)
    • justlen_*.sol - these are taken from the echidna-parade example
    • multi_simple_*.sol - sanity checks that verify that a fuzzer/tool can could in theory find bugs that require 9 or 10 transactions. Here the analyzer just needs to call 10 functions in the right order without any arguments. This is pretty easy for most analysis tools.
  • ./data/throughput contains the contracts we used for assessing the throughput. This is a selection of contracts with varying size. Note that we patched all vulnerabilities in these contracts, such that found vulnerabilities do not impact throughput measurements.
  • ./data/cov-max-testset contains the contracts we used for comparison of fuzzers based on code coverage.

Bug Detection

Tests

The following datasets contain basic synthetic test contracts to test the fuzzer's capabilities:

Fuzzing in more Detail

We utilize wrapper scripts to launch the actual fuzzer (AFL++ in our case). This is done automatically when using the efcfuzz launcher.

$ cd data/tests
$ make SimpleDAO.evm2cpp
$ cd ../../src/eEVM/
$ env AFL_BENCH_UNTIL_CRASH=1 ./fuzz/launch-aflfuzz.sh SimpleDAO

If you have tmux and tmuxp installed then for development and inspection the interactive version of the script might be useful:

$ ./fuzz/interactive-aflfuzz.sh -b SimpleDAO

This will then compile and fuzz for quite a while. You can then cd ./fuzz/out/SimpleDAO* to view the fuzzing results. Our wrapper scripts do some extra work on top of launching the afl-fuzz program, i.e., mostly post-processing the results. Additionally it will generated several convenience scripts to analyze the generated test cases.

  • ./a.sh - Print human-readable form of a test case, wrapper around efuzzcaseanalyzer.
  • ./r.sh - run a test case with the same settings as when the fuzzer was run.
  • ./m.sh - minimize a testcase with the same settings as when the fuzzer was run.
  • ./c.sh - analyze the "chain" of test cases that lead to the given test case. Useful for analyzing/optimizng the fuzzer. You can quickly see, which test case was produced by which chain of mutations on which queue entries. Requires fzf.

There are also some other convenience reports, such as

  • ./bugs and ./bugtypes that summarize any bugs that were identified.
  • ./crashes_min, which contains minimized crashes of all the afl-fuzz instances.

View EVM Basic Block Code Coverage

$ cat coverage-percent-all.evmcov
70.73170731707317

The script fuzz/evm-bb-coverage.sh will compute the basic block coverage given an AFL output directory. The harness can optionally dump a trace of basic blocks, which are then compared to a list of basic blocks that is output by evm2cpp (i.e., the .bb_list files in eEVM/contracts/).

By default we also compute the coverage that our default generic seeds (see eEVM/fuzz/generic_seeds produce:

$ cat coverage-percent-seeds.evmcov
10.5890

The list of covered basic blocks is stored in the file all.evmcov.

View Summary of Generated Testcases

efuzzcaseanalyzer can be used to view/summarize the generated testcases, e.g.,

$ efuzzcaseanalyzer -a ./contract.abi -s ./crashes_min/
Transactions Sequences:
--------------------------------------------------------------
TX [🪙]
    deposit()[🪙];
    withdraw(uint256)[↕️ ↩️ ];
    withdraw(uint256)[];
--------------------------------------------------------------
Number of fuzzcases: 1
Average number of TXs: 3
Number of unique TX sequences: 1
Number of unique TX sequences (consecutive deduplicated): 1

The summaries are usually stored in the files crashes_tx_summary and queue_tx_summary, but this last one can be a bit verbose.

Analyze a single crashing testcase

$ ./a.sh default/crashes/id:000000,...
# roughly equivalent to running
$ efuzzcaseanalyzer -a ./contract.abi default/crashes/id:000000,...
Block header:
  number: 0
  difficulty: 0
  gas_limit: 0
  timestamp: 0
  initial_ether: 0

TX with tx_sender: 54 (selector); call_value: 0x0; length: 36; block+=1; #returns=0
  func: withdraw(uint256)
  input: { Uint(80),  }
TX with tx_sender: 238 (selector); call_value: 0x246ddf979; length: 4; block+=1; #returns=0
  func: deposit()
  input: {  }
TX with tx_sender: 153 (selector); call_value: 0x3860e6373; length: 4; block+=1; #returns=0
  func: deposit()
  input: {  }
TX with tx_sender: 166 (selector); call_value: 0x0; length: 36; block+=1; #returns=1
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }
  returns:
    return val: 1; allows reenter: 2; data: 0x0000000000000000000000000000000000000000000000000000000000000001
TX with tx_sender: 166 (selector); call_value: 0x0; length: 36; block+=1; #returns=0
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }

And to get the actual result of the fuzz target, you can execute:

$ ./r.sh default/crashes/id:000000,sig:06,src:000000+000010,time:1584,EM-________SAO_______AD
# roughly equivalent to running
$ env EVM_DEBUG_PRINT=1 ./build/fuzz_multitx default/crashes/id:000000,sig:06,src:000000+000010,time:1584,EM-________SAO_______AD

[...]

account 0xc4b803ea8bc30894cc4672a9159ca000d377d9a3 has balance 0x100000000000000000000000000000001bc16d67562e80000( > 0x1000000000000000000000000000000000000000000000000)
Aborted (core dumped)

This gives you a lot of verbose output, including some parts of the execution traces of the contracts and the balance check result that the harness does.

Minimizing Crashing Inputs

Crashing inputs often do contain unrelated transactions due to the randomized testing approach. This can be mitigated by performing a minimization on the crashing input (i.e., reduce the input as long as it still causes a crash). If you want to minimize non-crashing inputs, then you can use the -M flag to enable minimization according to coverage as minimization criterion.

The following command will reduce the testcase and overwrite the file:

$ efuzzcaseminimizer -oa ./contract.abi ./build/fuzz_multitx ./default/crashes/id:000000,sig:06,src:000000+000010,time:1584,EM-________SAO_______AD

[..]

=== Before minimizing: ===
Block header:
  number: 0
  difficulty: 0
  gas_limit: 0
  timestamp: 0
  initial_ether: 0

TX with tx_sender: 54 (selector); call_value: 0x0; length: 36; block+=1; #returns=0
  func: withdraw(uint256)
  input: { Uint(80),  }
TX with tx_sender: 238 (selector); call_value: 0x246ddf979; length: 4; block+=1; #returns=0
  func: deposit()
  input: {  }
TX with tx_sender: 153 (selector); call_value: 0x3860e6373; length: 4; block+=1; #returns=0
  func: deposit()
  input: {  }
TX with tx_sender: 166 (selector); call_value: 0x0; length: 36; block+=1; #returns=1
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }
  returns:
    return val: 1; allows reenter: 2; data: 0x0000000000000000000000000000000000000000000000000000000000000001
TX with tx_sender: 166 (selector); call_value: 0x0; length: 36; block+=1; #returns=0
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }

=== After minimizing: ===
Block header:
  number: 0
  difficulty: 0
  gas_limit: 0
  timestamp: 0
  initial_ether: 15133991795

TX with tx_sender: 4 (selector); call_value: 0x246ddf979; length: 4; block+=0; #returns=0
  func: deposit()
  input: {  }
TX with tx_sender: 4 (selector); call_value: 0x0; length: 36; block+=1; #returns=1
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }
  returns:
    return val: 1; allows reenter: 2; data: 0x
TX with tx_sender: 0 (selector); call_value: 0x0; length: 36; block+=1; #returns=0
  func: withdraw(uint256)
  input: { Uint(37000000000000000000),  }

Reading the test-case format

The test case format is geared towards fuzzing and is not as straight forward to read. There are several subtleties you have to be aware of.

  • The test case format is considered as a "queue" of transactions that can be executed. As soon as any problem is encountered, processing of the test case will stop. This includes:
    • When a transaction reverts.
    • Any error is encountered by the harnessing code.
    • Any bug is triggered and detected. As a consequence, a printed test case does not necessarily correspond to what is executed - there can be transactions at the end that are not executed. Check the verbose output and use the minimizer to get rid of those!
  • Similarly, there can be too many returns or spurious reenter flags. Use the test case minimizer to get rid of those.
  • A contract is only reentered when there is another transaction in the list after the transaction that is supposed to do the reentrancy (i.e., there is another following entry int the queue).
  • Even if the reenter flag is set to something, this does not necessarily mean that the contract is reentered, just that the harness code will try to do it if possible. For example, if the contract does not perform a call the reenter flag is ignored as there is no possibility to reenter. Usually the minimizer will remove any spurious reenter flags.

In general many of those issues go away when you use the test case minimizer, so it is always a good idea to use this one before analyzing generated test cases.

Known False Alarms

We observed several types of false alarms that seem to be recurring when fuzzing contracts with EF/CF.

  • Contracts that pay out Ether by design. EF/CF's Ether-gains bug oracle will pick up these contracts as vulnerable, although they are operating as designed:
    • Gambling contracts: many gambling contracts feature some form of randomness, which is already a bad practice in Ethereum. However, some gambling contracts are implemented in a way that force you to guess e.g., the last two digits of the next blockhash or something similar. This can be enabled using a commitment scheme, i.e., the first transaction commits the user to a certain value and the second one triggers the guess and payout if won. These contracts are usually not exploitable in a real blockchain. However, in EF/CF's simulated blockchain, the fuzzer can adapt the commitment after the value in the second transaction is observed. This fact is important for EF/CF to reach better code coverage. However, it makes it also easy for EF/CF to identify a TX sequence that lets the fuzzer deterministically win in the gambling contract.
    • Contracts that pay out interest: There are many small contract that allow you to invest Ether and then pay out a certain percentage in interest every N blocks. EF/CF's simulated attacker is capable of waiting for N blocks and then receives the interest payout, which is again picked up by the Ether-gains bug oracle.
    • Airdrops: some token contracts enable airdrops, i.e., just handing out tokens to anyone that requests them until certain limits is reached. For example, airdrops are often enabled only for a short period of time. If such a contract is deployed within EF/CF, chances are high that the time limit is set such that airdrops are still enabled. EF/CF then picks up a Ether-gains if the airdropped tokens can be sold again.
  • Eager reporting of controllable DELEGATECALL: currently we report a controllable delegatecall as soon as it is invoked. However, there are multiple contracts, which feature functions that intentionally allow the caller to perform a delegatecall to an arbitrary address. However, these functions will unconditionally revert the transaction immediately after the delegatecall. This prevents any state updates or ether transfers from persisting. Typically such functions have words like "simulate" in their function names and then they are easy to spot.
    • This could be fixed in EF/CF by deferring reporting until the end of the execution. However, this complicates the bug oracle quite a bit.
    • Currently there are no plans to fix this.
  • Initializer callable: We observed that when fuzzing contracts exported from the blockchain, EF/CF is sometimes able to call initializer functions, even though the contract was already initialized. Normally, this should trigger a revert, but does not do so in EF/CF's EVM environment. Calling the initializer again often leads to trivial Ether gains, because e.g., the initializer sets an owner variable or something similar.
    • We are not yet sure what the root cause of this issue is. However, it is typically easily to spot, since the initializer function is typically called initializer, init, or similar.

Common Pitfalls

We did our best to make this somewhat usable, but it is still a research prototype. Do expect things breaking. Here are some common issues we observed:

  • Q: I get some weird compile error due to a TOKENPASTE macro.
    A: This often happens when efcfuzz guesses the wrong contract name (i.e., it guesses an abstract contract), try passing --name YourContract to specify the target contract.

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EF/CF - Extremely Fast smart Contract Fuzzing

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Apr 14, 2023

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