Beacon Fuzz - Update #02

Beacon Fuzz - Progress Update #2

Beacon Fuzz - Update #02

First Bugs, More Eth2 Clients

Sigma Prime is leading the development and maintenance of beacon-fuzz, a differential fuzzing solution for Eth2 clients. This post is part of our series of monthly status updates where we discuss current progress, interesting challenges encountered, and direction for future work. See #00 and the repository's README for more context.


The Beacon Fuzz team has been making steady progress over the last few weeks. Key achievements and points of interest include:

  • New bugs identified in two separate implementations
  • Additional state transition fuzzing targets implemented
  • A Proof-of-Concept simultaneously exercising both Prysm and ZRNT
  • Better internal tooling
  • Maintainability improvements

New Bugs Identified

Even in its early stages, beacon-fuzz is already identifying some bugs and providing value to the ecosystem:

  • Trinity#1497 - two validation functions (validate_voluntary_exit() and validate_proposer_slashing()) raise an IndexError given invalid Validator indices, when we had expected them to only raise ValidationError:

    There is currently some ambiguity whether this is a bug or we need to treat IndexError as an expected error result. The relevant test fixtures list IndexError as an expected exception, but the block importing in BeaconChainSyncer only explicitly checks for ValidationError. In any case, this does not crash the application, as all exceptions caused by block importing are caught and logged at a higher level. See the issue for more details.

  • Nimbus#703 - Merkle proof validation in process_deposit was not enforced:

    Passing an invalid deposit object (i.e. a deposit object with an invalid merkle proof) would lead to the deposit being succesfully processed. In other words, this bug could have allowed a malicious actor to artificially mint ETH on the Beacon Chain without actually depositing ETH to the Eth1 contract! The Nimbus team have also fixed the attestation validation crash discussed in our last post.

Thesse are listed in a Trophies section of our README, which will be updated as more bugs are found and can be disclosed.

Additional state transition targets

Fuzzing targets have been implemented for the remaining state transition block operations:

With this, the only remaining block-processing endpoints are process_randao() and process_eth1_data(), and per-epoch processing will be the primary focus for subsequent targets.

Big thanks to the Nimbus team for, amongst other things, their quick response accepting PRs that update their test harnesses.

process_deposit precondition

While implementing the deposit target, we noticed it is an interesting case where, unlike other operation processing functions, some relevant validation occurs outside process_deposit().

process_operations() contains

assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)

This has some effect on what input process_deposit() can expect, but only really implies the invariant state.eth1_deposit_index <= state.eth1_data.deposit_count.

This means passing a state where eth1_deposit_index >= eth1_data.deposit_count holds to process_deposit() is undefined behaviour; it could be reasonable to panic or abort.

We can enforce this precondition as part of pre-processing and, given how highly unlikely it is for random mutations to result in a correct Merkle proof (especially as a changed eth1_deposit_index can invalidate an otherwise correct proof), we might as well disable Merkle proof validation within the deposit fuzzer, allowing subsequent code to be exercised. If not feasible for all implementations to expose the ability to skip Merkle validation, pre-processing could be used to generate a correct proof as an alternative.

It would be reasonable to then exercise is_valid_merkle_branch() within its own dedicated fuzzer.

Progress integrating multiple Go clients (Prysm)

See our previous post for a detailed exploration of the problem and potential approaches.

Przmek (aka @cryptomental) has been plugging away, working on running multiple isolated Go clients, and has made significant progress! There is now a PoC in which ZRNT, Prysm, Nimbus and Lighthouse all exercise the shuffle target. We will need to sort out some PRs and update the fuzzers to spec v0.10.x before this is merged into master.

Some of his adventures are detailed below:

Modifying static libraries prior to linking

We'd previously made a naive attempt that renamed all symbols to avoid clashes. As discussed previously, this fails at link time because the cgo runtime references externally defined symbols in stdlib.h and libpthread.

A more precise approach explored by Przmek involves using objcopy --redefine-syms to rename only symbols that clash. While this resolved build errors, a runtime segfault occurred. Further troubleshooting indicated that the runtime performs code generation, where the generated code eventually references these symbols by fixed names. Any further work here would require modification to the underlying go runtime, resulting in a solution (if it exists) that is exceedingly unsupported and difficult to maintain.

Other unsuccessful approaches

  • That said, some experiments involved modifying clashing function names in go-fuzz-build/src/runtime and export lists of clashing symbols in the Go runtime's callbacks.go, and even renaming all clashing symbols in the runtime via sed.

Unfortunately, segmentation faults still occurred without narrowing things down.

  • Deleting duplicate symbols from the object files.

This resolved some build errors due to clashes, but there were still 2 Go runtimes being invoked that now reference each other.

  • Building static libraries with various combinations of arcane cgo linker flags, including -linkmode=external and direct linker arguments including -Bsymbolic, -Bnoobjreorder, --allow-multiple-definition, and -static.

These linker arguments either had no effect (when applied to go-fuzz-build because static libraries are only linked with the executable), or had unintended side-effects (applied "globally" and changing how every other part of the fuzzer is linked).

Successful PoC

The working concept involves using a combined GOPATH, and additional modifications to Guido Vranken's go-fuzz-build fork to allow shared library compilation via the c-shared build mode and application of the -BSymbolic linker and -fPIC compilation flags (which now apply only to the contents of the shared library).

It's also worth noting that Nim also currently makes use of Golang, wrapping go-libp2p for its current libp2p library. Przmek found that this also causes clashes when both Prysm and Nimbus were present, likely due to a shared dependency on go-libp2p. Using a more recent Nimbus commit (now v0.10.x) and building it as a shared library fixed those issues.

We expect to have Prysm running in master very shortly!

Tooling improvements

We added an extension to the seed corpora script: that converts all relevant Eth2 consensus tests and beaconstate objects to corpora (including for new state transitions). This now allows a single command to generate a complete set of starter corpora for a spec version.

Interestingly, this script has now encountered some difficulties if we still want it to be able to work for multiple spec versions. v0.9.4 introduced some naming differences with regards to the SSZ containers we use. As PySpec currently contains no VERSION package metadata, there is no reliable way for the script to programmatically determine the spec version, and which names to use. This will be provided via a user-supplied parameter for now.

Maintainability improvements

We've made several internal changes that improve quality of life, reduce overheads associated with adding additional targets, and ease crash investigation. These include:

  • A generalized Makefile shared between all fuzzing targets (

    This gets rid of a lot of copy-paste located in the per-target build process (with associated maintainability improvements), whilst still allowing per-target customization via an included Much time was previously spent checking that a change to the build was made correctly to all targets, so this is an important improvement to our build process.

  • Globally enable/disable BLS verification via Makefile variable:

    This allows an easy switch between fuzzing with BLS verification enabled or not, and is generally left disabled to improve coverage (exceedingly unlikely for a random mutation to result in a valid signature). It is also a PoC of the interface that will be used to enable/disable individual clients (to allow easy building of fuzzers that exercise some subset of clients available).

  • Print relevant clients when a difference is detected:

    Previous output only stated that a difference was detected, and what it was - not which clients were involved. Now, each client class contains a name string and, as the number of clients increase, this is quite a welcome improvement.

  • Nim and Rust C++ harness boilerplate put into a library:

    The C++ boilerplate used to interface with Rust and Nim clients was consistent across all block-operation harnesses so was extracted into library files, again reducing copy-pasted code.

  • Python FuzzerInit() function accepting runtime arguments - allows fuzzer to change configuration (e.g. BLS enable), opening up the ability for paths to harness and config files be set at runtime by the central C++.

As additional targets are implemented, costs involved with >= O(n) development processes exhaust more useful time and improvements become more important.

Next steps

  • Implementation of targets for epoch state transitions:
    • As these only take a beaconstate as input, providing a known, valid collection of beaconstate objects is not sufficient. We now need something to generate appropriate states - like a custom libprotobuf mutator.
    • Because a beaconstate is not untrusted input, there are many implicit preconditions and invariants that limit what states a function can expect. It can be reasonable for a client to abort when these preconditions are broken, so we want to only pass suitable states.
    • If unsuitable, we could try generating valid states for Epoch Transitions by performing a heap of block transitions via a reference spec.
  • Integration of the Java-based Teku/Artemis client:
    • Work on this is progressing smoothly and an initial proof-of-concept is imminent.
  • Update to Eth2 spec v0.10.1
  • Thanks to for providing OSS licensing. We will look to integrate some CI tooling as manually testing each fuzzer consumes more time
  • Proposal submission to OSS-Fuzz