Beacon Fuzz - Update #03

Beacon Fuzz - Progress Update #3

Beacon Fuzz - Update #03

New Fuzzing Engine, New Bugs

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 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.


Lots of Eth2-related fuzzing has happened at Sigma Prime over the past few weeks! Great progress was made by the Beacon Fuzz team:

  • New fuzzing engine: Honggfuzz
  • Coverage Reports and Beacon Fuzz Revamp in Rust
  • Critical SSZ bug identified in Lighthouse
  • Beacon State Fuzzing and Overflows
  • Java integration
  • New team member


Honggfuzz is an easy to use evolutionary fuzzer (developed and maintained by Google) with the following interesting features:

  • Multi-process and multi-threaded: Honggfuzz can leverage all available CPU cores with a single running instance (by automatically sharing the corpus information with all fuzzing processes);
  • Extremely fast with the support of persistent fuzzing: A standard LLVMFuzzerTestOneInput function can be tested with up to 1 million iterations per second!
  • Great track record of finding bugs (e.g OpenSSL): Honggfuzz is the only fuzzer that managed to identify a critical vulnerability in OpenSSL. The trophies list is quite impressive.
  • Keeps fuzzing even after the identification of a crash: identification of a crash doesn't mean that the fuzzing stops, as opposed to the behaviour of libFuzzer (painful for continuous fuzzing).

In the latest benchmarks published by Google via their project FuzzBench, Honggfuzz seems to be more efficient in terms of reached coverage (code blocks) than AFL and libFuzzer.


Source: fuzz bench

We've decided to experiment with Honggfuzz to leverage its different set of mutation algorithms (compared to libFuzzer and AFL). This was a great decision considering that this new fuzzing engine helped us identify the bugs described in this article.


Screenshot illustrating Honggfuzz exercising the process deposit function of Lighthouse

Coverage Reports and Beacon Fuzz Revamp

Over the past few weeks, we decided to get a better view on the code coverage of our fuzzing inputs. As such, code coverage reports were produced using Kcov, indicating a coverage of more than 84% using the eth2spec tests:


Coverage reports produced using Kcov

To generate interesting samples/inputs, we've created a CLI tool, currently named all_fuzz that allows:

  • Automatic fuzzing of lighthouse harnesses (without user interaction)
  • Swapping between multiple fuzzing engines (Honggfuzz, AFL, libFuzzer)
  • Multi-threading
  • Automatic and pseudo-random selection of a BeaconState per fuzzing thread
  • Crash report/detection

This allowed the identification of the two bugs described in the following sections.

We've also started rewriting Beacon Fuzz in Rust (from C++) as the team is more familiar/experienced with this language.

Critical SSZ Bug in Lighthouse

Honggfuzz found a crash in Lighthouse related to our SSZ decoding which failed to ensure that all offsets were in-bounds for variable length types.

When decoding a specific, custom, mutated BeaconState SSZ file, Lighthouse tries to allocate too much memory, causing a memory allocation failure which leads to a panic. The affected function in our SSZ crate is pub fn decode_list_of_variable_length_items<T: Decode>

In fact, this is an attack vector that Paul had identified last year.

We thought we had a test internally that would catch this particular case, but as described in this issue, the error we were catching in the relevant unit test was in fact unrelated to the test case we wanted to cover.

This bug was quickly addressed by Paul and a fix has been submitted in this Pull Request (to be merged soon).

Beacon State Fuzzing and Overflows

When fuzzing the BeaconState struct in debug mode with Honggfuzz, a panic was triggered by a multiplication overflow when attempting to get the Beacon proposer index for a mutated BeaconState.

Indeed, the underlying issue stems from these particular lines (see the function compute_proposer_index):

let effective_balance = self.validators[candidate_index].effective_balance;
if effective_balance * MAX_RANDOM_BYTE
    >= spec.max_effective_balance * u64::from(random_byte)
    return Ok(candidate_index);
i += 1;

The mutations performed by Honggfuzz triggered an integer overflow in the condition if effective_balance * MAX_RANDOM_BYTE >= spec.max_effective_balance * u64::from(random_byte).

The Beacon State is currently considered as a trusted container: it is internally constructed and iterated by clients and cannot (at this stage) be provided as an externally user-supplied input. This might however change as future syncing strategies might enable beacon nodes to request BeaconState objects from peers for faster syncing (as opposed to requesting block batches).

This is important as some of the bugs that our fuzzing effort will identify won't necessarily be exploitable (including the overflow described above). However, it's important to clarify the overflow assumptions of the eth2 specification, and this bug lead to an interesting discussion.

As a result, the remerkleable Python package maintained by protolambda and used in the python executable eth2 spec has been updated to provide stronger guarantees on overflows.

Java Integration

The team has also been making great progress in integrating Teku (formerly known as Artemis/Harmony):

  • New fuzzing harnesses have been written in Java to exercise the relevant state transition functions. These harnesses compile, and the relevant CLASSPATH is passed to the fuzzer during build;
  • Similar to the Python module, the C++ fuzzing endpoint launches an (in-process) Java runtime and interacts with a Teku harness via the Java Native Interface (JNI). Embedding all these runtimes can bloat the central process, but avoids overheads and complications involved with passing corpora and coverage data via IPC, as well as reliably ensuring signals and crashes are managed by the fuzzer.
  • All currently supported harnesses have been implemented for Teku, and the integration will be merged to master upon completion of beacon_fuzz support for spec v0.11.

New Team Member

Patrick has recently joined the Beacon Fuzz crew and has been instrumental in achieving the milestones decribed in this article. We're really glad to have him onboard are look forward to working with him closely on Beacon Fuzz over the coming weeks and months!

Next steps

  • Update to Eth2 spec v0.11 (nearing completion)
  • Process interesting corpora on all available implementations
  • Continue revamp of Beacon Fuzz in Rust
  • Integrate the use of the Arbitrary trait for grammar-based fuzzing
  • Proposal submission to OSS-Fuzz