Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

CometBFT is a BFT implementation used by Cosmos blockchains. In this, block timestamps are derived from a weighted median of validator votes. In theory, this should ensure that the median timestamp falls within the range of honest validators, even when 1/3 are malicious. Commit is used for the finalized proof within a block being accepted and bundling the block ID with a set of signatures. Each commit signature is a validator's vote attesting to that block.

The Commit structure stores the entirety of the block information. The signatures are a list of CommitSig objects, each containing an address, timestamp, and signature. When performing commit signature verification, the index of the signature is used to find the amount of voting power. When computing the median time, the validator address is used instead. If the address is not present in the current validator set, then it's simply skipped.

This small difference allows different values to be used for different things. For signature verification, the ValidatorAddress doesn't matter; it's only the index of the signature. So, the submitter of a block can use an invalid ValidatorAddress to force the lookup of an invalid value for the median time difference! The example exploit makes the attacker's validator address the ONLY valid address and index, allowing them to set the block timestamp arbitrarily.

On most chains, it's possible to cause a chain halt by forcing an overflow on the time of the block. In the case of chains with time-based rewards, increasing the time horizon enables the creation of large amounts of assets. Notably, Babylon and Celestia would result in a significant token inflation. The "$600M" portion feels slightly exaggerated. These funds become unusable almost immediately after exploitation.

The vulnerability is pretty rad! Bad input validation on all fields leads to a weird edge case that breaks everything. Awesome find! To fix this, they suggest performing address validation on the index and returning an error if the address cannot be found. Anytime errors are silently being ignored, it's probably going to be a problem!