Before Bitcoin Core v25.0, a peer sending mutated blocks could clear the download state of other peers that also announced the block to us, which would hinder block propagation.
This issue is considered Medium severity.
Details
Bitcoin Core treats a block as mutated when, for example, the Merkle root in the header or the witness commitment in the coinbase transaction doesn’t match the transactions in the block.
Before Bitcoin Core v25.0, a peer could clear the block download state of
other peers by sending an unrequested mutated block. This was a problem for, for
example, compact block relay. After receiving a compact block and while waiting
for a response to a getblocktxn request to reconstruct the full block,
receiving the mutated block would let Bitcoin Core forget about the compact
block reconstruction state. A blocktxn response arriving after the mutated
block couldn’t be used to reconstruct the block. This hindered block propagation.
This was fixed in #27608 by making sure that a peer can only affect its own block download state and not the download state of other peers.
Attribution
Credit goes to Suhas Daftuar for noticing the problem and working on a fix.
Timeline
- 2023-05-08 - A problem with mutated blocks is first reported in the #bitcoin-core-dev IRC channel.
- 2023-05-10 - Fix is merged (#27608)
- 2023-05-25 - v25.0 is released
- 2024-10-09 - Public disclosure