This weblog put up discloses a risk in opposition to the Ethereum community that was current from the Merge up till the Dencun onerous fork.

Background

Previous to the merge, totally different message dimension limits for RPC communication had been set to guard shoppers from denial-of-service (DOS) assaults. These limits, utilized to messages obtained through HTTP endpoints, had been carried over to the engine API, which performs an important function in connecting Execution and Consensus Layer shoppers throughout block manufacturing. Because of the engine API’s involvement in block manufacturing, it grew to become potential for blocks to be produced that surpassed the RPC dimension limits of some shoppers however remained throughout the acceptable vary for others.

If an attacker creates a message that exceeds the scale restrict of the shopper with the bottom setting, whereas nonetheless adhering to the gasoline restrict necessities, after which waits for a block to be produced, it might lead to a state of affairs the place some shoppers regard the block as legitimate, whereas others reject it, issuing a HTTP error code “413: Content material Too Massive.”

Impression

An attacker that would craft these messages would be capable of pressure the vast majority of nodes (=geth) to reject blocks {that a} minority would settle for. These blocks could be forked away and the proposer would miss out on rewards.

At first we thought that it was solely potential to create these blocks through the use of builders or a modified model of a shopper. Geth has a builtin restrict of 128KB for transactions, which implies that a giant transaction just like the one below dialogue wouldn’t find yourself within the transaction swimming pools of any geth node. It was nonetheless potential to nonetheless set off the restrict by having a shopper with a better restrict suggest the block and the CL requesting validation of this proposed greater block.

We proposed an answer in briefly reducing the RPC restrict on all shoppers to the bottom worth (5MB). This may make the block invalid and an attacker could be very restricted within the chaos they’ll trigger within the community because the majority of the nodes would reject their blocks.

Nonetheless on February seventh we found that it was potential to create a block that will hit the 5MB restrict with a bunch of transactions which can be beneath the 128KB restrict and never exceed 30 million gasoline.

This can be a greater subject as a result of we realized an attacker might create a bunch of excessive paying transactions and ship them to the community. Since he outpays everybody else within the mempool, each node (even geth nodes) would come with the assault transactions of their block thus making a block that will not be accepted by the vast majority of the community, leading to a variety of forks (all being deemed legitimate by the minority nodes) and the chain retains reorging again and again.

Afterward February seventh, we got here to the conclusion that everybody elevating their RPC limits could be the safer different.

Timeline

• 2024-02-06 13:00: Toni (EF), Pari (EF) and Justin (Besu) attempt to submit a specificly grinded transaction to the community. The transaction contributes to as much as 2.7 MB blocks when snappy compressed.
• 2024-02-06 13:25: Pari receives errors from his native Geth node though the transaction ought to be legitimate.
• 2024-02-06 15:14: Justin managed to place the transaction in a block and submitted it by the Besu shopper.
• 2024-02-06 20:46: Sam (EF) alerts Pari (particular due to mysticryuujin on X), Toni and Alex about sure Sepolia nodes struggeling.
• 2024-02-06 21:05: Staff double checks with Marius from Geth and confirms the bug.
• 2024-02-06 21:10: The gang will get collectively to debug it
• 2024-02-07 23:40: We determined for all shoppers to restrict their RPC request restrict to 5MB
• 2024-02-07 6:40: We found that there could be a much bigger subject and the assault may be executed with transactions lower than 128KB dimension.
• 2024-02-07 10:00: We determined for all shoppers to extend the RPC request restrict.
• 2024-02-07 21:00: The repair was merged in geth.
• 2024-02-09: Geth was launched

Whereas Geth was the one shopper affected by this bug, different shoppers have additionally up to date their defaults to be protected of this assault even when gasoline limits are elevated.
The shopper groups indicated that the next updates have the protected rpc limits:

Geth: v1.13.12

Nethermind: v1.25.4

Besu: 24.1.2

Erigon: v2.58.0

Reth: v0.1.0-alpha.18