Sepolia Incident

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


Previous to the merge, totally different message dimension limits for RPC communication had been set to guard purchasers from denial-of-service (DOS) assaults. These limits, utilized to messages acquired through HTTP endpoints, had been carried over to the engine API, which performs an important function in connecting Execution and Consensus Layer purchasers 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 purchasers however remained throughout the acceptable vary for others.

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


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 can be forked away and the proposer would miss out on rewards.

To start with we thought that it was solely potential to create these blocks by utilizing builders or a modified model of a consumer. Geth has a builtin restrict of 128KB for transactions, which signifies that an enormous transaction just like the one beneath dialogue wouldn’t find yourself within the transaction swimming pools of any geth node. It was nevertheless potential to nonetheless set off the restrict by having a consumer with the next restrict suggest the block and the CL requesting validation of this proposed greater block.

We proposed an answer in quickly reducing the RPC restrict on all purchasers to the bottom worth (5MB). This is able to make the block invalid and an attacker can be very restricted within the chaos they will 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 might hit the 5MB restrict with a bunch of transactions which might be beneath the 128KB restrict and never exceed 30 million gasoline.

This can be a greater concern as a result of we realized an attacker may 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 might not be accepted by the vast majority of the community, leading to quite a lot of forks (all being deemed legitimate by the minority nodes) and the chain retains reorging over and over.

In a while February seventh, we got here to the conclusion that everybody elevating their RPC limits can be the safer different.


  • 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 needs to be legitimate.
  • 2024-02-06 15:14: Justin managed to place the transaction in a block and submitted it by the Besu consumer.
  • 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: Group 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 purchasers to restrict their RPC request restrict to 5MB
  • 2024-02-07 6:40: We found that there is likely to be a much bigger concern and the assault might be executed with transactions lower than 128KB dimension.
  • 2024-02-07 10:00: We determined for all purchasers 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 consumer affected by this bug, different purchasers have additionally up to date their defaults to be protected of this assault even when gasoline limits are elevated.
The consumer 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

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