Unknown attacker causes headaches during Pectra upgrade on Sepolia

An Ethereum developer says the recent Pectra upgrade of the Sepolia testnet ran into errors, which was made worse after an attacker used an “edge case” to cause the mining of empty blocks.

Pectra rolled out on its final testnet, Sepolia, at 7:29 am on March 5, but Ethereum developer Marius van der Wijden said in a March 8 post that the team immediately started seeing error messages on their geth node and empty blocks being mined.

The error was because the deposit contract triggered the wrong type of event — a transfer event instead of a deposit, according to Van der Wijden.

A fix was rolled out, but van der Wijden says they missed one edge case, and an unknown user exploited it by sending a 0-token transfer to the deposit address, which triggered the error again. 

“After a few minutes we saw a lot of empty blocks again, so we looked again into the transaction pools and found another offending transaction that triggered the same edge cases,” he said.

“First we thought that someone from the trusted validators has made a mistake, but we quickly realized that this transaction originated from a new account recently funded by the faucet.”  

The ERC-20 standard does not forbid a zero token transfer; this allows anyone, even if they don’t own any tokens, to transfer to another address, which the unknown user realized, van der Wijden said.

“The only way to stop the attack would be to filter out all transactions that interact with the deposit contract. So we made the following private fix, which we deployed to a few of the DevOps nodes.”

“We suspected that the attacker was reading some of our chats, so we decided not to publicize the fix, but only update a few nodes that we controlled in order to get more full blocks on the network,” he added. 

By 2 pm, all the nodes had been updated with the fix, and the unknown user transaction was mined successfully.

Van der Wijden said they never lost finalization during the incident, and the issue was isolated to Sepolia because they were using a token-gated deposit contract instead of the normal mainnet deposit contract.

Previously, the developers tested the Pectra upgrade on the Holesky testnet on Feb. 26, which also encountered issues.

As a result, the developers have decided to postpone the Pectra upgrade until more tests can be done.

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The Pectra fork follows the network’s Dencun upgrade, which slashed transaction fees for layer-2 networks and improved the economics of Ethereum rollups. The Dencun hard fork rolled out on March 13, 2024.

The Ethereum Foundation recently a new leadership structure with two co-directors of the foundation, Hsiao-Wei Wang and Tomasz Stańczak, taking the helm. 

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