Ethereum Sets Glamsterdam Sepolia Test for October 6, Flags Attack Risk

Key Takeaways

• Sepolia’s public test is scheduled for October 6, Hoodi testing is tentatively set for October 27, and mainnet activation remains unscheduled.
• Ethereum developers have warned that attackers could exploit free test ETH and disposable builder identities to win Sepolia block auctions and withhold transaction data.
• The warning concerns testnet operations, with no mainnet funds at risk from the described attack, and highlights the need for clients to identify and reject malicious builders.

Ethereum developers have scheduled the Glamsterdam upgrade’s Sepolia test for October 6, 2026. However, the announcement comes with a warning that malicious block builders could interfere with rollout preparations, CoinDesk reported.

Developers warn that attackers could use free test ETH to outbid legitimate builders, then withhold the transaction data they promised to deliver. Repeating the process under different identities could disrupt Sepolia and prevent developers from properly testing the upgrade.

Ethereum consensus developer Potuz raised the concern during the September 17 developer call. He said, “I can just spin up a thousand builders, rotate them, offer very high bids, and not produce payloads.”

What is Glamsterdam?

Glamsterdam is the next major update on the Ethereum network, designed to fit more activity into each block without overwhelming the computers that verify it. Alongside changes to gas pricing, the upgrade aims to support a block gas limit of around 200 million. The additional capacity could ease congestion, although fees would still depend on demand and the type of transaction.

The upgrade also brings coordination between validators and specialized block builders into Ethereum’s protocol. Builders assemble blocks of transactions and bid to have them included. Once a validator selects a bid, the winning builder is expected to deliver the promised transaction data.

Why Block Builders Matter

The issue involves enshrined proposer-builder separation, or ePBS, a change to bring the relationship between block proposers and builders into Ethereum’s protocol.

Under EIP-7732, the validator proposing a block selects a builder’s signed commitment, which specifies the promised transaction data and payment.

The design separates agreement on a block from verification of its transactions. This gives validators more time to check transaction execution and reduces reliance on trusted intermediaries that currently help coordinate outsourced block construction.

A committee of validators checks whether the payload arrives on time. However, detecting missing data itself does not make that data available for processing.

The proposal explicitly accommodates blocks whose transaction payloads never arrive. It also identifies withholding as a potential cause of missed slots and worse user experience, with penalties among the mitigations discussed.  

This distinction helps explain the testing challenge: a payment mechanism can protect a proposer’s compensation without ensuring every builder delivers usable transaction data. Where bidding carries little economic cost, software must also cope with participants who have no financial incentive to cooperate.

Shorter Review Window Before Sepolia

Client teams now have until September 29 to release Sepolia-compatible software. This leaves seven days before the fork instead of the usual 14-day review period. Developers accepted the shorter window because Sepolia is relatively centralized and easier to recover if problems occur, CoinDesk reported.

Builder software used by Titan and Ultrasound had also yet to complete a Glamsterdam upgrade transition in testing.

Developers will reassess Hoodi’s tentative October 27 date after Sepolia testing. No date has been confirmed for Glamsterdam’s activation on Ethereum mainnet.

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