Ethereum warns of builder abuse ahead of Glamsterdam testnet activation

Ethereum developers confirmed that the Glamsterdam upgrade will be activated on the Sepolia testnet on October 6, followed later by the Hoodi testnet. The rollout comes with a warning that the free test ether available on Sepolia could enable malicious builders to repeatedly win block‑auction bids and withhold transaction payloads.

Enshrined Proposer‑Builder Separation (ePBS) and the abuse scenario

The core of the concern is EIP‑7732, which introduces Enshrined Proposer‑Builder Separation (ePBS). Under ePBS, builders submit signed bids for the right to supply an execution payload, and proposers commit to the winning bid. On a public network where test ETH has no market value, an attacker could create many disposable builder identities, submit inflated bids, win the auction, and then refuse to deliver the payload.

Potential impact on Sepolia

Because test ETH is cheap, the economic barrier that limits such behavior on mainnet is removed. Developers noted that existing client circuit‑breakers may only react after several missed payloads and may not block individual abusive builders quickly enough. The risk is framed as a public‑testnet availability issue rather than a direct threat to mainnet ETH.

Testing timeline and client readiness

Devnet‑11 successfully completed a “happy‑path” rehearsal, raising the gas limit from 60 million to 200 million and confirming finality. The Sepolia client review window is compressed to seven days: client teams are expected to release Sepolia‑ready software by September 29, leaving only a week before the October 6 activation. This is half the usual 14‑day review period, but developers accepted the risk due to Sepolia’s relatively centralized validator set.

Roadmap outlook

Ethereum’s public roadmap still lists the mainnet Glamsterdam activation for the fourth quarter of 2026, without a confirmed epoch or timestamp. The Sepolia upgrade serves as the first public test of the ePBS design, with the Hoodi testnet slated to follow and provide an open validator environment for further staking and protocol testing.