Ethereum developers have scheduled public testing of the Glamsterdam upgrade on Sepolia for Oct. 6 and warned that an attacker could use free testnet ETH and disposable builder identities to win block bids before withholding transaction data. The scenario could disrupt block production and undermine testing of Glamsterdam infrastructure, but would not directly put Ethereum mainnet funds at risk. Glamsterdam’s enshrined proposer-builder separation design moves the link between validators and block builders into Ethereum’s protocol: builders bid for the right to supply blocks, while validators accept winning bids and expect the transaction payloads to follow. An attacker could potentially use a one-time builder ID or create multiple accounts, submit high bids and refuse to reveal payloads. Developers are evaluating safeguards, including fallback block construction and ways to reject abusive builders. Glamsterdam also targets higher Layer 1 capacity, with gas-pricing changes intended to support a block gas limit near 200 million while keeping verification manageable. A private rehearsal reached that level without losing finality, while Devnet-11 previously increased its gas limit from 60 million to 200 million. Client teams have until Sept. 29 to publish Sepolia-ready software, leaving seven days for review before the test instead of the usual 14. Titan and Ultrasound have not completed a Glamsterdam fork transition with their production builder software. Testing on Hoodi is expected to begin on Oct. 27, while mainnet activation remains unscheduled and depends on test results and infrastructure stability; the roadmap continues to target the fourth quarter of 2026.