CoinDesk
Ethereum developers warned that malicious builders could exploit free test ether and disposable identities to win Sepolia block auctions, then withhold the transaction payloads. The warning came as developers confirmed an Oct. 6 public test for the Glamsterdam upgrade.
The attack would not threaten Ethereum mainnet funds, but it could disrupt the infrastructure testing required before the upgrade moves forward. A builder can submit bids far above legitimate rivals, win repeatedly and refuse to reveal the transactions behind accepted blocks.
Developers said clients may need to identify and reject individual builders, rather than relying only on fallback blocks after multiple missing payloads. A private rehearsal recently completed the Glamsterdam transition and lifted the block gas limit toward 200 million without losing finality.
Client teams must release Sepolia-ready software by Sept. 29, leaving seven days before the fork instead of the usual 14-day security review window. Hoodi testing is tentatively set for Oct. 27, while Ethereum has not scheduled mainnet activation.
Frequently asked questions
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What attack did Ethereum developers identify ahead of Glamsterdam?
A malicious operator could use free test ether and disposable builder identities to submit high bids, win Sepolia block auctions and withhold transaction payloads.
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Would the fake-builder attack put Ethereum mainnet funds at risk?
No. The threat is confined to Sepolia, where it could disrupt infrastructure testing but would not endanger mainnet funds.
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When is the public Sepolia test for Glamsterdam scheduled?
The public Sepolia test is scheduled for Oct. 6, with client teams expected to release Sepolia-ready software by Sept. 29.
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How are Ethereum clients expected to respond to malicious builders?
Developers said clients may need to identify and reject individual builders, rather than relying only on fallback blocks after missing payloads.
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What are the next testing and mainnet dates for Glamsterdam?
Hoodi testing is tentatively planned for Oct. 27. Ethereum has not yet scheduled Glamsterdam's mainnet activation.
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