Ethereum developers confirmed an October 6, 2026, public test of the Glamsterdam upgrade, an update expected to increase network throughput. They also warned that a vulnerability in Sepolia’s new block auction system could complicate infrastructure testing before the upgrade reaches the Ethereum mainnet.

CoinDesk reported the plans and warning, citing an Ethereum core developers call.

An attacker could create many fake builder identities, outbid legitimate participants and then fail to deliver transactions after winning an auction. Because test Ethereum has no meaningful value, such an attack could be carried out relatively cheaply on a testnet.

Glamsterdam targets higher throughput

Glamsterdam will be Ethereum’s next major upgrade after Fusaka, which launched on the mainnet on December 3, 2025. Ethereum previously activated Pectra on May 7, 2025, marking the network’s biggest upgrade since The Merge.

Glamsterdam is designed to fit more activity into each block without overloading the computers that verify the network. Along with changes to gas accounting, the upgrade is expected to pave the way for a block gas limit of around 200 million, increasing the number of payments and trades that can be processed in a single block.

The upgrade will bring validators’ interactions with specialized block builders into the Ethereum protocol. Builders assemble blocks and compete against one another. After a validator accepts the highest bid, it expects to receive the corresponding set of transactions.

Ethereum consensus-layer developer Potuz described how that system could be disrupted:

“I can just spin up a thousand builders, rotate them, offer very high bids, and not produce payloads. Any teenager can do this.”

Developers said existing protections generally switch to locally built blocks only after several instances of missing data. They said clients also need the ability to block specific builders so that an attacker cannot continue the disruption under a new identity.

Sepolia testing window shortened

The warning followed a large-scale private Glamsterdam rehearsal in which developers raised the block gas limit toward 200 million without losing finality.

Client teams have until September 29 to release software compatible with Sepolia. That leaves seven days before the October 6 fork, half the standard 14-day period allocated for security checks and bug-bounty testing.

Developers accepted the shorter schedule because Sepolia is a relatively centralized network and would be easier to recover if problems emerged. However, block-building software from the Titan and Ultrasound teams has not yet completed its transition to Glamsterdam.

A subsequent public test on the Hoodi network is tentatively scheduled for October 27. Developers will make a final decision based on the Sepolia results, while a date for activating Glamsterdam on the Ethereum mainnet has not been set.

Ethereum launched the Platåberget testnet a month ago to validate Glamsterdam before its planned deployment on Sepolia and Hoodi.

Source: Incrypted