Ethereum’s Glamsterdam Testnet Launch Set for October 6 Amid Builder Attack Concerns

Key Points

• Ethereum’s Glamsterdam network upgrade will deploy on the Sepolia testnet starting October 6, 2026
• Core developers issued warnings about malicious actors potentially exploiting free test ether to create fake builder identities
• Bad actors could generate numerous counterfeit builders, secure auction bids, and refuse to release transaction information
• Development teams face a September 29 deadline to ship Sepolia-compatible software, providing just seven days for review
• No mainnet deployment timeline has been established, though Ethereum aims for Q4 2026 implementation

Ethereum’s core development team has officially set October 6 as the launch window for Glamsterdam’s initial public testing phase on the Sepolia testnet. The announcement included cautionary notes regarding a possible attack vector that may interfere with testing procedures.

ETHEREUM’S NEXT MAJOR UPGRADE IS MOVING FORWARD.

Glamsterdam is scheduled to hit the Sepolia testnet on October 6, followed by a tentative Hoodi test on October 27.

Developers have identified a potential weakness where attackers could create fake builder identities, submit huge… pic.twitter.com/79L6vjSf4I

— MANDO CT (@MandoCT) September 18, 2026

The security issue revolves around Glamsterdam’s block construction mechanism. This upgrade implements Enshrined Proposer-Builder Separation, known as ePBS. This framework allows dedicated builders to compete through auction systems to deliver transaction blocks to network validators.

While mainnet participation requires actual capital investment, Sepolia operates with freely available test ether, fundamentally altering the threat landscape.

Understanding the Potential Exploit

During a September 17 development call, consensus developer Potuz outlined the vulnerability. A malicious party could generate numerous fraudulent builder profiles, place inflated bids to secure block auctions, then deliberately withhold the required transaction information.

I am not sure you are prepared for Glamsterdam on Ethereum pic.twitter.com/O57rRe7XDr

— Ben {chmark} Adams ⟠ (@ben_a_adams) September 16, 2026

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

While actual funds remain secure, such an attack could paralyze testnet operations and prevent developers from accurately assessing the upgrade’s functionality under authentic network conditions.

Current protective measures appear insufficient. Certain client protections only activate after multiple payload failures. Developers emphasized the necessity for builder-specific banning mechanisms to prevent attackers from circumventing restrictions through identity rotation.

Core Objectives of the Glamsterdam Upgrade

Glamsterdam represents Ethereum’s upcoming significant network enhancement. The upgrade aims to substantially expand network throughput capacity per block, with gas limit targets approaching 200 million, compared to the 60 million observed in recent testing environments.

Additionally, the upgrade expands the transaction payload data distribution timeframe from approximately two seconds to roughly nine seconds, providing nodes with additional processing capacity for larger block sizes.

A closed testing environment designated Devnet-11 successfully executed a Glamsterdam transition on September 16. The network operated 84,000 validators across diverse client implementations and pushed gas limits toward the 200 million threshold while maintaining finality.

Implementation Schedule and Future Steps

Development teams must deliver Sepolia-compatible client software by September 29. This creates only a seven-day buffer before the October 6 fork activation, representing half of Ethereum’s standard 14-day allocation for security audits and bug identification.

Developers approved this compressed timeline because Sepolia operates with a managed validator configuration that allows simpler recovery procedures should complications arise.

Following Sepolia deployment, testing on the Hoodi testnet is provisionally scheduled for October 27. This timeline remains contingent on Sepolia’s performance outcomes.

No mainnet activation schedule has been established. Ethereum’s official development roadmap designates Glamsterdam for Q4 2026, though specific timestamps or epoch numbers remain unconfirmed. Production builder implementations from Titan and Ultrasound teams have not yet successfully completed Glamsterdam fork transitions.