Ethereum confirms October 6 Glamsterdam testnet launch amid security warnings

Ethereum developers have scheduled the Glamsterdam upgrade to activate on the Sepolia testnet on October 6, 2026, targeting a full mainnet launch in late 2026. While private rehearsals on Devnet-11 successfully raised block gas limits to 200 million, developers warn that the new Enshrined Proposer-Builder Separation (ePBS) mechanism is vulnerable to disruption. On public testnets, bad actors can use free test ether to win block auctions and withhold transaction payloads, highlighting the need for stronger client-side safeguards.

Glamsterdam Sepolia testnet launch

• ▪Ethereum's public roadmap targets the fourth quarter of 2026 for the Glamsterdam upgrade's mainnet activation, though no official mainnet date has been scheduled
• ▪Ethereum client software teams have until September 29, 2026 to release Sepolia-ready software, leaving seven days for review instead of the standard 14 days.
• ▪Ethereum developers confirmed that the Glamsterdam upgrade will activate on the Sepolia testnet on October 6, 2026.
• ▪Ethereum's Hoodi testnet public Glamsterdam test is tentatively planned for October 27, 2026, depending on the performance of Ethereum's Sepolia testnet Glamsterdam test on October 6, 2026

Enshrined Proposer-Builder Separation mechanism

• ▪Enshrined Proposer-Builder Separation (ePBS), introduced via EIP-7732, separates the job of building transaction blocks from validating them within Ethereum's consensus rules.
• ▪Under the Enshrined Proposer-Builder Separation design, block builders submit bids to supply execution payloads, and the winning builder must release the underlying transactions once a proposer commits to the bid.
• ▪EIP-7732 extends the execution-payload propagation window from approximately two seconds to around nine seconds to allow nodes more time to distribute and validate larger payloads.

Builder abuse vulnerability

• ▪A malicious builder could repeatedly win auctions by bidding aggressively with free test ether and then withhold transaction payloads, effectively freezing testnet traffic.
• ▪Ethereum developers warned that cheap or free test ether on the Sepolia testnet could allow malicious actors to create disposable builder identities and win block auctions.
• ▪Ethereum consensus developer Potuz stated that any teenager could spin up and rotate a thousand builders to disrupt Ethereum's Sepolia public test network during the Glamsterdam upgrade testing
• ▪The Glamsterdam upgrade's ePBS builder abuse vulnerability does not threaten real mainnet Ethereum funds, but it can disrupt infrastructure testing on public testnets such as Sepolia

Client circuit breaker safeguards

• ▪Ethereum consensus developer Potuz noted that client software needs updates to identify and reject individual abusive builders to prevent them from returning under new identities.
• ▪Ethereum developers chose the Sepolia testnet for the first public Glamsterdam test, scheduled for October 6, 2026, because its validator set is controlled by a smaller, permissioned group of teams, making it easier to recover if software breaks
• ▪Existing Ethereum client software safeguards for the Glamsterdam Sepolia test may fail to prevent builder abuse because some client circuit breakers only switch to locally built blocks after several payloads are missed

Devnet-11 gas limit testing

• ▪Prior to Devnet-11, a private rehearsal network that began September 14, 2026, developers experienced finality and node offline issues during testing on Devnet-8, Devnet-9, and the Platåberget environment
• ▪The 200 million gas limit used in Devnet-11 is a test parameter and does not represent a confirmed commitment for the Ethereum mainnet.
• ▪Devnet-11, a private rehearsal network that launched on September 14, 2026, completed its Gloas transition on September 16, 2026.
• ▪Devnet-11, a private rehearsal network that began on September 14, 2026, successfully raised its block gas limit from 60 million to 200 million without losing finality across 84,000 validators