**Title: Navigating Challenges: The Pectra Upgrade on Sepolia Testnet and the Unexpected Exploit**
The Pectra upgrade on the Sepolia testnet encountered some surprising challenges, resulting in the production of empty blocks. Developers quickly discovered that an attacker was taking advantage of a previously overlooked edge case in the ERC-20 standard, which led to a coordinated effort to address the issue.
When the Pectra upgrade was deployed at 7:29 AM UTC on March 5, developers faced bugs in their geth nodes, alongside the troubling occurrence of mined empty blocks. The root of the problem stemmed from an unexpected interaction with the deposit agreement, which triggered unintended log events. This situation resulted in invalid blocks, preventing transactions from being processed correctly. Fortunately, the Ethereum team swiftly pinpointed the issue: the deposit contract, responsible for processing staking deposits, erroneously emitted an ERC-20 transfer event instead of the anticipated deposit event.
I recently penned an article detailing the Pectra incident on Sepolia, which is quite a fascinating tale of edge cases, teamwork, and an attacker who made things significantly more challenging for us! If you’re interested, take a look (4-minute read): https://t.co/0ezGnm0Z8j — MariusVanDerWijden (@vdWijden) March 9, 2025.
Since the network’s logic required uniform handling of all deposit contract logs, this inconsistency led to errors in block production. Developers noticed that any block containing a transaction linked to the contract was being rejected, resulting in only empty blocks being produced. A patch was proposed to disregard invalid logs, but the developers faced a significant hurdle: implementing it without causing a chain split. They managed to schedule the update for 14:00 UTC, allowing teams ample time to prepare. In the meantime, to minimize disruption, they replaced the problematic transactions with higher-paying ones, ensuring that blocks could still be generated.
However, just when the team thought they had resolved the issue, empty blocks reemerged. Upon further investigation, they discovered a new transaction exploiting the same vulnerability. Initially, the team suspected an internal error, only to later realize that an external party had uncovered the flaw. The attacker took advantage of a lesser-known aspect of the ERC-20 standard: zero-token transfers. This method enabled anyone, even those without token ownership, to trigger transfer events, circumventing the original fix.
In response to this new challenge, the developers acted swiftly. They deployed a private patch to block transactions that interacted directly with the deposit contract, quietly rolling it out to a limited number of DevOps nodes to keep the chain operational while working on a more comprehensive solution. The attacker’s strategy suggested they had access to developer forums, complicating matters further. They could potentially adjust their approach by executing transactions through intermediary contracts if the patch became public knowledge. This situation compelled developers to manage their response discreetly while ensuring that enough full blocks were produced.
Source: Github Ethereum Dev Plan Deeper Reviews Before Mainnet Rollout
