Testnets play a crucial role in Ethereum’s development process, serving as experimental environments to detect and resolve issues before they impact mainnet. Holesky, one of Ethereum’s key testnets, recently encountered a significant issue while undergoing Pectra upgrade that caused disruption but also provided valuable insights for future improvements.
This postmortem aims to break down the root cause of the issue, its impact, the response from developers, and key takeaways.
Overview of the Holesky Testnet Incident
During a recent Holesky fork, a configuration issue affected three majority clients on the network, preventing them from properly tracking deposit contract addresses. However, minority clients continued to produce valid blocks, highlighting a disparity in client configurations.
Despite the temporary chaos, the issue was quickly identified, and fixes were rolled out to restore network functionality. While testnet failures might seem problematic, they serve their intended purpose—uncovering edge cases and strengthening Ethereum’s robustness before such issues reach mainnet.
Source: eipsinsight.com
What Went Wrong?
The core of the issue revolved around Ethereum’s Execution Layer (EL) clients, which failed to configure the correct deposit contract addresses. This was critical for the Pectra Requests Hash calculation, a mechanism essential for validator deposits. The affected clients forgot to add the correct address, leading to inconsistencies in deposit tracking.
One particular client did not track any address at all, further compounding the issue. Because of this misconfiguration, the network could not correctly identify validator deposits, leading to a breakdown in consensus among Holesky clients. This issue was specific to Holesky as a named testnet, while other networks remained unaffected.
Client teams quickly responded with updates, ensuring that future forks would not face the same issue. Fixes were implemented in Hyperledger Besu, Nethermind, and Go-Ethereum. These updates helped restore proper configuration tracking and prevent similar issues from occurring again.
Why Did the Issue Occur?
Ethereum networks, including Mainnet, Sepolia, Devnets, and Holesky, have different deposit contract addresses. Mainnet uses a standard deposit contract address, while Sepolia has a unique deposit contract to facilitate validator onboarding using tokens. In contrast, Holesky and Devnets use a merged genesis configuration. Some EL clients did not account for these variations, leading to an inconsistency in tracking deposit contracts.
Impact of Pectra Fork & EIP-6110
The Pectra fork introduced several new system contract configurations. One of the key changes, EIP-6110, shifted deposit detection responsibilities from the Consensus Layer (CL) to the Execution Layer (EL). Previously, EL clients did not need to track deposit contracts because the CL handled deposit readings directly. However, with Pectra, EL clients were required to track deposit contracts explicitly.
This change meant that any misconfiguration in deposit contract tracking could lead to network inconsistencies. The bug surfaced in Holesky because the deposit contract addresses on Holesky and Sepolia were different from those on Mainnet. This discrepancy was not immediately obvious to EL clients, leading to an error in deposit validation.
Why This Would Not Have Been Caught in Devnets or Shadowforks?
This issue was unique to Holesky’s testnet setup. Devnets use predefined genesis configurations, which meant that the deposit contract address was always correctly specified. Similarly, Shadowforks replicate mainnet behavior, making Holesky’s misconfiguration irrelevant in those environments. Other testnets, such as Ephemery, were also unaffected due to differences in how they are configured.
Since Holesky was the only network affected, the issue was difficult to detect in other testing environments. This highlights the importance of having diverse testnets to catch unique configuration issues before they impact live deployments.
Whats important to note with todays Holesky fork ist that this issue would've NEVER happened on mainnet or devnets.
— MariusVanDerWijden (@vdWijden) February 25, 2025
It is very specific to how testnets are configured in the client.
Will This Affect Mainnet?
The good news is that this issue will not affect Ethereum’s mainnet. The misconfiguration was specific to Holesky’s testnet setup, and mainnet does not suffer from the same deposit contract tracking issues. However, this event provides valuable insights for Ethereum’s development process.
One key takeaway is the need for EL clients to properly configure deposit contract addresses. The misconfiguration in Holesky revealed an oversight in how clients handle different network setups. Moving forward, client teams must ensure that deposit contract tracking is properly implemented before deploying updates.
Testnet failures can serve as stress tests for real-world mainnet resilience. By analyzing how the Holesky failure unfolded, developers can identify potential weaknesses in Ethereum’s client configurations and address them before they cause more significant problems. Additionally, manually verifying client configurations before network upgrades can prevent similar issues from arising in the future.
How Holesky Recovers?
To restore Holesky’s network functionality, developers have outlined a series of steps. First, both Execution Layer (EL) and Consensus Layer (CL) clients need to be resynchronized with updated releases. This will ensure that all clients use the correct deposit contract addresses.
Additionally, any validator slashing databases need to be deleted. This step is crucial to prevent further issues related to the misconfiguration. Finally, client teams are encouraged to verify configurations before future forks to avoid similar incidents.
Effects of this Incident
One major challenge that arose from the Holesky incident was that an invalid chain was justified, though it was not finalized. Because the majority client had a bug, validators attested to an invalid state. This created a situation where the network would need to either leak or slash clients that supported the incorrect chain.
The immediate goal is to get the correct chain to finalize before further slashing occurs. Until that happens, affected validators will continue facing potential penalties for attesting to an invalid chain. The situation underscores the importance of having robust mechanisms in place to detect and mitigate similar misconfigurations in the future.
Why This Bug Was a Unique Testing Opportunity?
While the issue caused temporary disruption, it was also a great real-world test for Ethereum’s resilience. In previous non-finality devnet tests, researchers simulated similar conditions, but this live failure on Holesky provided an opportunity to observe the impact in real time.
Through this incident, developers could analyze how the network behaved under misconfiguration. The issue also allowed researchers to validate p2p layer performance and study how clients recover from such disruptions. By observing Holesky’s failure and recovery process, Ethereum’s developers can refine their understanding of how the network might react if similar issues ever arise on mainnet.
Thanks to community efforts, some related issues were identified before they could escalate. The Lodestar & Chiado Testnet teams identified a related bug early, reducing the scope of potential chaos. Their findings helped prevent additional misconfigurations, allowing developers to address the problem before it spread further.
This incident highlights the importance of running multiple testnets, devnets, and shadowforks before mainnet deployments. By catching issues in early-stage testing environments, developers can ensure that mainnet remains stable and secure.
For further insights & developer discussions, follow the updates from: @parithosh_j on X
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