On Friday and Saturday, a series of events unfolded as a 13-block chain reorganization occurred on the Litecoin network, effectively rewinding approximately 32 minutes of network activity. This reorganization was in response to a denial-of-service attack that exploited a vulnerability in the Mimblewimble Extension Block (MWEB) protocol, allowing invalid transactions to bypass nodes that had not been updated. The Litecoin Core v0.21.5.4 release has since been made available, containing crucial security updates. According to the Litecoin Foundation, the bug has been fully patched, and the network is now operating as normal.
However, an examination of the litecoin-project GitHub repository reveals that the consensus vulnerability was privately patched between March 19 and 26, roughly four weeks prior to the attack. A separate denial-of-service vulnerability was patched on the morning of April 25, with both fixes being incorporated into the release 0.21.5.4 the same afternoon, after the attack had commenced. The term 'zero-day' refers to a vulnerability that is unknown to defenders at the time of an attack.
In this instance, Litecoin's commit history indicates that the consensus vulnerability was known and patched privately a month before the exploit, but the fix had not been publicly disclosed or mandated for all mining pools. This created a window of opportunity where some miners were running the patched code, while others were still running the vulnerable version, and the attackers seemed to be aware of the difference. Alex Shevchenko, CTO of NEAR Foundation's Aurora project, has raised concerns about the timeline of events.
Blockchain data reveals that the attacker pre-funded a wallet 38 hours before the exploit via a Binance withdrawal, with the destination address already set up to swap LTC for ETH on a decentralized exchange. Shevchenko argues that the denial-of-service attack and the MWEB bug were separate components, with the DoS designed to take patched mining nodes offline, allowing the unpatched ones to form a chain that included the invalid transactions.
The fact that the network automatically handled the 13-block reorganization once the DoS stopped suggests that enough hashrate was running updated code to eventually overpower the attack, but only after the unpatched fork had run for 32 minutes. This incident highlights the differences in how various networks respond to exploits, with newer chains having more centralized validator sets that can coordinate upgrades quickly, whereas older proof-of-work networks like Litecoin and Bitcoin rely on independent mining pools choosing when to upgrade, creating a window of vulnerability when a security patch needs to be implemented rapidly. As of Sunday morning, the Litecoin Foundation has not publicly addressed the GitHub timeline.
The amount of LTC pegged out during the invalid block window and the value of any swaps completed before the reorganization reversed them have not been disclosed.