On Friday and Saturday, a series of events unfolded as a 13-block chain reorganization occurred on the Litecoin network, effectively rewinding 32 minutes of activity. This was in response to a denial-of-service attack that leveraged a vulnerability within the Mimblewimble Extension Block protocol.
The attack allowed invalid transactions to bypass nodes that had not been updated, prior to the network's longest valid chain correcting them. Following the incident, Litecoin Core v0.21.5.4 was released, with users advised to upgrade due to important security updates. According to the Litecoin Foundation, the bug was fully patched and the network resumed normal operation by Sunday morning. However, security researchers have pointed out discrepancies in the timeline of events, suggesting that the vulnerability was privately patched between March 19 and 26, roughly four weeks before the attack.
The consensus vulnerability that enabled the invalid transactions was known and patched, but the fix had not been publicly broadcast or mandated for all mining pools, creating a window of vulnerability. The attackers seemingly exploited this knowledge, targeting unpatched nodes. Blockchain data revealed that the attacker had pre-funded a wallet 38 hours prior to the exploit, with plans to swap LTC for ETH on a decentralized exchange. The denial-of-service attack and the MWEB bug were distinct components, with the former designed to take patched nodes offline, allowing the unpatched ones to form a chain that included the invalid transactions.
The network's automatic handling of the 13-block reorganization once the DoS stopped suggests that enough hashrate was running updated code to eventually overpower the attack. This incident highlights the differences in how various networks respond to exploits, with newer chains capable of coordinating upgrades quickly, while older proof-of-work networks like Litecoin rely on independent mining pools to upgrade, creating potential windows of vulnerability.