On Friday and Saturday, a series of events unfolded as a 13-block chain reorganization occurred on the Litecoin network, effectively reversing approximately 32 minutes of network activity. This was in response to an attack that leveraged a vulnerability within the Mimblewimble Extension Block (MWEB) protocol, allowing invalid transactions to bypass nodes that had not been updated.
The attack was facilitated by a denial-of-service vulnerability that targeted major mining pools, enabling the invalid transactions to temporarily gain acceptance before the network self-corrected. Following the incident, Litecoin Core version 0.21.5.4 was released, advising all users to upgrade due to important security updates. According to the Litecoin Foundation, the bug has been fully patched, and the network has returned to normal operation.
However, security researchers have pointed out discrepancies in the timeline of events, suggesting that the vulnerability was known and privately patched between March 19 and 26, roughly four weeks before the attack. The consensus vulnerability and a separate denial-of-service vulnerability were both addressed in the 0.21.5.4 release, which was made available after the attack had begun. The term 'zero-day' refers to a vulnerability that is unknown to defenders at the time of an attack, but in this case, the vulnerability was known and patched, albeit not publicly disclosed or mandated for all mining pools.
This created a window of opportunity for attackers, who seemingly targeted nodes running the unpatched version of the code. Further analysis by blockchain experts, including Alex Shevchenko, CTO of NEAR Foundation's Aurora project, highlighted that the attacker had pre-funded a wallet 38 hours before the exploit, using a Binance withdrawal, and had set up the destination address 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 mining nodes offline, allowing the unpatched nodes 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.
This incident highlights the differences in how various networks respond to exploits, with newer chains capable of coordinating upgrades quickly and older proof-of-work networks like Litecoin and Bitcoin relying on independent mining pools to choose when to upgrade, creating potential windows of vulnerability.