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 network activity. This was in response to an attack that leveraged a vulnerability in the Mimblewimble Extension Block (MWEB) protocol, allowing for invalid transactions to temporarily slip through nodes that had not been updated. The bug enabled a denial-of-service attack against major mining pools, but the network's longest valid chain eventually corrected the invalid transactions. Following the incident, Litecoin Core v0.21.5.4 was released, advising all users 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 known and privately patched between March 19 and 26, roughly four weeks before the attack.
This private patching created a window of vulnerability, as some miners ran the updated code while others continued to run the vulnerable version, which the attackers seemingly exploited. The attack involved a pre-funded wallet and a complex sequence of events, including a denial-of-service attack designed to take patched mining nodes offline, allowing unpatched nodes to form a chain that included invalid transactions. The automatic handling of the 13-block reorganization by the network, once the DoS stopped, indicates that enough hashrate was running updated code to eventually overpower the attack. This incident highlights the differences in how various networks respond to exploits and the challenges faced by older proof-of-work networks like Litecoin, where independent mining pools have the discretion to choose when to upgrade, potentially creating windows of vulnerability.