Late Friday and Saturday, a chain reorganization of 13 blocks on the Litecoin network unwound approximately 32 minutes of network activity. This was in response to an attack exploiting a vulnerability in the Mimblewimble Extension Block (MWEB) protocol, which allowed for a denial-of-service attack against major mining pools. The attackers were able to slip invalid MWEB transactions past nodes that hadn't been updated, before the network's longest valid chain corrected them.
A security update, Litecoin Core v0.21.5.4, has been released, with all users advised to upgrade. According to the Litecoin Foundation, the bug has been fully patched and the network is operating normally. However, security researchers have pointed out that the timeline of events, as shown in the litecoin-project GitHub repository, indicates the consensus vulnerability was known and privately patched between March 19 and 26, roughly four weeks before the attack.
The fix had not been publicly announced or required for all mining pools, creating a window of vulnerability. The attackers appeared to have knowledge of which miners were running the patched code and which were still vulnerable. Blockchain data revealed that the attacker had pre-funded a wallet 38 hours before the exploit, with the destination address configured to swap LTC for ETH on a decentralized exchange. The denial-of-service attack and the MWEB bug were separate components, designed to take patched mining nodes offline and allow unpatched ones to form a chain that included 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. The incident highlights the differences in how various networks respond to exploits, with newer chains able to coordinate upgrades quickly and older proof-of-work networks like Litecoin and bitcoin relying on independent mining pools to choose when to upgrade, creating a window of vulnerability.