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 a denial-of-service attack that leveraged 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 was made available, advising all users to upgrade due to important security updates. According to the Litecoin Foundation, the bug was fully patched, and the network returned to normal operation by Sunday morning. However, an examination of the litecoin-project GitHub repository reveals that the consensus vulnerability was privately patched between March 19 and 26, more than four weeks before the attack.
A separate denial-of-service vulnerability was addressed on April 25, with both fixes being included in the release 0.21.5.4 after the attack had commenced. The term 'zero-day' refers to a vulnerability that is unknown to defenders at the time of an attack, but in this case, the consensus vulnerability was known and patched a month prior to the exploit. This created a window of opportunity where some miners were running patched code, while others were still vulnerable, and it appears the attackers were aware of this discrepancy.
Concerns have been raised by prominent researchers, including Alex Shevchenko, CTO of NEAR Foundation's Aurora project, who pointed out that blockchain data showed the attacker had pre-funded a wallet 38 hours before the exploit and had configured the destination address to swap LTC for ETH on a decentralized exchange. The denial-of-service attack and the MWEB bug were separate components, with the DoS designed to take patched mining nodes offline so the unpatched ones would form the chain that included the invalid transactions.
The network's ability to automatically handle 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 attacks on various networks are handled, with newer chains having more centralized validator sets that can coordinate upgrades quickly, while older proof-of-work networks like Litecoin and Bitcoin rely on independent mining pools choosing when to upgrade, creating a window of vulnerability when security patches need to be applied.
As of Sunday morning, the Litecoin Foundation had not publicly addressed the GitHub timeline, and the amount of LTC pegged out during the invalid block window, along with the value of any swaps completed before the reorganization reversed them, remains undisclosed.