Litecoin Recovers from Denial-of-Service Attack by Rewriting 13 Blocks

Late Friday and Saturday, a series of events unfolded on the Litecoin network, starting with a denial-of-service attack that exploited a vulnerability in its Mimblewimble Extension Block protocol, allowing for the temporary inclusion of invalid transactions. The network responded with a 13-block chain reorganization to reverse the effects of the attack, effectively rewinding 32 minutes of network activity. This reorganization occurred after attackers utilized a vulnerability to enable a denial-of-service attack against major mining pools, which allowed invalid transactions to bypass nodes that had not been updated. The Litecoin Core v0.21.5.4 was subsequently released, containing crucial security updates and advising all users to upgrade. According to the Litecoin Foundation, the bug was fully patched, and the network resumed normal operations by Sunday morning. However, security researchers have pointed out discrepancies between the Foundation's narrative and the timeline of patches as shown in the litecoin-project GitHub repository. The repository indicates 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 patched on April 25, with both fixes being included in the release 0.21.5.4 after the attack had begun. The term 'zero-day' refers to a vulnerability that is unknown to the defenders at the time of an attack, but in this case, the vulnerability was known and patched a month prior to the exploit. This created a window of vulnerability where some miners were running patched code, while others were still using the vulnerable version, and the attackers seemed to be aware of this distinction. Alex Shevchenko, CTO of NEAR Foundation's Aurora project, highlighted concerns in a thread, noting 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 seen as separate but complementary components of the exploit, with the DoS designed to take patched mining nodes offline, allowing the unpatched ones 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 having more centralized validator sets that can coordinate 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. As of Sunday morning, the Litecoin Foundation had not publicly addressed the GitHub timeline, and details regarding the amount of LTC pegged out during the invalid block window and the value of any swaps completed before the reorganization have not been disclosed.