Litecoin has published a detailed postmortem on two linked MWEB incidents that hit the network in March and April 2026: an inflation exploit that briefly created 85,034.47285734 LTC through a malformed pegout, and a later 13-block reorg triggered when a second exploit attempt interacted with patched and unpatched mining infrastructure.
The account, published April 28 by David Burkett on Litecoin.com, says the original bug has now been fixed and that Litecoin Core 0.21.5.4 addresses the later failure mode that helped an invalid chain grow before it was reorged out. The postmortem also states that no confirmed user funds were ultimately lost in the March exploit, though some third-party services were impacted during the April reorg event.
How the MWEB Bug Enabled an Inflation Exploit
According to Burkett’s postmortem, the March issue came from a consensus validation gap in Litecoin’s Mimblewimble Extension Block implementation. MWEB inputs are supposed to match the actual output they spend, including metadata used in balance and spend checks, but that check “was not fully enforced in the block connection path.” In practice, that meant an attacker who mined a block, or controlled a miner, could bypass normal mempool and block-building checks and make “a small MWEB input appear to support a much larger pegout.”
Litecoin said a chain scan later showed the bug had already been exploited at block height 3,073,882. The malicious pegout created 85,034.47285734 LTC and sent it to a transparent address before the funds were split into three transparent-chain outpoints. Burkett wrote: “The inflated March outputs were still intact when discovered. This allowed developers and miners to contain the funds before they were dispersed.” He added that upgraded miners then rejected at least one attempted spend from the frozen outputs.
The postmortem says the actor later cooperated after being contacted and signed a recovery transaction that returned 84,184.47278630 LTC to a recovery address, while 850 LTC was paid to the actor as an agreed bounty and roughly 0.00007104 LTC went to fees. Litecoin said Charlie Lee, creator of Litecoin, bought 850 LTC to make the amount whole. Burkett wrote: “The full 85,034.47285734 LTC was then pegged back into MWEB in a single transaction. The resulting MWEB output was frozen, which restored the MWEB balance while ensuring those funds cannot be spent again.” The rebalance peg-in was mined at block 3,078,098, and the network now carries that frozen output to neutralize the exploit’s economic effect.
Why Litecoin Saw a 13-Block Invalid Chain Reorg
The April reorg stemmed from a second attempt to use the same exploit path after the March fix was already in place, according to Litecoin. This time, upgraded nodes rejected the malformed MWEB block, but a separate issue involving mutated MWEB block data caused some patched mining nodes to stall. Burkett explained the problem this way: “When an upgraded node received a mutated MWEB block over P2P, it could fail while applying the MWEB body and classify the failure as BLOCK_MUTATED. The node could then retain the bad serialized block data for that block hash.”
That behavior mattered because the same block hash could later arrive with valid data, but the stored mutated bytes could interfere with block processing and mining RPC flows such as submitblock. Burkett wrote: “During the April incident, this caused upgraded mining nodes to reject the bad block but also become unable to continue normal mining operations quickly enough. Unupgraded miners, which did not enforce the MWEB fix, continued extending the invalid chain until upgraded miners coordinated and overtook it.” The invalid chain ran from block 3,095,931 through 3,095,943, for a total of 13 blocks.
Litecoin says those 13 blocks were later reorged out by miners following correct validation rules, and it stressed that this “was therefore not a rollback of valid Litecoin history.” Still, some third-party systems processed transactions on the invalid chain before the reorg completed. The postmortem specifically names NEAR Intents, where an attacker swapped 11,000 LTC for 7.78814476 BTC, and THORChain, where 10 LTC was swapped for 0.00719957 BTC, with both LTC inputs later disappearing from the valid chain after the reorg. Litecoin Core 0.21.5.4, released April 25, now erases stored block data for mutated blocks so valid data for the same hash can later be accepted, and Litecoin is urging users and services to upgrade to that version or later.
AI Transparency Note: This article was prepared with the assistance of an AI system based on the sources listed and was reviewed, edited, and approved by a human editor before publication. All quotes, data points, and factual claims are intended to be grounded in the cited source material; however, errors cannot be ruled out entirely.
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