Cosmos Labs has disclosed that attackers exploited a critical Cosmos EVM vulnerability across six blockchain networks between Aug. 20 and Aug. 25, converting stolen tokens into about $5.72 million in assets through decentralized and centralized exchanges.
Summary
- Attackers exploited a critical Cosmos EVM flaw across six networks between Aug. 20 and Aug. 25, converting stolen tokens into about $5.72 million in other assets.
- Cosmos Labs first received the vulnerability report in April but initially concluded that production networks were not at risk and handled the fix through its silent patch process.
- MANTRA lost 720.9 million tokens worth about $3.6 million, while TAC and KiiChain later suffered separate attacks using the same method.
- The first attack began about 20 hours after patched Cosmos EVM versions were released without a vulnerability specific advisory to network operators.
- Cosmos Labs coordinated with 40 chains during the response and helped 13 networks patch or halt before they could be attacked.
Cosmos Labs said in a technical post-mortem published Friday that the flaw had first been reported through its bug bounty program on April 25, nearly four months before the attacks began. Its testers were unable to reproduce the exploit against configurations used by known production Cosmos EVM networks and concluded at the time that live user funds were not at risk.
Based on that assessment, developers handled the vulnerability through a silent public patch instead of privately distributing a security fix to affected chains. Cosmos Labs merged the fix in May without telling network operators which vulnerability it addressed.
The assessment later proved incorrect after independent researchers established in early August that the bug affected all Cosmos EVM chains. Cosmos Labs then obscured the fix to make reverse engineering more difficult and released patched versions at 7:01 p.m. ET on Aug. 19.
Release notes referred to “important” security fixes without describing the vulnerability. The first known attack began at 3:06 p.m. ET on Aug. 20, about 20 hours after the patched software became available.
Cosmos EVM flaw allowed attackers to drain large accounts
The vulnerability involved an integer underflow in Cosmos EVM, the ecosystem’s Ethereum-compatible framework built from the open-source Evmos codebase.
An attacker could first create an account containing locked tokens and delegate more tokens to a validator than the account was able to spend. Subtracting the delegated amount caused the balance to fall below zero, making the value wrap around to the maximum possible figure of 2^256-1 base units.
The attacker could then use the inflated balance against another account. Sending the amount to a target pushed its recorded balance past the same numerical ceiling, causing an overflow that wrapped the value back down and left the attacker holding the target’s tokens.
No additional tokens were created through the process, according to Cosmos Labs, and total token supply remained effectively unchanged. MANTRA said the exploit changed its supply by only one base unit, the smallest divisible denomination of the token.
Cosmos Labs said attackers targeted accounts holding large balances, including burn addresses and multisignature wallets created when networks launched. Its advisory classified the flaw as critical and identified Cosmos EVM releases before v0.6.2 and v0.7.2 as vulnerable.
The incident followed another security disclosure involving Cosmos software earlier this year. Crypto.news previously reported that a researcher had disclosed a CometBFT flaw in April that could cause nodes to stall during block synchronization. The CVSS 7.1 issue did not allow direct asset theft.
Networks had about 20 hours after the patch
Once independent researchers confirmed the Cosmos EVM flaw could affect production chains, Cosmos Labs prepared the security releases that went live on Aug. 19.
Network operators were not given a vulnerability-specific warning explaining what the upgrade fixed. MANTRA later said 20 hours was not enough to assess, build, test and coordinate a state-breaking upgrade across its 38 independent validators.
“Twenty hours was not a realistic window in which to assess, build, test and coordinate a state-breaking upgrade across 38 independent validators, particularly without a vulnerability-specific advisory,” MANTRA wrote in its post-mortem.
Another disclosure occurred before the first theft. At 3:16 a.m. ET on Aug. 20, a Push Chain developer publicly submitted a code change describing the vulnerability and its exploitation path. The filing credited the finding to an audit by security firm Hacken and listed versions considered vulnerable.
The submission said no released version contained the fix, though its version table omitted v0.6.2 and v0.7.2, which Cosmos Labs had published roughly eight hours earlier.
Cosmos Labs described publication of an exact exploitation path by a downstream developer as “highly unusual” and said such disclosures can raise the risk that a vulnerability will be exploited.
MANTRA placed the public security finding 11 hours and 45 minutes before the attacker’s first probe. However, the attacker’s wallet had been funded almost four hours before the finding was filed.
“We state the timing as fact and draw no conclusion from it,” MANTRA said.
A withdrawal of 472.70 MANTRA from a customer account at a centralized exchange funded the gas fees used throughout the attack, according to the network.
MANTRA lost $3.6 million before halting its chain
MANTRA suffered the largest publicly disclosed loss from the attacks, with 720.9 million MANTRA tokens then valued at about $3.6 million taken from two addresses.
One was the network’s burn address. The second was a dormant multisignature wallet left from an earlier incentive campaign.
No automated warning was generated when tokens first moved from the burn address because MANTRA’s monitoring systems treated the address as immovable and did not watch it for outgoing transactions.
The attack remained undetected for almost four hours, giving the attacker time to drain the dormant multisig wallet.
MANTRA halted the network at 7:13 p.m. ET on Aug. 20. About 38 million stolen MANTRA remained frozen in the attacker’s wallet, but 94.7% of the stolen tokens had already been transferred to one centralized exchange deposit address through 15 transactions.
The chain remained unable to process transactions for roughly 30 hours. Crypto.news reported during the interruption that MANTRA halted transactions while engineering and security teams investigated the incident and exchanges suspended deposits and withdrawals.
Validators later deployed patched software and resumed block production without rolling back the chain or altering user balances. Version 8.4.0 included the Cosmos EVM security fix.
MANTRA had added native EVM support to its mainnet in September 2025 alongside CosmWasm compatibility, allowing Solidity applications and Cosmos-native smart contracts to operate on the network.
No stolen MANTRA tokens had been recovered as of Aug. 28, according to the project.
Its circulating supply increased by about 720.9 million tokens because assets held in accounts previously classified as unspendable, including the burn address, became tradable after being moved by the attacker.
TAC and KiiChain were hit after MANTRA
The same method was used against TAC on Aug. 22, according to Cosmos Labs. Nearly 3 billion TAC were taken from the network’s staking pool.
TAC is designed to bring decentralized finance applications to TON and Telegram users. Around 1.2 billion of the stolen tokens were sold on BNB Chain for roughly $950,000.
KiiChain was attacked that evening, losing approximately 148 million KII. About 64.6 million tokens were sold for roughly $1.6 million.
Cosmos Labs estimated that around 54% of the stolen KII remains recoverable onchain if the network is restored.
In its Aug. 23 technical post-mortem, KiiChain criticized how the vulnerability had been communicated to downstream networks. The project said Cosmos Labs did not provide advance notice, identify the release as security critical or initially tell affected chains to halt.
“A patch takes days to review, build, test and roll out across a validator set. A halt takes minutes,” KiiChain wrote. “The only measure that would have contained the risk immediately was a clear instruction to stop producing blocks, and that instruction came after the damage was done.”
Cosmos Labs recommended that vulnerable networks halt on Aug. 22, after MANTRA, TAC and KiiChain had already been hit.
KiiChain disputed part of the technical assessment as well, saying three upstream defects were needed to carry out the exploit and that only the underflow had been publicly patched.
MANTRA reached a different conclusion after testing the fix against a working reproduction of the exploit. Its post-mortem described the underflow repair as “the control that closes this attack path.”
Cosmos Labs described two chained vulnerabilities but did not address KiiChain’s claim that another upstream defect remains unresolved.
Three other Cosmos EVM networks were attacked
Three further chains were exploited with the same method, though Cosmos Labs did not identify them in its report.
Nesa may have been one of the affected networks. Bitvavo suspended NES deposits and withdrawals on Aug. 24, citing a critical consensus vulnerability that had been exploited to make vulnerable nodes accept invalid blocks.
Blockchain analytics firm Bubblemaps identified Nesa as one of the affected chains in an Aug. 26 analysis. The firm said an attacker bought about $250,000 worth of NES, bridged it to Nesa, used the flaw to increase the balance about 200-fold and transferred roughly $50 million in NES back to Ethereum.
Most attempted swaps suffered extreme slippage as liquidity was removed from trading pools, leaving the attacker with about $60,000 in profit, according to Bubblemaps.
The wallet had originally been funded through Monero. Bubblemaps said differences in the funding method and the attacker’s behavior meant a separate party may have been responsible for the Nesa exploit.
The remaining two affected chains have not been publicly identified.
Cosmos Labs said it coordinated with 40 networks during its response and worked with 13 others to patch the vulnerability or halt before they were attacked.
The firm said it does not maintain a complete registry of the more than 115 public blockchains operating across the Cosmos ecosystem. Its response uncovered 11 Cosmos EVM deployments that had not previously been registered with the team.
MANTRA, meanwhile, is being acquired by existing backer Inveniam Capital Partners, which had made a $20 million strategic investment in the project in August 2025. The transaction is expected to close in the third quarter of 2026, with MANTRA Chain, its token and related infrastructure set to continue operating under Inveniam’s ownership.
