CVE-2021-25835: 
vulnerability analysis and mitigation

Cosmos Network Ethermint <= v0.4.0 is affected by a cross-chain transaction replay vulnerability in the EVM module. Since ethermint uses the same type of chainIDEpoch for ethereum compatibility, a verified signature in ethereum remains valid in ethermint with identical message content and chainIDEpoch (Github Issue).

The vulnerability stems from both Ethermint and Ethereum using the same signature scheme, with users having identical public/private key pairs on both chains. Different chainIDs can be parsed to the same chainIDEpoch (e.g., 'ethereum-1' and 'ethermint-1'). As a result, if a message is successfully executed on Ethereum with a specific chainIDEpoch (e.g., mainnet with epoch 1), it can be replayed on Ethermint with the same chainIDEpoch when the nonce check is passed, and vice versa (Github Issue).

An attacker could replay transactions across chains when the victim has accounts on both chains with the same public key. For example, if a victim transfers funds on one chain, the attacker could replay that transaction on another chain to duplicate the transfer, potentially leading to unauthorized fund movements (Github Issue).

The vulnerability can be exploited when: 1) Two chains exist with the same chainIDEpoch (e.g., 'Ethereum-1' and 'Ethermint-1'), 2) The victim has accounts on both chains with the same public key, 3) The victim's transaction nonce on the target chain is equal to or less than the nonce of the transaction being replayed (Github Issue).