CVE-2018-11239 in Hexagon HXG
Summary
by MITRE
An integer overflow in the _transfer function of a smart contract implementation for Hexagon (HXG), an Ethereum ERC20 token, allows attackers to accomplish an unauthorized increase of digital assets by providing a _to argument in conjunction with a large _value argument, as exploited in the wild in May 2018, aka the "burnOverflow" issue.
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Analysis
by VulDB Data Team • 02/07/2020
The vulnerability identified as CVE-2018-11239 represents a critical integer overflow flaw within the Hexagon (HXG) ERC20 token smart contract implementation on the Ethereum blockchain. This issue stems from improper input validation within the _transfer function, creating a scenario where attackers can manipulate the token transfer mechanism to artificially inflate their digital asset balances. The vulnerability specifically manifests when an attacker supplies a large _value argument alongside a carefully crafted _to argument, enabling unauthorized asset accumulation through mathematical overflow conditions.
The technical execution of this vulnerability operates through a classic integer overflow attack vector where the smart contract fails to properly validate the magnitude of the _value parameter before performing arithmetic operations. When the _transfer function processes a sufficiently large value that exceeds the maximum representable integer value for the data type in use, the overflow causes the arithmetic operation to wrap around to a smaller value, effectively allowing attackers to bypass normal transfer limitations and accumulate additional tokens beyond their legitimate holdings. This flaw aligns with CWE-190, which specifically addresses integer overflow and underflow conditions in software implementations.
The operational impact of this vulnerability extends beyond simple asset manipulation to represent a fundamental security breach in the token's integrity mechanism. In the wild exploitation documented in May 2018, attackers leveraged this vulnerability to increase their token balances without authorization, creating an unprecedented situation where the token supply could be artificially inflated. The burnOverflow moniker reflects the specific method by which attackers could effectively burn tokens from other users while simultaneously minting new tokens for themselves, fundamentally compromising the token economics and potentially affecting the entire ecosystem's trust model.
Mitigation strategies for this vulnerability require immediate smart contract upgrades that implement proper input validation and overflow protection mechanisms. The recommended approach involves adding explicit checks to validate that the _value parameter does not exceed reasonable limits before processing transfers, implementing safe arithmetic operations using libraries such as OpenZeppelin's SafeMath, and conducting comprehensive security audits of all token contracts. Additionally, this vulnerability demonstrates the importance of adhering to established security frameworks and standards such as those outlined in the Ethereum Smart Contract Security Best Practices and aligns with ATT&CK framework techniques related to smart contract exploitation and financial gain through code manipulation. Organizations should also implement continuous monitoring and automated security testing to identify similar vulnerabilities in their deployed smart contracts and maintain robust incident response procedures to address exploitation attempts.