CVE-2020-24837 in ZCFees
Summary
by MITRE • 02/10/2021
An integer underflow has been found in the latest version of ZCFees. The variables 'currPeriodIdx' and 'lastPeriodExecIdx' are both unsigned integers, and the result of the minus operation may be a negative integer which leads to an underflow. The attackers can modify the current timestamp of the transaction somehow and block the execution of the process function.
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Analysis
by VulDB Data Team • 02/27/2021
The vulnerability identified as CVE-2020-24837 represents a critical integer underflow condition within the ZCFees smart contract implementation that fundamentally compromises the system's operational integrity. This flaw exists in the handling of time-based execution logic where the unsigned integer variables currPeriodIdx and lastPeriodExecIdx are subjected to subtraction operations that can produce negative results. The underlying issue stems from the absence of proper bounds checking when performing arithmetic operations between unsigned integers, creating a scenario where legitimate transaction processing can be disrupted through manipulation of timestamp values. The vulnerability manifests when attackers can artificially alter transaction timestamps to manipulate the arithmetic result, leading to unexpected behavior in the contract's execution flow. This type of vulnerability falls under CWE-191, which specifically addresses integer underflow conditions that occur when a subtraction operation results in a value that cannot be represented in the target data type. The operational impact extends beyond simple denial of service as it creates a potential attack vector that could be exploited to prevent legitimate contract functions from executing properly, effectively creating a persistent blockage of system operations.
The technical exploitation of this vulnerability requires attackers to manipulate the blockchain timestamp within transaction contexts to force the arithmetic operation to produce an underflow condition. When unsigned integers underflow, they wrap around to extremely large positive values due to their binary representation, causing the contract logic to evaluate incorrectly and potentially leading to the process function being blocked entirely. The attack surface becomes particularly dangerous because blockchain transactions are inherently timestamp-dependent, and any manipulation of these timestamps can trigger the underflow condition. This vulnerability directly relates to ATT&CK technique T1499.004, which involves network denial of service through manipulation of system resources, and specifically targets the availability aspect of the smart contract's operational framework. The flaw demonstrates poor defensive programming practices where the system fails to validate that subtraction operations will produce valid results within the expected range of unsigned integer values, creating a direct path for malicious actors to disrupt contract functionality.
Mitigation strategies for this vulnerability must address both the immediate code-level fix and broader architectural considerations for smart contract development. The primary remediation involves implementing proper bounds checking before any arithmetic operations between unsigned integers, ensuring that the subtraction operation cannot produce negative results that would cause underflow. Developers should incorporate validation logic that verifies currPeriodIdx is greater than or equal to lastPeriodExecIdx before performing the subtraction, or alternatively use signed integer types when negative results are possible. Additionally, implementing timestamp validation mechanisms that prevent manipulation of transaction timestamps beyond reasonable temporal boundaries can significantly reduce exploitability. The solution must also consider the broader context of blockchain security practices and incorporate defensive programming patterns that align with industry standards for smart contract development. Organizations should conduct comprehensive code reviews focusing on arithmetic operations involving unsigned integers, particularly in time-based execution logic, to identify similar vulnerabilities across their smart contract portfolios. This remediation approach addresses the core issue while maintaining system functionality and ensuring that legitimate contract operations can proceed without disruption from timestamp manipulation attacks.