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6 Common Vulnerabilities In Smart Contracts

Smart contracts are useful for peer-to-peer transactions in blockchains. They are also helpful for trade finance and insurance to improve claim processing, stock taking, and record keeping. 

Although these blockchain applications are useful in various industries, they may not always be safe and prone to attacks. The effects of smart contract attacks are very devastating and can cause significant losses to a project’s investors.

This article will address various vulnerabilities in smart contracts, how they occur, and their effects on them. We will also highlight how to mitigate these smart contract vulnerabilities. 

Common Smart Contract Vulnerabilities

Reentrancy Attacks

A reentrancy attack is any procedure that, in its execution, can be interrupted in the middle, reentered, and the previous and the latter functions continue to the end. Therefore, a reentrancy attack occurs when an untrusted external force, known as an attacker, repetitively calls its targets’ withdrawal function. 

The contract does not automatically update the account balance and recurs the withdraw function until it is washed clean. The attacker manipulates the withdraw function and associates it with their smart contract that recursively withdraws the amounts. Reentrancy attacks completely drain ether from your smart contract and interfere with your contracts.

Reentrancy attacks can be mitigated by updating the contract before adding another contract. This vulnerability can also be avoided by understanding the difference between the call, transfer, and send functions since attackers only maximize those. 

This smart contract vulnerability can also be mitigated by marking all untrusted functions and using a mutex. Mutex locks the contract on its state, with only the contract owner being able to edit.

Denial Of Service Attacks

DOS attack is meant to stop the host’s services to the client. Untrusted external contracts also play a significant role in DOS attacks. DOS attack in ether occurs when transactions are stopped due to system failures. The attacker may overload the target computer with many requests that the target cannot handle and can’t serve its clients.

In September 2016, two DOS attacks were conducted on the Ether networks to slow down the processes. When a contract tries to do a refund, it reverts. When this happens, criminals can become leaders by ensuring that all transactions with them fail. DoS attacks slow down the operation of a contract. They also lead to system failure and chain transaction failures. 

This vulnerability can be mitigated by avoiding making contracts with untrusted parties. DoS attacks can also be prevented using pull payments instead of push payments and software-defined networking to configure rules to block any DOS attacks.        

Gas Limit

It’s the maximum amount a smart contract owner is willing to pay to process his/ her payments. If, at any point, they exceed the gas limit, the transaction fails. An attacker can take advantage of this by sending DOS attacks. This attack will stop a chain of other operations in line.

The main effect of gas limit vulnerability is slowing down all transactions. However, the gas limit vulnerability can be mitigated by setting higher limits to make faster processing of your transactions by miners.


Frontrunning is overtaking an unconfirmed blockchain transaction. Frontrunning occurs due to the blockchain’s transparency property. Unconfirmed blockchain transactions are visible in the mempool, but only before the miner includes them in a block. 

Transactions in mempool can be easily monitored by interested parties and can be overtaken by paying higher transaction fees. For developers to mitigate against frontrunning, they need to redesign the blockchain.

Integer Errors

Smart contracts generally express numbers as integers because they do not have floating-point support. Therefore, one must step down to small units when integers represent values in smart contracts. Stepping down to small units when using integers is vital to allow for accuracy. 

Expressing integers in small units may cause the integers to overflow. When done wrongly, integer arithmetic may lead to a lack of precision. Developers can use safe math libraries to mitigate their code from this smart contract vulnerability.

Other Logic Bugs

Logic bug errors may result from simple typing errors, misunderstanding of the specification, or a programming mistake. These logic bugs negatively affect a smart contract’s functionality and security. 

This smart contract vulnerability can be mitigated if you understand the contract’s specifications and have insight into the project’s intended functionality. The issue can also be corrected when you thoroughly understand the code base of the transaction.

Final Thoughts

These vulnerabilities have caused many smart contract projects to lose money. Constant loss of funds in smart contracts has made these projects aware of the need to take security seriously. Therefore, developers must employ vital tools to mitigate these smart contract vulnerabilities. In addition, the thorough auditing of smart contracts is also critical to help rectify any weaknesses.

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There are so many other vulnerabilities that could lead to a project’s downfall. However, knowing these common ones and how to prevent them can go a long way in helping developers ensure the success of their smart contract projects.

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