Common vulnerability patterns in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Common vulnerability patterns

Start: Smart Contract Deployment

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User Interaction with Contract

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Check for Vulnerabilities

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Reentrancy

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Funds Drained

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Security Patch

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End

This flow shows how a smart contract is deployed, users interact, vulnerabilities may occur, and how they lead to problems before a patch fixes them.

Execution Sample

Blockchain / Solidity

contract Vulnerable {
  mapping(address => uint) public balances;
  function withdraw(uint amount) public {
    require(balances[msg.sender] >= amount);
    (bool success,) = msg.sender.call{value: amount}("");
    require(success);
    balances[msg.sender] -= amount;
  }
}

This contract allows users to withdraw funds but is vulnerable to reentrancy attacks because it sends funds before updating balances.

Execution Table

Step	Action	Condition/Evaluation	State Change	Result/Output
1	User calls withdraw(10)	balances[user] >= 10 is True	No change yet	Proceed to send funds
2	Contract sends 10 ether to user	Call to user succeeds	Funds sent but balances not updated	User receives 10 ether
3	User's fallback calls withdraw again (reentrancy)	balances[user] still >= 10 is True	No balance deducted yet	Funds sent again
4	Balances updated after calls	balances[user] -= 10	Balance reduced by 10 only once	User drained more funds than allowed
5	Attack ends when balance < amount	balances[user] < 10	No more withdrawals allowed	Contract drained
6	Security patch: update balance before sending	balances[user] -= amount before call	Balance updated early	Reentrancy prevented

💡 Execution stops when user's balance is insufficient or attack is prevented by patch.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
balances[user]	100	100	100	100	90	90
funds sent	0	0	10	20	20	20
call success	N/A	N/A	True	True	True	True

Key Moments - 3 Insights

Why does the contract send funds before updating the balance cause a problem?

Because in step 2 and 3 of the execution_table, the contract sends funds before reducing the balance, allowing the user to call withdraw multiple times and drain funds.

What stops the attack eventually?

In step 5, the user's balance becomes less than the withdrawal amount, so the require condition fails and no more funds are sent.

How does updating the balance before sending funds prevent reentrancy?

As shown in step 6, updating the balance early means the condition fails on reentrant calls, stopping multiple withdrawals.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what allows the user to withdraw funds again?

A The balance is updated before sending funds

B The balance is not updated before sending funds

C The call to user fails

D The require condition fails

💡 Hint

Check the 'State Change' column at step 3 showing balance not updated yet.

At which step does the contract finally reduce the user's balance?

A Step 2

B Step 3

C Step 4

D Step 6

💡 Hint

Look at the 'State Change' column where balances[user] -= 10 happens.

If the contract updated balance before sending funds, which step would change?

A Step 2 would update balance before sending

B Step 3 would allow reentrancy

C Step 5 would never be reached

D Step 4 would send more funds

💡 Hint

Refer to step 6 in execution_table describing the security patch.

Concept Snapshot

Common vulnerability patterns in blockchain contracts:
- Reentrancy: Sending funds before updating state allows repeated calls.
- Integer Overflow: Arithmetic errors cause wrong balances.
- Unchecked Calls: Ignoring call success leads to silent failures.
Always update state before external calls and check all conditions.

Full Transcript

This visual execution shows a common vulnerability pattern called reentrancy in blockchain smart contracts. The contract lets users withdraw funds but sends money before updating their balance. This allows a user to call withdraw repeatedly before the balance is reduced, draining more funds than allowed. The execution table traces each step: user calls withdraw, funds are sent, fallback triggers another withdraw, and balance updates happen too late. The variable tracker shows balances and funds sent at each step. Key moments explain why sending funds before updating balance is dangerous and how updating balance first stops the attack. The quiz tests understanding of these steps. The snapshot summarizes key vulnerability patterns and best practices to avoid them.