Solidity Security Patterns

Smart contracts are not programs — they are state machines executing in an adversarial environment.

This repository models real-world failure modes in Solidity systems and demonstrates how they are exploited, mitigated, and measured.

Each module follows the same discipline:

• break the system
• prove the exploit
• apply the fix
• verify the invariant

The focus is not patterns — it is decision-making under EVM constraints: correctness, security, and cost.

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Why This Matters

These vulnerabilities are not theoretical:

• reentrancy has drained protocols
• access control bugs have led to full fund loss
• MEV exploits reorder execution in production
• gas inefficiencies directly impact protocol viability

Understanding these tradeoffs is required to ship secure systems on-chain.

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System Overview

The repository contains paired implementations:

• vulnerable contracts (real attack surfaces)
• secure versions (mitigations applied)
• test suites proving both exploitability and correctness

Focus areas:

• reentrancy
• arithmetic safety
• access control
• transaction ordering (MEV)
• gas optimization

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Security Case Studies

Reentrancy

Vulnerability
State is updated after an external call.

Exploit
Attacker re-enters withdraw() via receive() before balance is cleared, draining funds.

Root Cause
Control flow is transferred before state transition is finalized.

Invariant Violation
User balance must be reduced before any external interaction.

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Reentrancy Mitigation

Fix Strategy

• Checks-Effects-Interactions (CEI)
  Eliminates the root cause.

• ReentrancyGuard
  Blocks nested execution at runtime.

Validation

• Exploit succeeds on vulnerable contract
• Exploit fails on protected implementations

Insight

Reentrancy is not a race condition — it is a control flow problem.

Security requires:

• correct state ordering
• runtime enforcement

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Integer Arithmetic

Models Compared

• Legacy (wraparound)
• Checked (Solidity 0.8+)
• Unchecked (manual responsibility)

Root Cause

The EVM uses modular arithmetic (mod 2²⁵⁶).
Safety is introduced by the compiler, not the execution layer.

Failure Mode

Unchecked arithmetic without bounds leads to silent state corruption.

Invariant

Arithmetic must remain within defined system bounds.

Insight

Overflow is not removed — it is abstracted behind compiler checks.

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Access Control

Vulnerability

Missing authorization enables:

• ownership takeover
• unrestricted withdrawals

Root Cause

Authorization boundary is not enforced.

Failure Mode

Unauthorized actors can mutate state and extract funds.

Fix

• onlyOwner modifier
• custom errors

Invariant

Only authorized entities may perform privileged actions.

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tx.origin Anti-Pattern

Vulnerability

Authorization based on tx.origin.

Exploit

Malicious contract forwards a call:

• tx.origin = victim
• msg.sender = attacker

Check passes → attacker gains access.

Fix

Use msg.sender.

Insight

Authorization must follow execution context, not transaction origin.

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MEV / Front-Running

Vulnerability

User input is visible in the mempool before execution.

Attackers can reorder transactions by bidding higher gas.

Root Cause

Execution ordering is adversarial.

Failure Mode

State transitions depend on publicly observable input.

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Commit-Reveal Mitigation

Approach

• commit (hidden intent)
• reveal (verified execution)

keccak256(abi.encodePacked(msg.sender, value, salt))

Security Properties

• hides intent
• binds commitment to sender
• prevents copying attacks

Limitations

Does not guarantee fairness — only removes reactive advantage.

Invariant

Outcome must not depend on observable input before execution.

Insight

Security sometimes requires hiding information, not validating it.

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Gas Analysis

Optimization is not about cleaner code — it is about reducing expensive EVM operations.

Results

Function	Inefficient	Optimized	Δ Gas	Δ %
Deployment	564860	462483	-102377	-18.1%
sumBalanceTimes	6963	3906	-3057	-43.9%
sumNumbers	17858	15190	-2668	-14.9%
setNumbers	158984	157450	-1534	-1.0%
setBalance	44437	44437	0	0%

Interpretation

sumBalanceTimes (-44%)
Eliminates repeated SLOAD via caching.

sumNumbers (-15%)
Reduces loop and memory overhead.

setNumbers (-1%)
Dominated by SSTORE. Writes outweigh optimizations.

Deployment (-18%)
Reduced via immutable and custom errors.

Key Insight

Gas cost follows EVM primitives:

• optimize SLOAD → high impact
• optimize SSTORE → limited impact
• optimize syntax → marginal gains

Limitations

Optimization does not solve:

• expensive storage writes
• poor state modeling
• write-heavy designs

If the model is inefficient, optimization will not fix it.

Measure first. Optimize what matters.

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Engineering Principles

• State is the primary attack surface
• Execution order defines security boundaries
• External calls transfer control
• Authorization must be explicit
• Compiler safety is not a guarantee
• Storage dominates cost
• Measure before optimizing

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Project Structure

solidity-security-patterns/
├── src/
│   ├── gas/
│   │   ├── GasInefficient.sol
│   │   └── GasOptimized.sol
│   ├── mev/
│   │   ├── VulnerableGame.sol
│   │   └── CommitRevealGame.sol
│   ├── AccessControlVulnerable.sol
│   ├── AccessControlFixed.sol
│   ├── AccessControlBrokenTxOrigin.sol
│   ├── Attacker.sol
│   ├── TxOriginAttacker.sol
│   ├── OverflowLegacy.sol
│   ├── OverflowUnchecked.sol
│   ├── OverflowChecked.sol
│   ├── VulnerableBank.sol
│   ├── GuardedBank.sol
│   └── SafeBank.sol
├── test/
│   ├── gas/
│   │   └── GasOptimization.t.sol
│   ├── mev/
│   │   ├── VulnerableGame.t.sol
│   │   └── CommitRevealGame.t.sol
│   ├── AccessControl.t.sol
│   ├── AccessControlTxOrigin.t.sol
│   ├── Overflow.t.sol
│   └── Reentrancy.t.sol
├── foundry.toml
└── README.md

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Running Tests

forge test

forge test --match-path test/Reentrancy.t.sol
forge test --match-path test/AccessControl.t.sol
forge test --match-path test/mev/VulnerableGame.t.sol

forge test --match-path test/gas/GasOptimization.t.sol --gas-report

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Tech Stack

• Foundry
• Solidity ^0.8.20
• WSL2 / Ubuntu