Reading Solidity for Non-Programmers

How to read a smart contract on Etherscan, identify common patterns, spot red flags, walk through real safe and exploited contracts, and use free tools to understand what you are signing.

25 min · advanced · part of Smart Contracts: Programmable Money

Why You Should Be Able to Read a Contract

Every time you click "approve" or "swap" or "stake" in a DeFi application, you are authorizing a smart contract to execute on your behalf. Most users have no idea what the contract actually does. They click because the button says "approve" and the wallet pop-up looks normal. This is dangerous. Some of the worst losses in DeFi history came from users signing contracts that did exactly what was advertised on the surface but had additional behavior buried in the code. Sometimes the contract was malicious by design (a rug pull). Sometimes the contract was buggy (the DAO hack, the Parity wallet bug). Sometimes the contract was deceptive (the Bybit hack of February 2025 used a substituted Safe upgrade contract that re-routed signing). You do not need to be a Solidity developer to defend yourself. You need to know enough to read what a contract claims to do, identify the patterns that signal "this is normal" or "this is suspicious," and use free tools to check the actual behavior before signing. This lesson is for the user who has clicked "approve" hundreds of times without understanding the underlying mechanics. We will walk through how to find a contract on Etherscan, how to read the basic structure, what red flags to look for, and what tools exist to translate complex transactions into plain English. By the end you will not be a contract auditor, but you will be able to make a meaningful security check before authorizing transfers of significant value. Module 5 lesson 1 covered Solidity syntax in general. This lesson is the practical follow-up: now that you know what Solidity looks like, how do you actually use that knowledge in a real situation?

Also in this lesson

Reading a Contract on Etherscan
Common Patterns: Constructor, Modifier, Mapping, Event
Red Flags: Backdoors, Owner Mints, Unbounded Loops, Dangerous Transfers
Walkthrough: Uniswap V2 Router (Safe Contract)
Walkthrough: The DAO 2016 (Exploited Contract)
Free Tools: Tenderly, OpenZeppelin Defender, Phalcon, Etherscan Simulator
For Deeper Reading

Key terms

Etherscan verified contract: A contract whose source code has been published and matched against the deployed bytecode by Etherscan. Indicated by a green checkmark. Unverified contracts hide their behavior and are a red flag for non-trivial interactions.
Read Contract / Write Contract tabs: Etherscan UI sections that let you call view functions (Read) or send transactions (Write) directly through the explorer. Useful for sanity-checking state without signing transactions.
Constructor: Solidity function that runs once when a contract is deployed and never again. Sets initial state. Replaced by initialize() in upgradeable contracts.
Modifier: Reusable Solidity check applied to functions, e.g., onlyOwner. Common modifiers: onlyOwner, nonReentrant, whenNotPaused. Look for them on functions that change critical state.
Mapping: Solidity hash table from key to value. Used for balances (mapping(address => uint256)), allowances (mapping(address => mapping(address => uint256))), and many other lookups.
Event: Solidity log entry emitted to the transaction record. Does not modify state. Indexed parameters can be filtered. Wallets and Etherscan use events to display transaction history.
Reentrancy: Vulnerability where an external call allows a malicious contract to re-enter the calling function before its state is updated. Exploited in the DAO hack (June 2016, $60M); still seen in modern hacks (Cream $130M Oct 2021).
Checks-Effects-Interactions pattern: Best-practice ordering inside a function: validate inputs (checks), update state (effects), then make external calls (interactions). Prevents reentrancy by design.
Owner mint function: Function callable by the contract owner that creates new tokens. Sometimes legitimate (LSTs, stablecoins) but often a rug-pull capability. Always check what address holds owner rights.
Tenderly: Smart contract simulation and observability platform at tenderly.co. Free tier includes a transaction simulator that runs unsigned transactions against current chain state to predict behavior.
Phalcon by BlockSec: Security-focused transaction explainer at explorer.phalcon.xyz. Traces execution, identifies token flows, and flags suspicious patterns. Free for individual transaction analysis.
OpenZeppelin Defender: Smart contract management platform at defender.openzeppelin.com. Provides Sentinels (event monitors), Admin (multisig UI), and Autotasks (scripted automations). Free tier available.

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Educational only — not financial advice.