Zokyo Auditing Tutorials
  • 🔐Zokyo Auditing Tutorials
  • 📚Tutorials
    • 🏃Tutorial 1: Front-Running
      • 🚀Prerequisites
      • 📘Understanding Front-Running
      • 👓Examples
      • ⚒️Mitigation Steps
      • 🏦Resource Bank to more front running examples
      • 🤝Front-Running Conclusion
    • 🧱Tutorial 2: Unsafe Casting
      • 🚀Prerequisites
      • 📘Understanding Casting
      • 👓Examples
      • 🤝Unsafe Casting Conclusion
    • 👍Tutorial 3: Approvals and Safe Approvals
      • 🚀Prerequisites
      • 📘Understanding Approvals
      • 👓Vulnerability Examples
        • 🔁ERC20 Approval Reset Requirement
        • 😴Ignoring Return Values from ERC20 approve() Function: Potential Miscount of Successful Approvals
        • 🚫Improper use of Open Zeppelins safeApprove() for Non-zero Allowance Increments
        • 🥾Omitted Approval for Contract Interactions Within a Protocol
        • 🤦‍♂️Failing to Reset Token Approvals in Case of Failed Transactions or other actions
        • 💭Miscellaneous
        • ERC20 Approve Race Condition Vulnerability
      • ⚒️Spot the Vulnerability
      • 🤝Approvals and Safe Approvals Conclusion
    • ⛓️Tutorial 4: Block.chainid, DOMAIN_SEPARATOR and EIP-2612 permit
      • 🚀Prerequisites
      • 📘Understanding Block.chainid and DOMAIN_SEPARATOR
      • 👓Examples
      • ⚒️General Mitigation Steps
      • 🤝Tutorial 4 Conclusion
  • 💰Tutorial 5: Fee-On-Transfer Tokens
    • 🚀Prerequisites
    • 📘Understanding Fee-On-Transfer
    • 👓Examples
    • 📘Links to more fee-on-transfer vulnerability examples
    • 🤝Fee-On-Transfer Tokens: Conclusion
  • 🌴Tutorial 6: Merkle Trees
    • 🚀Prerequisites
    • 📘Understanding Merkle Trees
    • 🔎Verification within a Merkle Tree:
    • 📜Merkle Proofs Within Smart Contracts
    • 🖋️Merkle Proof Solidity Implementation
    • 🛑Vulnerabilities When Using Merkle Trees
    • 💀Example Vulnerabilities
    • 🧠Exercise
    • 🤝Merkle Trees Conclusion
  • 🌳Tutorial 7: Merkle-Patricia Trees
    • 🚀Prerequisites
    • 📘Understanding Merkle-Patricia Trees
    • 📕Understanding Merkle-Patrica Trees pt.2
    • 🔎Verification within a Merkle-Patricia Tree
    • 🛑Vulnerabilities When Using Merkle-Patricia Trees
    • 💀Example Vulnerability
    • 🤝Merkle-Patricia Trees: Conclusion
  • 🔁Tutorial 8: Reentrancy
    • 🚀Prerequisites
    • 📘Understanding Reentrancy
    • ⚒️Mitigation
    • 💀The DAO Hack: An In-depth Examination
    • 👓Examples
    • 🏦Resource Bank To More Reentrancy Examples
    • 🤝Conclusion: Reflecting on the Reentrancy Vulnerability
  • 🔂Tutorial 9: Read-Only Reentrancy
    • 🚀Prerequisites
    • 📘Understanding Read-Only Reentrancy
    • 🔨Mitigating Read-Only Reentrancy
    • 👓Real World Examples
    • 🏦Resource Bank To More Reentrancy Examples
    • 🤝Read-Only Reentrancy: Conclusion
  • 🚆Tutorial 10: ERC20 transfer() and safeTransfer()
    • 🚀Prerequisites
    • 📘Understanding ERC20 transfer() and safeTransfer()
    • 👓Examples
    • 🤝Conclusion
  • 📞Tutorial 11: Low level .call(), .transfer() and .send()
    • 🚀Prerequisites
    • 📘Understanding .call, .transfer, and .send
    • 🛑Understanding the Vulnerabilities of .transfer and .send
    • 👓Examples
    • 🤝Low level .call(), .transfer() and .send() conclusion
  • ☎️Tutorial 12: Delegatecall Vulnerabilities in Precompiled Contracts
    • 🚀Prerequisites
    • 📳Understanding Delegatecall
    • ⛰️EVM, L2s, Bridges, and the Quest for Scalability
    • 🏗️Understanding Precompiles in the Ethereum Virtual Machine (EVM)
    • 💻Custom Precompiles
    • 💀Potential Vulnerabilities in EVM Implementations: Overlooked DelegateCall in Precompiled Contracts
    • 👓Real World Examples
    • 🤝Delegatecall and Precompiles: Conclusion
  • 🌊Tutorial 13: Liquid Staking
    • 🚀Prerequisites
    • 📘Understanding Liquid Staking
    • 💀Understanding Liquid Staking Vulnerabilities
    • 🛑Example Vulnerability
    • 🐜Example Vulnerability 2
    • 🕷️Example Vulnerability 3
    • 🤝Liquid Staking: Conclusion
  • 🚿Tutorial 14: Slippage
    • 🚀Prerequisites
    • 📘Understanding Slippage in Automated Market Makers (AMMs)
    • 💀Understanding the "Lack of Slippage Check" Vulnerability in Automated Market Makers (AMMs) and DEXs
    • 😡On-Chain Slippage Calculations Vulnerability
    • 📛0 slippage tolerance vulnerability
    • 👓Real World Examples
    • 🏦Resource bank to more slippage vulnerabilities
    • 🤝Slippage Conclusion
  • 📉Tutorial 15: Oracles
    • 🚀Prerequisites
    • 📘Understanding Oracles
    • 📈Types of price feeds
    • 😡Flash Loans
    • 💀Understanding Oracle Vulnerabilities
      • ⛓️The Danger of Single Oracle Dependence
      • ⬇️Using Deprecated Functions
      • ❌Lack of return data validation
      • 🕐Inconsistent or Absent Price Data Fetching/Updating Intervals
    • 🔫Decentralized Exchange (DEX) Price Oracles Vulnerabilities
    • 🛑Found Vulnerabilities In Oracle Implementations
      • ⚖️Newly Registered Assets Skew Consultation Results
      • ⚡Flash-Loan Oracle Manipulations
      • ⛓️Relying Only On Chainlink: PriceOracle Does Not Filter Price Feed Outliers
      • ✍️Not Validating Return Data e.g Chainlink: (lastestRoundData)
      • 🗯️Chainlink: Using latestAnswer instead of latestRoundData
      • 😭Reliance On Fetching Oracle Functionality
      • 🎱Wrong Assumption of 18 decimals
      • 🧀Stale Prices
      • 0️⃣Oracle Price Returning 0
      • 🛶TWAP Oracles
      • 😖Wrong Token Order In Return Value
      • 🏗️miscellaneous
    • 🤝Oracles: Conclusion
  • 🧠Tutorial 16: Zero Knowledge (ZK)
    • 🚀Prerequisites
    • 📚Theory
      • 🔌Circom
      • 💻Computation
      • 🛤️Arithmetic Circuits
      • 🚧Rank-1 Constraint System (R1CS)
      • ➗Quadratic Arithmetic Program
      • ✏️Linear Interactive Proof
      • ✨ZK-Snarks
    • 🤓Definitions and Essentials
      • 🔑Key
      • 😎Scalar Field Order
      • 🌳Incremental Merkle Tree
      • ✒️ECDSA signature
      • 📨Non-Interactive Proofs
      • 🏝️Fiat-Shamir transformation (or Fiat-Shamir heuristic)
      • 🪶Pedersen commitment
    • 💀Common Vulnerabilities in ZK Code
      • ⛓️Under-constrained Circuits
      • ❗Nondeterministic Circuits
      • 🌊Arithmetic Over/Under Flows
      • 🍂Mismatching Bit Lengths
      • 🌪️Unused Public Inputs Optimized Out
      • 🥶Frozen Heart: Forging of Zero Knowledge Proofs
      • 🚰Trusted Setup Leak
      • ⛔Assigned but not Constrained
    • 🐛Bugs In The Wild
      • 🌳Dark Forest v0.3: Missing Bit Length Check
      • 🔢BigInt: Missing Bit Length Check
      • 🚓Circom-Pairing: Missing Output Check Constraint
      • 🏹Semaphore: Missing Smart Contract Range Check
      • 🔫Zk-Kit: Missing Smart Contract Range Check
      • 🤖Aztec 2.0: Missing Bit Length Check / Nondeterministic Nullifier
      • ⏸️Aztec Plonk Verifier: 0 Bug
      • 🪂0xPARC StealthDrop: Nondeterministic Nullifier
      • 😨a16z ZkDrops: Missing Nullifier Range Check
      • 🤫MACI 1.0: Under-constrained Circuit
      • ❄️Bulletproofs Paper: Frozen Heart
      • 🏔️PlonK: Frozen Heart
      • 💤Zcash: Trusted Setup Leak
      • 🚨14. MiMC Hash: Assigned but not Constrained
      • 🚔PSE & Scroll zkEVM: Missing Overflow Constraint
      • ➡️PSE & Scroll zkEVM: Missing Constraint
      • 🤨Dusk Network: Missing Blinding Factors
      • 🌃EY Nightfall: Missing Nullifier Range Check
      • 🎆Summa: Unconstrained Constants Assignemnt
      • 📌Polygon zkEVM: Missing Remainder Constraint
    • 💿ZK Security Resources
  • 🤝Tutorial 17 DEX's (Decentralized Exchanges)
    • 🚀Prerequisites
    • 📚Understanding Decentralized Exchanges
    • 💀Common Vulnerabilities in DEX Code
      • 🔎The "Lack of Slippage Check" Vulnerability in Automated Market Makers (AMMs) a
      • 😡On-Chain Slippage Calculations Vulnerability
      • 📛Slippage tolerance vulnerability
      • 😵How Pool Implementation Mismatches Pose a Security Risk to Decentralized Exchanges (DEXs)
      • 🏊‍♂️Vulnerabilities in Initial Pool Creation - Liquidity Manipulation Attacks
      • 🛑Vulnerabilities In Oracle Implementations
        • ⚖️Newly Registered Assets Skew Consultation Results
        • ⚡Flash-Loan Oracle Manipulations
        • ⛓️Relying Only On Chainlink: PriceOracle Does Not Filter Price Feed Outliers
        • ✍️Not Validating Return Data e.g Chainlink: (lastestRoundData)
        • 🗯️Chainlink: Using latestAnswer instead of latestRoundData
        • 😭Reliance On Fetching Oracle Functionality
        • 🎱Wrong Assumption of 18 decimals
        • 🧀Stale Prices
        • 0️⃣Oracle Price Returning 0
        • 🛶TWAP Oracles
        • 😖Wrong Token Order In Return Value
        • 🏗️miscellaneous
      • 🥶Minting and Burning Liquidity Pool Tokens
      • 🎫Missing Checks
      • 🔞18 Decimal Assumption
        • 📌Understanding ERC20 Decimals
        • 💀Examples Of Vulnerabilities To Do With Assuming 18 Decimals
      • 🛣️Incorrect Swap Path
      • The Importance of Deadline Checks in Swaps
    • 🤝Conclusion
  • 🤖Tutorial 18: Proxies
    • 🚀Prerequisites
    • 📥Ethereum Storage and Memory
    • 📲Ethereum Calls and Delegate Calls
    • 💪Upgradability Patterns in Ethereum: Enhancing Smart Contracts Over Time
    • 🔝Proxy Upgrade Pattern in Ethereum
    • 🌀Exploring the Landscape of Ethereum Proxies
      • 🪞Transparent Proxies
      • ⬆️UUPS Proxies
      • 💡Beacon Proxies
      • 💎Diamond Proxies
  • 🔞Tutorial 19: 18 Decimal Assumption
    • 🚀Prerequisites
    • 📌Understanding ERC20 Decimals
    • 💀Examples Of Vulnerabilities To Do With Assuming 18 Decimals
    • 🤝Conclusion
  • ➗Tutorial 20: Arithmetic
    • 🚀Prerequisites
    • 🕳️Arithmetic pitfall 1: Division by 0
    • 🔪Arithmetic pitfall 2: Precision Loss Due To Rounding
    • 🥸Arithmetic pitfall 3: Erroneous Calculations
    • 🤝Conclusion
  • 🔁Tutorial 21: Unbounded Loops
    • 🚀Prerequisites
    • ⛽Gas Limit Vulnerability
    • 📨Transaction Failures Within Loops
    • 🤝Conclusion
  • 📔Tutorial 22: `isContract`
    • 🚀Prerequisites
    • 💀Understanding the 'isContract()` vulnerability
    • 🤝Conclusion
  • 💵Tutorial 23: Staking
    • 🚀Prerequisites
    • 💀First Depositor Inflation Attack in Staking Contracts
    • 🌪️Front-Running Rebase Attack (Stepwise Jump in Rewards)
    • ♨️Rugability of a Poorly Implemented recoverERC20 Function in Staking Contracts
    • 😠General Considerations for ERC777 Reentrancy Vulnerabilities
    • 🥏Vulnerability: _lpToken and Reward Token Confusion in Staking Contracts
    • 🌊Slippage Checks
    • 🌽The Harvest Functionality in Vaults: Issues and Best Practices
  • ⛓️Tutorial 24: Chain Re-org Vulnerability
    • 🚀Prerequisites
    • ♻️Chain Reorganization (Re-org) Vulnerability
    • 🧑‍⚖️Chain Re-org Vulnerability in Governance Voting Mechanisms
  • 🌉Tutorial 25: Cross Chain Bridges Vulnerabilities
    • 🚀Prerequisites
    • ♻️ERC777 Bridge Vulnerability: Reentrancy Attack in Token Accounting
      • 🛑Vulnerability: Withdrawals Can Be Locked Forever If Recipient Is a Contract
    • 👛The Dangers of Not Using SafeERC20 for Token Transfers
    • Uninitialized Variable Vulnerability in Upgradeable Smart Contracts
    • Unsafe External Calls and Their Vulnerabilities
    • Signature Replay Attacks in Cross-Chain Protocols
  • 🚰Tutorial 26: Integer Underflow and Overflow Vulnerabilities in Solidity (Before 0.8.0)
    • 🚀Prerequisites
    • 💀Understanding Integer Underflow and Overflow Vulnerabilities
    • 🤝Conclusion
  • 🥏Tutorial 27: OpenZeppelin Vulnerabilities
    • 🚀Prerequisites
    • 🛣️A Guide on Vulnerability Awareness and Management
      • 🤝Conclusion
  • 🖊️Tutorial 28: Signature Vulnerabilities / Replays
    • 🚀Prerequisites
    • 🔏Reusing EIP-712 Signatures in Private Sales
    • 🔁Replay Attacks on Failed Transactions
    • 📃Improper Token Validation in Permit Signature
  • 🤝Tutorial 29: Solmate Vulnerabilities
    • 🔏Lack of Code Size Check in Token Transfer Functions in Solmate
  • 🧱Tutorial 30: Inconsistent block lengths across chains
    • 🕛Incorrect Assumptions about Block Number in Multi-Chain Deployments
  • 💉Tutorial 31: NFT JSON and XSS injection
    • 📜Vulnerability: JSON Injection in tokenURI Functions
    • 📍Cross-Site Scripting (XSS) Vulnerability via SVG Construction in Smart Contracts
  • 🍃Tutorial 32: Merkle Leafs
    • 🖥️Misuse of Merkle Leaf Nodes
  • 0️Tutorial 33: Layer 0
    • 📩Lack of Force Resume in LayerZero Integrations
    • ⛽LayerZero-Specific Vulnerabilities in Airdropped Gas and Failure Handling
    • 🔓Understanding the Vulnerability of Blocking LayerZero Channels
    • 🖊️Copy of Understanding the Vulnerability of Blocking LayerZero Channels
  • ♻️Tutorial 34: Forgetting to Update the Global State in Smart Contracts
  • ‼️Tutorial 35: Wrong Function Signature
  • 🛑Tutorial 36: Handling Edge Cases of Banned Addresses in DeFi
  • Tutorial 37: initializer and onlyInitializing
  • ➗Tutorial 38: Eigen Layer
    • 📩Denial of Service in NodeDelegator Due to EigenLayer's maxPerDeposit Check
    • 📈Incorrect Share Issuance Due to Strategy Updates in EigenLayer Integrations
    • 🔁nonReentrant Vulnerability in EigenLayer Withdrawals
  • ⚫Tutorial 39: Wormhole
    • 📩Proposal Execution Failure Due to Guardian Set Change
  • 💼Tutorial 40: Uniswap V3
    • 📩Understanding and Mitigating Partial Swaps in Uniswap V3
    • 🌊Underflow Vulnerability in Uniswap V3 Position Fee Growth Calculations
    • ➗Handling Decimal Discrepancies in Uniswap V3 Price Calculations
  • 🔢Tutorial 41: Multiple Token Addresses in Proxied Tokens
    • 🔓Understanding Vulnerabilities Arising from Tokens with Multiple Entry Points
  • 🤖Tutorial 42: abiDecoder v2
    • 🥥Vulnerabilities from Manipulated Token Interactions Using ABI Decoding
  • ❓Tutorial 43: On-Chain Randomness
    • Vulnerabilities in On-Chain Randomness and How It Can Be Exploited
  • 😖Tutorial 44: Weird ERC20 Tokens
    • Weird Token List
  • 🔨Tutorial 45: Hardcoded stable coin values
  • ❤️Tutorial 46: The Risks of Chainlink Heartbeat Discrepancies in Smart Contracts
  • 👣Tutorial 47: The Risk of Forgetting a Withdrawal Mechanism in Smart Contracts
  • 💻Tutorial 48: Governance and Voting
    • Flash Loan Voting Exploit
    • Exploiting Self-Delegation
    • 💰Missing payable Keyword in Governance Execute Function
    • 👊Voting Multiple Times by Shifting Delegation
    • 🏑Missing Duplicate Veto Check
  • 📕Tutorial 49: Not Conforming To EIP standards
    • 💎Understanding EIP-2981: NFT Royalty Standard
    • 👍Improper Implementation of EIP-2612 Permit Standard
    • 🔁Vulnerabilities of Missing EIP-155 Replay Attack Protection
    • ➡️Vulnerabilities Due to Missing EIP-1967 in Proxy Contracts
    • 🔓Vulnerability of Design Preventing EIP-165 Extensibility
    • 🎟️The Dangers of Not Properly Implementing ERC-4626 in Yield Vaults
    • 🔁EIP-712 Implementation and Replay Attacks
  • ⏳Tutorial 50: Vesting
    • 🚔Vulnerability of Allowing Unauthorized Withdrawals in Vesting Contracts
    • 👊Vulnerability of Unbounded Timelock Loops in Vesting Contracts
    • ⬆️Vulnerability of Incorrect Linear Vesting Calculations
    • ⛳Missing hasStarted Modifier
    • 🔓Vulnerability in Bond Depositor's Vesting Period Reset
  • ⛽Tutorial 51: Ethereum's 63/64 Gas Rule
    • 🛢️Abusing Ethereum's 63/64 Gas Rule to Manipulate Contract Behavior
  • 📩Tutorial 52: NPM Dependency Confusion and Unclaimed Packages
    • 💎Exploiting Unclaimed NPM Packages and Scopes
  • 🎈Tutorial 53: Airdrops
    • 🛄Claiming on Behalf of Other Users
    • 🧲Repeated Airdrop Claims Vulnerability
    • 🍃Airdrop Vulnerability – Merkle Leaves and Parent Node Hash Collisions
  • 🎯Tutorial 54: Precision
    • 🎁Vulnerabilities Due to Insufficient Precision in Reward Calculations
    • Min-Shares: Fixed Minimum Share Values for Tokens with Low Decimal Precision
    • Vulnerability Due to Incorrect Rounding When the Numerator is Not a Multiple of the Denominator
    • Vulnerability from Small Deposits Being Rounded Down to Zero Shares in Smart Contracts
    • Precision Loss During Withdrawals from Vaults Can Block Token Transfers Due to Value < Amount
    • 18 Decimal Assumption Scaling: Loss of Precision in Asset Conversion Due to Incorrect Scaling
  • Tutorial 55: AssetIn == AssetOut, FromToken == ToToken
    • 🖼️Vulnerability: Missing fromToken != toToken Check
  • 🚿Tutorial 56: Vulnerabilities Related to LP Tokens Being the Same as Reward Tokens
    • 🖼️Vulnerabilities Caused by LP Tokens Being the Same as Reward Tokens
  • Tutorial 57: Unsanitized SWAP Paths and Arbitrary Contract Call Vulnerabilities
    • 📲Arbitrary Contract Calls from Unsanitized Paths
  • Tutorial 58: The Risk of Infinite Approvals and Arbitrary Contract Calls
    • 🪣Exploiting Infinite Approvals and Arbitrary Contract Calls
  • Tutorial 59: Low-Level Calls in Solidity Returning True for Non-Existent Contracts
    • Low-Level Calls Returning True for Non-Existent Contracts
  • 0️⃣Tutorial 60: The Impact of PUSH0 and the Shanghai Hardfork on Cross-Chain Deployments > 0.8.20
    • PUSH0 and Cross-Chain Compatibility Challenges
  • 🐍Tutorial 61: Vyper Vulnerable Versions
    • Vyper and the EVM
  • ⌨️Tutorial 62: Typos in Smart Contracts — The Silent Threat Leading to Interface Mismatch
    • Vyper and the EVM
  • ☁️Tutorial 63: Balance Check Using ==
    • The Vulnerability: == Balance Check
  • 💍Tutorial 64: Equal Royalties for Unequal NFTs
    • Understanding the Problem: Equal Royalties for Unequal NFTs
  • 🖼️Tutorial 65: ERC721 and NFTs
    • The Risk of Using transferFrom Instead of safeTransferFrom in ERC721 Projects
    • ❄️Why _safeMint Should Be Used Instead of _mint in ERC721 Projects
    • The Importance of Validating Token Types in Smart Contracts
    • 📬Implementing ERC721TokenReceiver to Handle ERC721 Safe Transfers
    • NFT Implementation Deviating from ERC721 Standard in Transfer Functions
    • NFT Approval Persistence after Transfer
    • 🎮Gameable NFT Launches through Pseudo-Randomness
    • 2️⃣Protecting Buyers from Losing Funds Due to Claimed NFT Rewards on Secondary Markets
    • ♻️Preventing Reentrancy When Using SafeERC721
    • 🖊️Preventing Re-use of EIP-712 Signatures in NFT Private Sales
  • 2️⃣Tutorial 66: Vulnerability Arising from NFTs Supporting Both ERC721 and ERC1155 Standards
  • 📷Tutorial 67: ERC1155 Vulnerabilities
    • ♻️Preventing Reentrancy in OpenZeppelin's SafeERC1155
    • 🛫Vulnerabilities in OpenZeppelin's ERC1155Supply Contract
    • Understanding Incorrect Token Owner Enumeration in ERC1155Enumerable
    • Avoiding Breaking ERC1155 Composability with Improper safeTransferFrom Implementation
    • 💍Ensuring Compatibility with EIP-2981 in ERC1155 Contracts
  • 🪟Informational Vulnerabilities
  • ⛽Gas Efficiency
  • 💻Automation Tools
  • 🔜Out Of Gas (Coming Soon)
  • 🔜DEX Aggregators (Coming Soon)
  • 🔜Bribes (Coming Soon)
  • 🔜Understanding Compiled Bytecode (coming soon)
  • 🔜Deployment Mistakes (coming soon)
  • 🔜Optimistic Roll-ups (coming soon)
  • 🔜Typos (coming soon)
  • 🔜Try-Catch (coming soon)
  • 🔜NFT Market-place (coming soon)
  • 🔜Upgrade-able Contracts (coming soon)
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  1. Tutorial 55: AssetIn == AssetOut, FromToken == ToToken

Vulnerability: Missing fromToken != toToken Check

Introduction

In token swap protocols, especially those handling complex operations like cross-chain swaps or margin trading, ensuring proper validation of input parameters is critical to maintaining the integrity and security of the system. One of the key checks often overlooked is confirming that the input token (fromToken) and output token (toToken) are different.

Without this validation, attackers can manipulate the swap process by specifying the same token for both fromToken and toToken, leading to potential exploits where they withdraw funds or circumvent important security mechanisms such as time-locks or cooling periods. This type of vulnerability is particularly dangerous because it can drain liquidity or enable an attacker to withdraw funds they should not have access to, ultimately compromising the financial security of the protocol.


Vulnerability: Missing fromToken != toToken Check

How the Exploit Works

When the contract responsible for token swapping (or any similar operation) does not check whether the input token and output token are the same, it allows attackers to manipulate the process. Typically, during a swap, the fromToken should be exchanged for a different toToken. If this check is missing, however, the attacker can specify the same token for both, causing the swap logic to misbehave and enabling them to withdraw funds without undergoing the proper swap process.

Attack Flow

  1. No Input Validation: The swap function does not verify that fromToken and toToken are different.

  2. Fake Pair Setup: The attacker calls the function with a crafted pair, where the same token is used for both input and output.

  3. Fake Swap Process: The swap proceeds with fake reserves, allowing the attacker to bypass proper fund transfers or security checks, such as time-locks.

  4. Circumventing Security Mechanisms: The attacker can bypass conditions like cooling-off periods or other time-based restrictions on withdrawals by exploiting the faulty logic.

  5. Fund Withdrawal: The attacker eventually ends up withdrawing the funds or circumventing security restrictions, leaving the system vulnerable.

Tutorial: Vulnerability Arising from Missing fromToken != toToken Check

Introduction

In token swap protocols, especially those handling complex operations like cross-chain swaps or margin trading, ensuring proper validation of input parameters is critical to maintaining the integrity and security of the system. One of the key checks often overlooked is confirming that the input token (fromToken) and output token (toToken) are different.

Without this validation, attackers can manipulate the swap process by specifying the same token for both fromToken and toToken, leading to potential exploits where they withdraw funds or circumvent important security mechanisms such as time-locks or cooling periods. This type of vulnerability is particularly dangerous because it can drain liquidity or enable an attacker to withdraw funds they should not have access to, ultimately compromising the financial security of the protocol.


Vulnerability: Missing fromToken != toToken Check

How the Exploit Works

When the contract responsible for token swapping (or any similar operation) does not check whether the input token and output token are the same, it allows attackers to manipulate the process. Typically, during a swap, the fromToken should be exchanged for a different toToken. If this check is missing, however, the attacker can specify the same token for both, causing the swap logic to misbehave and enabling them to withdraw funds without undergoing the proper swap process.

Attack Flow

  1. No Input Validation: The swap function does not verify that fromToken and toToken are different.

  2. Fake Pair Setup: The attacker calls the function with a crafted pair, where the same token is used for both input and output.

  3. Fake Swap Process: The swap proceeds with fake reserves, allowing the attacker to bypass proper fund transfers or security checks, such as time-locks.

  4. Circumventing Security Mechanisms: The attacker can bypass conditions like cooling-off periods or other time-based restrictions on withdrawals by exploiting the faulty logic.

  5. Fund Withdrawal: The attacker eventually ends up withdrawing the funds or circumventing security restrictions, leaving the system vulnerable.


Example: Faulty Swap Logic

Let’s consider a simplified version of the vulnerable swap logic. Below, the swapTokens function does not check whether fromToken is the same as toToken:

function swapTokens(
    address fromToken, 
    address toToken, 
    uint256 amountIn, 
    address recipient
) external {
    // Vulnerability: No check to ensure fromToken != toToken
    uint256 amountOut = calculateAmountOut(fromToken, toToken, amountIn);

    // Transfer the input tokens from the user to the contract
    IERC20(fromToken).transferFrom(msg.sender, address(this), amountIn);

    // Transfer the output tokens from the contract to the user
    IERC20(toToken).transfer(recipient, amountOut);
}

function calculateAmountOut(
    address fromToken, 
    address toToken, 
    uint256 amountIn
) internal view returns (uint256) {
    // Calculation logic (vulnerable when fromToken == toToken)
    return amountIn * getExchangeRate(fromToken, toToken);
}

In this case:

  • The contract fails to check whether the fromToken and toToken are the same.

  • An attacker could call this function with the same token for both fromToken and toToken.

  • If the swap logic allows, the attacker can manipulate the reserves, making it appear as if a valid trade occurred, when in reality, no actual token exchange took place.

This allows the attacker to bypass the intended security mechanisms, such as cooling-off periods for withdrawals, or to trick the contract into performing fake swaps.


Potential Impact

Failing to validate that the fromToken and toToken are different can have severe consequences, including:

  1. Liquidity Drain: The attacker could repeatedly use this technique to drain liquidity from the pools, causing significant financial loss for liquidity providers and the protocol.

  2. Bypassing Security Mechanisms: This exploit can be used to bypass important security features, such as time-locks or cooldown periods that are meant to protect user funds and the protocol’s stability.

  3. Economic Loss: Even if liquidity is not fully drained, this exploit can disrupt the economic balance of the protocol, leading to price slippage or other market manipulation strategies that could harm regular users.

Mitigation Strategies

To prevent this type of vulnerability, it is essential to include a check that ensures the fromToken and toToken are not the same

Conclusion

The absence of a fromToken != toToken check introduces a critical vulnerability in swap-based protocols. Attackers can use this oversight to drain liquidity, bypass security measures, or manipulate the economic functions of the protocol. Developers and auditors must always include proper input validation to ensure that protocols function as intended, safeguarding against exploits and preserving user trust.

By implementing straightforward checks, like ensuring the fromToken and toToken are different, DeFi protocols can protect themselves from these types of exploits and prevent significant financial loss.

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Last updated 8 months ago

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