external-call-safety

External Call Safety

Detect vulnerabilities arising from unsafe interactions with external contracts and non-standard token behaviors that break protocol assumptions. Covers OWASP SC06 (Unchecked External Calls) plus the entire "weird ERC20" problem space.

When to Use

• Auditing any contract that calls external contracts (token transfers, cross-contract interactions)
• Reviewing protocols that support arbitrary/user-supplied ERC20 tokens
• Analyzing ETH payment distribution logic (airdrops, reward distribution, refunds)
• Verifying low-level call safety (call, delegatecall, staticcall)
• When a protocol claims to support "any ERC20 token"

When NOT to Use

• Reentrancy-specific analysis (use reentrancy-pattern-analysis — though there is overlap)
• Oracle/price feed analysis (use oracle-flashloan-analysis)
• Pure access control review (use semantic-guard-analysis)

Part 1: External Call Safety

Vulnerability Class 1: Unchecked Return Values

Low-level calls (call, delegatecall, staticcall) return a boolean indicating success. If unchecked, failed calls are silently ignored.

// VULNERABLE: Return value not checked
function withdraw(uint256 amount) external {
    balances[msg.sender] -= amount;
    payable(msg.sender).call{value: amount}(""); // Can fail silently!
    // User's balance decreased but ETH not sent
}

// SAFE: Check return value
function withdraw(uint256 amount) external {
    balances[msg.sender] -= amount;
    (bool success, ) = payable(msg.sender).call{value: amount}("");
    require(success, "Transfer failed");
}

Detection Algorithm:

For each low-level call expression:
  1. Is the return value captured? (bool success, bytes memory data) = ...
  2. Is the success boolean checked? require(success) or if(!success) revert
  3. If not captured or not checked → UNCHECKED RETURN VALUE

Severity:
  - ETH transfer unchecked → CRITICAL (funds lost)
  - Token operation unchecked → HIGH (state desync)
  - Non-financial call unchecked → MEDIUM

Vulnerability Class 2: Gas Stipend Limitations

// DANGEROUS: transfer() and send() forward only 2300 gas
payable(recipient).transfer(amount); // Reverts if recipient needs > 2300 gas
payable(recipient).send(amount);     // Returns false, often unchecked

// SAFE: Use call() with gas
(bool success, ) = payable(recipient).call{value: amount}("");
require(success, "Transfer failed");

Why 2300 gas is dangerous:

• Contracts with receive() or fallback() that do more than emit an event will fail
• EIP-1884 changed SLOAD gas cost, breaking some existing contracts
• Multi-sig wallets and smart contract wallets often need more gas

Vulnerability Class 3: Return Data Bomb

A malicious contract can return extremely large data to consume the caller's gas.

// Vulnerable to return data bomb
(bool success, bytes memory data) = untrustedContract.call(calldata);
// If untrustedContract returns 1MB of data, copying it costs massive gas

// SAFE: Limit return data or ignore it
(bool success, ) = untrustedContract.call(calldata); // Ignore return data
// Or use assembly to limit return data size

Vulnerability Class 4: Delegatecall to Untrusted Contract

// CRITICAL: delegatecall executes untrusted code in OUR storage context
function execute(address target, bytes calldata data) external {
    target.delegatecall(data); // Untrusted code can overwrite ANY storage
}

// delegatecall should ONLY be used with trusted, immutable targets

Part 2: Token Integration Safety ("Weird ERC20" Tokens)

Issue 1: Fee-on-Transfer Tokens

Some tokens deduct a fee during transfer() and transferFrom(). The recipient receives less than the specified amount.

// VULNERABLE: Assumes received amount equals input amount
function deposit(uint256 amount) external {
    token.transferFrom(msg.sender, address(this), amount);
    balances[msg.sender] += amount; // Credits MORE than actually received!
}

// SAFE: Check actual balance change
function deposit(uint256 amount) external {
    uint256 balanceBefore = token.balanceOf(address(this));
    token.transferFrom(msg.sender, address(this), amount);
    uint256 balanceAfter = token.balanceOf(address(this));
    uint256 actualReceived = balanceAfter - balanceBefore;
    balances[msg.sender] += actualReceived; // Credits actual amount
}

Known fee-on-transfer tokens: STA, PAXG, USDT (fee currently 0 but can be activated), RFI/SAFEMOON forks.

Issue 2: Rebasing Tokens

Rebasing tokens change all balances proportionally without transfers. Protocol's accounting desynchronizes from actual balances.

// VULNERABLE: Stores absolute balance amounts
function deposit(uint256 amount) external {
    token.transferFrom(msg.sender, address(this), amount);
    userDeposit[msg.sender] = amount; // After rebase, actual balance differs!
}

// Mitigation options:
// 1. Store shares instead of amounts
// 2. Wrap rebasing token (wstETH pattern)
// 3. Explicitly state: "rebasing tokens not supported"

Known rebasing tokens: stETH, AMPL, OHM, YAM, BASED.

Issue 3: Missing Return Values

Some tokens don't return a boolean from transfer()/transferFrom()/approve(), breaking the ERC20 standard.

// VULNERABLE: Assumes return value exists
bool success = token.transfer(recipient, amount); // Reverts if token returns nothing

// SAFE: Use SafeERC20
using SafeERC20 for IERC20;
token.safeTransfer(recipient, amount); // Handles missing return values

Known tokens with missing returns: USDT, BNB, OMG, KNC (legacy versions).

Issue 4: Tokens with Callbacks (ERC-777)

ERC-777 tokens trigger tokensToSend() on the sender and tokensReceived() on the recipient during transfers, enabling reentrancy.

ERC-777 callback hooks:
  transfer() → calls tokensReceived() on recipient
  transferFrom() → calls tokensToSend() on sender, tokensReceived() on recipient
  send() → calls tokensToSend() on sender, tokensReceived() on recipient

ANY of these can re-enter the calling contract!

Cross-reference: See reentrancy-pattern-analysis for detailed ERC-777 reentrancy detection.

Issue 5: Unsafe Approve Pattern

// VULNERABLE: Approve race condition
token.approve(spender, newAmount);
// Between the approval TX and the spending TX, the spender can:
// 1. Spend the OLD allowance
// 2. Then spend the NEW allowance
// Total spent: oldAmount + newAmount (double spending)

// SAFE: Reset to zero first, or use increaseAllowance
token.approve(spender, 0); // Reset
token.approve(spender, newAmount); // Set new

// Or use SafeERC20
token.safeIncreaseAllowance(spender, amount);

// ALSO DANGEROUS: Some tokens (USDT) revert on non-zero to non-zero approve
token.approve(spender, newAmount); // REVERTS if current allowance != 0
// MUST reset to 0 first for USDT

Issue 6: Tokens with Blacklists

Some tokens can blacklist addresses, causing transfers to/from those addresses to revert.

// VULNERABLE: Assumes transfer always succeeds for valid amounts
function distribute(address[] calldata users, uint256[] calldata amounts) external {
    for (uint i = 0; i < users.length; i++) {
        token.transfer(users[i], amounts[i]); // Reverts if ANY user is blacklisted
        // Entire batch fails!
    }
}

// SAFE: Handle per-user failures
function distribute(address[] calldata users, uint256[] calldata amounts) external {
    for (uint i = 0; i < users.length; i++) {
        try IERC20(token).transfer(users[i], amounts[i]) {
            // Success
        } catch {
            // Log failure, skip this user, don't block others
        }
    }
}

Known blacklist tokens: USDC, USDT, TUSD.

Issue 7: Tokens with Max Supply / Transfer Limits

Some tokens have maximum transfer amounts per transaction or maximum holding amounts per address.

// Protocol may assume any amount can be transferred
// But some tokens: require(amount <= maxTransferAmount)
// This can brick protocols that batch large transfers

Part 3: Payment Pattern Analysis

Push vs Pull Pattern

PUSH (Dangerous):
  Contract sends funds TO recipients
  - Can fail if recipient is a contract that reverts
  - Can be DoS'd by one malicious recipient
  - Gas costs unpredictable

PULL (Safe):
  Recipients claim funds FROM contract
  - Each claim is independent
  - One user's failure doesn't affect others
  - Gas costs predictable per claim

Detection:

For each function that sends ETH or tokens to external addresses:
  If sending to user-supplied addresses in a loop → PUSH pattern
  If sending to individual addresses via claim function → PULL pattern
  PUSH pattern with untrusted recipients → HIGH risk of DoS

Workflow

Task Progress:
- [ ] Step 1: Find all external calls (call, delegatecall, staticcall, transfer, send)
- [ ] Step 2: Verify return values are checked for all external calls
- [ ] Step 3: Identify all token interactions and classify token assumptions
- [ ] Step 4: Check for fee-on-transfer compatibility (balance before/after pattern)
- [ ] Step 5: Check for rebasing token compatibility
- [ ] Step 6: Verify SafeERC20 usage for tokens with missing return values
- [ ] Step 7: Check approve patterns for race conditions and USDT compatibility
- [ ] Step 8: Analyze payment distribution pattern (push vs pull)
- [ ] Step 9: Score findings and generate report

Output Format

## External Call Safety Report

### Finding: [Title]

**Function:** `functionName()` at `Contract.sol:L42`
**Category:** [Unchecked Return | Fee-on-Transfer | Rebasing | Missing Return | Callback | Approve Race | DoS]
**Severity:** [CRITICAL | HIGH | MEDIUM]

**Issue:**
[Description of the unsafe external call or token integration issue]

**Affected Tokens:**
[List of known tokens that trigger this issue, e.g., USDT, USDC, stETH]

**Vulnerable Code:**
[Code snippet]

**Attack Scenario:**
1. [Step-by-step exploitation]

**Recommendation:**
[Use SafeERC20, balance-before-after, pull pattern, etc.]

Quick Detection Checklist

• Are ALL low-level call return values checked (require(success))?
• Does the protocol use SafeERC20 for all token interactions?
• Does the deposit function use balance-before-after pattern for fee-on-transfer tokens?
• Does the protocol explicitly handle or reject rebasing tokens?
• Does approve() reset to 0 before setting new allowance (USDT compatibility)?
• Are batch payment operations using pull pattern (not push)?
• Is delegatecall only used with trusted, immutable targets?
• Are return data sizes from untrusted contracts limited?
• Does the protocol handle token blacklisting gracefully?

For weird ERC20 catalog, see {baseDir}/references/weird-erc20.md. For call safety patterns, see {baseDir}/references/call-safety-patterns.md.

Rationalizations to Reject

• "We only support standard ERC20 tokens" → USDT is the most used token and it's non-standard (no return value, fee capability)
• "The call will always succeed" → Smart contract wallets, blacklisted addresses, and gas changes can cause failures
• "We trust the token contract" → Token contracts can be upgraded (proxies) or have hidden features
• "transfer() is safe enough" → 2300 gas stipend breaks with gas repricing EIPs; use call()
• "We checked the token before listing" → Fee-on-transfer can be toggled on after listing (USDT has this capability)
• "Rebasing tokens are rare" → stETH is one of the largest tokens by TVL