Recursive Internal Function Calls

id: LS26M
title: Recursive Internal Function Calls
baseSeverity: M
category: logic
language: solidity
blockchain: [ethereum, polygon, bsc, arbitrum, optimism]
impact: Stack overflows or gas exhaustion leading to DoS or unpredictable execution
status: draft
complexity: medium
attack_vector: internal
mitigation_difficulty: medium
versions: [">=0.5.0", "<=0.8.25"]
cwe: CWE-674
swc: SWC-128

📝 Description

• Recursive internal function calls occur when a function calls itself directly or indirectly (via another function) without properly defined termination conditions.
• Unexpected Denial-of-Service (DoS) for contracts, especially during looping reward logic or tree traversals
• Solidity has a limited stack depth (1024 frames) and block gas limits, so recursive calls must be tightly bounded. When state changes or complex loops are involved, recursion becomes both dangerous and unpredictable.

🚨 Vulnerable Code

mapping(address => address[]) public referrals;

function getAllReferrals(address user) public view returns (uint256 count) {
    address[] memory refs = referrals[user];
    count += refs.length;
    for (uint256 i = 0; i < refs.length; i++) {
        count += getAllReferrals(refs[i]); // ❌ unbounded recursion
    }
}

🧪 Exploit Scenario

Step-by-step exploit process:

1. The protocol includes a recursive function like getAllReferrals() that walks referral trees or governance hierarchies.
2. An attacker invites a deep referral chain (e.g., 50+ addresses deep).
3. They then call a function that relies on recursive logic (getAllReferrals), triggering deep nesting.
4. Execution fails with a stack overflow or out-of-gas exception.

Assumptions:

• The contract includes recursive function logic for tree traversal, multi-hop reward aggregation, or ancestry checking.
• There are no hard limits or gas-aware break conditions.
• Recursion occurs on-chain rather than off-chain (e.g., in view or public functions).

✅ Fixed Code

function getAllReferrals(address user, uint256 maxDepth) public view returns (uint256 count) {
    return _getReferrals(user, 0, maxDepth);
}

function _getReferrals(address user, uint256 depth, uint256 maxDepth) internal view returns (uint256 count) {
    if (depth >= maxDepth) return 0;
    address[] memory refs = referrals[user];
    count += refs.length;
    for (uint256 i = 0; i < refs.length; i++) {
        count += _getReferrals(refs[i], depth + 1, maxDepth); // ✅ recursion bounded
    }
}

🧭 Contextual Severity

- context: "Default"
  severity: M
  reasoning: "Recursive logic introduces unnecessary gas risk if unbounded."
- context: "High-throughput staking protocol"
  severity: H
  reasoning: "Staker inputs could cause cascading DoS across reward functions."
- context: "Static-use or dev-only contract"
  severity: L
  reasoning: "Recursive path unlikely to be triggered in production usage."

🛡️ Prevention

Primary Defenses

• Avoid on-chain recursion; use iteration or off-chain indexing.
• Cap recursion depth and validate it in each call frame.
• Cache traversed paths or flatten data ahead of time.

Additional Safeguards

• Use event-based systems for tree updates and flattening.
• Integrate The Graph or off-chain workers to precompute trees.
• Add gas budgeting logic for traversal inside view or public functions.

Detection Methods

• Manual inspection for internal functions calling themselves or mutually recursive pairs.
• Static analysis for unbounded recursion paths.
• Tools: Slither (unbounded-loop), MythX, Foundry fuzzing on graph inputs

🕰️ Historical Exploits

• Name: dForce Referral Tree Overflow
• Date: 2021-06
• Loss: ~$90,000
• Post-mortem: Link to post-mortem
• Name: DAOStack DelegateChain Loop
• Date: 2020
• Loss: ~$20,000
• Post-mortem: Link to post-mortem

📚 Further Reading

• CWE-674: Uncontrolled Recursion
• SWC-128: DoS with Block Gas Limit
• The Graph – Off-chain Indexing for Solidity
• Solidity Docs – Stack Depth and Gas Considerations

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✅ Vulnerability Report

id: LS26M
title: Recursive Internal Function Calls
severity: M
score:
impact: 3 
exploitability: 3  
reachability: 4  
complexity: 2   
detectability: 4  
finalScore: 3.25

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📄 Justifications & Analysis

• Impact: Affects reward, voting, or referral systems where the function is used for traversal.
• Exploitability: Anyone can trigger it by calling the recursive path with deep nesting.
• Reachability: Present in projects using tree or DAG-based logic.
• Complexity: Constructing a deep tree takes effort, but calling the function is simple.
• Detectability: Easy to catch in audits with control-flow tools or Slither plugins.