CVE-2025-27104
📋 TL;DR
This vulnerability in Vyper smart contract language allows multiple evaluation of iterator expressions in for loops, enabling side-effect consumption where reads in iterators can interleave with writes in loop bodies. This leads to unexpected program behavior and potential logic manipulation in smart contracts. All Vyper users writing smart contracts for EVM blockchains are affected.
💻 Affected Systems
- vyper
📦 What is this software?
Vyper by Vyperlang
⚠️ Risk & Real-World Impact
Worst Case
Smart contract logic manipulation leading to unauthorized fund transfers, contract state corruption, or denial of service in critical DeFi applications.
Likely Case
Unexpected contract behavior causing incorrect calculations, failed transactions, or minor financial losses in less critical applications.
If Mitigated
Limited impact if contracts don't use vulnerable iterator patterns or are deployed on testnets only.
🎯 Exploit Status
Exploitation requires specific vulnerable code patterns in smart contracts; not all Vyper contracts are vulnerable.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 0.4.1
Vendor Advisory: https://github.com/vyperlang/vyper/security/advisories/GHSA-h33q-mhmp-8p67
Restart Required: No
Instructions:
1. Upgrade Vyper to version 0.4.1 or later using pip: pip install vyper>=0.4.1
2. Recompile all existing smart contracts with the patched version
3. Redeploy contracts if they were deployed with vulnerable versions
🔧 Temporary Workarounds
Avoid vulnerable iterator patterns
allManually review and refactor code to avoid using ifexp expressions in iterator targets of for loops with SArray/DArray iterables.
🧯 If You Can't Patch
- Audit all smart contracts for vulnerable iterator patterns and refactor them
- Limit deployment of new contracts and monitor existing ones for unexpected behavior
🔍 How to Verify
Check if Vulnerable:
Check if any smart contracts use patterns like: for s: uint256 in ([read(), read()] if True else []) with SArray/DArray iterablesCheck Version:
vyper --versionVerify Fix Applied:
Verify Vyper version is 0.4.1 or higher: vyper --version📡 Detection & Monitoring
Log Indicators:
- Unexpected contract state changes
- Failed transactions with gas estimation errors
- Contract behavior deviating from expected logic
Network Indicators:
- Unusual transaction patterns to vulnerable contracts
- Increased failed transactions on specific contracts
SIEM Query:
Not applicable for smart contract vulnerabilities