CVE-2025-27104

Executive Summary

CVE-2025-27104 is a critical vulnerability in the Vyper smart contract language (Pythonic language for the EVM) that allows multiple evaluations of a single expression in the iterator target of a for loop. This flaw can lead to unexpected program behavior, including the interleaving of reads and writes within loop bodies, potentially enabling attackers to manipulate contract state or extract sensitive information. The vulnerability affects all versions prior to 0.4.1, and no official patch or workaround is currently available. The EPSS score is 0.0%, indicating low exploitation activity at this time, but the risk remains significant for deployed contracts.

Supply Chain & Package Security

Vyper is distributed as a Python package via PyPI. Developers using Vyper should monitor for the patched release (0.4.1) and update their development environments immediately. The vulnerability is in the compiler itself, meaning any contract compiled with an affected version is potentially vulnerable. Supply chain risks include:

• Compromised Compiler: An attacker could backdoor the Vyper compiler to inject malicious code into compiled contracts.
• Dependency Confusion: Malicious packages mimicking Vyper could be uploaded to PyPI.
• Version Pinning: Teams pinning Vyper versions without monitoring for security updates are at risk.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (Unavailable) CVSS Vector: UNKNOWN

While no official CVSS score is provided, the vulnerability is assessed as High based on:

• Attack Vector: Network (via malicious contract deployment or interaction)
• Attack Complexity: Low (requires understanding of Vyper loop semantics)
• Privileges Required: None (anyone can deploy a contract)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High (potential data leakage)
• Integrity Impact: High (state manipulation)
• Availability Impact: Low (no direct DoS, but logic errors can cause contract failure)

Threat Level: High – Exploitation could lead to loss of funds or contract compromise.

Deep Dive & Mechanics

The vulnerability lies in the Vyper compiler's handling of for loop iterators. In Vyper, for loops can iterate over range() or iterable types like static arrays (SArray) and dynamic arrays (DArray). The compiler attempts to enforce that iterator expressions are side-effect-free by checking is_constant during code generation. However, this check is insufficient.

Root Cause: For SArray iterators, the iter_list is instantiated inside the body of a repeat IR (intermediate representation) node. This means the iterator expression can be evaluated multiple times during loop execution, even though it appears only once in the source code. This allows the iterator to consume side effects produced by the loop body, such as reading a storage variable that is updated within the loop.

Example Scenario:

# Vulnerable contract
storage_var: uint256

@external
def vulnerable_loop() -> uint256:
    for s: uint256 in ([self.storage_var, self.storage_var] if True else []):
        self.storage_var = s + 1
    return self.storage_var

In this example, the iterator [self.storage_var, self.storage_var] is evaluated multiple times. The first iteration reads storage_var, then the loop body updates it. The second iteration reads the updated value, leading to unexpected behavior.

Impact:

• State Manipulation: Attackers can craft loops that read and write storage variables in unintended orders.
• Data Leakage: Sensitive data stored in contract state could be exposed through repeated reads.
• Logic Errors: Contracts relying on deterministic loop behavior may fail.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-0.4.1):

# Example of vulnerable iterator
for s: uint256 in ([read(), read()] if True else []):
    # loop body with side effects
    write(s)

Patched Code (Expected in 0.4.1): The patch will likely enforce that iterator expressions are evaluated only once, either by:

• Caching the iterator result before the loop begins.
• Moving iterator evaluation outside the repeat IR node.
• Adding stricter compile-time checks to prevent side-effect consumption.

Hypothetical Patched Example:

# Patched behavior: iterator evaluated once
let iter = [read(), read()] if True else []
for s: uint256 in iter:
    write(s)

Exploitation & Lab Setup

Red Team (Attacker)

Objective: Exploit the vulnerability to manipulate contract state or extract data.

Lab Setup:

1. Install vulnerable Vyper version (e.g., 0.3.10):

   pip install vyper==0.3.10
   

2. Create a vulnerable contract:

   # vulnerable.vy
   @external
   def exploit() -> uint256:
       secret: uint256 = 42
       for s: uint256 in ([secret, secret] if True else []):
           secret = s + 1
       return secret
   

3. Compile and deploy to a testnet (e.g., Ganache):

   vyper vulnerable.vy > vulnerable_bytecode.txt
   

4. Interact with the contract to observe unexpected behavior.

Exploit Code (Conceptual):

from web3 import Web3

w3 = Web3(Web3.HTTPProvider('http://localhost:8545'))
contract = w3.eth.contract(address='0x...', abi=abi)
result = contract.functions.exploit().call()
print(f"Unexpected result: {result}")  # Should be 43, but may be different

Blue Team (Defender)

Detection:

• Monitor for contracts using for loops with complex iterator expressions (e.g., containing ifexp or function calls).
• Analyze bytecode for patterns indicating multiple iterator evaluations.
• Use static analysis tools to flag suspicious loop constructs.

Mitigation:

• Upgrade to Vyper 0.4.1 once released.
• Avoid using side-effect-producing expressions in loop iterators.
• Implement runtime checks to detect unexpected state changes.

Indicators of Compromise (IOCs)

• Contract Bytecode: Presence of repeat IR nodes with iterator expressions that include storage reads or function calls.
• Transaction Patterns: Repeated calls to a contract that produce non-deterministic loop behavior.
• Logs: Unexpected state changes in storage variables during loop execution.
• Source Code: Vyper contracts containing for loops with iterators like [func(), func()] or [storage_var, storage_var].

Detection Engineering

YARA Rule

rule Detect_Vyper_For_Loop_Vulnerability {
    meta:
        description = "Detects Vyper contracts with vulnerable for loop iterators"
        author = "Valters IT"
        date = "2025-03-01"
        cve = "CVE-2025-27104"
    strings:
        $for_loop = "for" ascii
        $iterator_pattern = /\[.*,.*\]\s+if\s+/ ascii
        $storage_read = /self\.\w+/ ascii
    condition:
        $for_loop and $iterator_pattern and $storage_read
}

Sigma Rule

title: Vyper For Loop Iterator Vulnerability
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects Vyper contracts with vulnerable for loop iterators
author: Valters IT
date: 2025-03-01
logsource:
    category: application
    product: vyper
detection:
    selection:
        SourceCode|contains: 
            - 'for '
            - 'if True'
            - 'self.'
    condition: selection
falsepositives:
    - Legitimate use of conditional iterators
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-27104 Vyper For Loop Iterator Vulnerability"; flow:to_server; content:"for"; content:"if True"; content:"self."; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Vyper: Update to version 0.4.1 as soon as it is released.
2. Audit Contracts: Review all deployed contracts for vulnerable for loop patterns.
3. Static Analysis: Use tools like Slither or Mythril to detect suspicious iterator expressions.

Long-Term Mitigation:

• Compiler Hardening: Ensure iterator expressions are evaluated only once at compile time.
• Runtime Guards: Add checks to prevent side-effect consumption in loops.
• Developer Training: Educate Vyper developers about safe loop patterns.

Workarounds:

• No known workarounds exist. Avoid using complex iterators in for loops until patched.

Valters IT
Securing the Future of Smart Contracts