CVE-2025-27105

Executive Summary

CVE-2025-27105 is a critical vulnerability in the Vyper smart contract language, a Pythonic language for the Ethereum Virtual Machine (EVM). The flaw resides in the handling of AugAssign statements (e.g., +=, -=) when the target is a dynamic array (DynArray) and the right-hand side (rhs) modifies the same array. Vyper caches the target location to avoid double evaluation, but fails to re-evaluate bounds checks during the write phase. This can lead to out-of-bounds writes, potentially allowing an attacker to corrupt memory, overwrite adjacent storage slots, or execute arbitrary code within the EVM context. The vulnerability is patched in Vyper version 0.4.1, and all users are strongly advised to upgrade. No workarounds are available.

Supply Chain & Package Security

Vyper is a critical component in the Ethereum smart contract supply chain. Developers using Vyper to write DeFi protocols, NFTs, or other dApps are directly affected. A compromised smart contract could lead to loss of funds, manipulation of on-chain state, or complete contract takeover. The vulnerability is in the compiler itself, meaning any contract compiled with a vulnerable version is at risk. The package is distributed via PyPI (vyper), and users should verify their installed version and upgrade immediately.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the technical details, the vulnerability is severe. A reasonable CVSS vector would be:

• Attack Vector (AV): Network (the vulnerability is exploitable via crafted smart contract code)
• Attack Complexity (AC): Low (the exploitation path is straightforward)
• Privileges Required (PR): None (no special privileges needed to deploy a malicious contract)
• User Interaction (UI): None (no user action required beyond deploying the contract)
• Scope (S): Changed (the vulnerability can affect other parts of the contract state)
• Confidentiality (C): High (memory corruption can leak sensitive data)
• Integrity (I): High (attacker can overwrite contract state)
• Availability (A): High (can cause contract to crash or behave unpredictably)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H Base Score: 10.0 (Critical)

Deep Dive & Mechanics

The vulnerability is in the Vyper compiler's handling of AugAssign statements. In Vyper, AugAssign (e.g., x += y) is compiled by first caching the target location (the memory or storage slot of x) to avoid evaluating the target expression twice. This is a common optimization. However, when the target is a dynamic array (DynArray) and the right-hand side (rhs) expression modifies the same array, a race condition occurs.

Step-by-step exploitation:

1. The compiler caches the target location (e.g., the pointer to the array element).
2. The rhs expression is evaluated, which may modify the array (e.g., by appending or removing elements).
3. The cached target location is now stale because the array's underlying memory may have been reallocated or shifted.
4. The write operation uses the stale cached location, writing to an out-of-bounds memory region.

This can lead to:

• Memory corruption: Overwriting adjacent storage slots.
• Arbitrary write: If the attacker can control the offset, they can write to any storage slot.
• Contract takeover: By overwriting critical state variables like the owner address or a delegatecall target.

The vulnerability is specific to DynArray types because they are dynamically sized and can change their memory layout during evaluation.

Vulnerable vs. Patched Code

Vulnerable Code (Vyper < 0.4.1):

@external
def vulnerable_function(arr: DynArray[uint256, 10]):
    # arr is a dynamic array
    # AugAssign with rhs that modifies arr
    arr[0] += self.modify_and_return(arr)  # rhs modifies arr

@internal
def modify_and_return(arr: DynArray[uint256, 10]) -> uint256:
    arr.append(1)  # This changes the array's memory layout
    return 5

In this example, the compiler caches the location of arr[0] before evaluating self.modify_and_return(arr). The modify_and_return function appends to arr, which may cause a memory reallocation. The cached location is now invalid, and the write to arr[0] may write to an arbitrary memory location.

Patched Code (Vyper >= 0.4.1):

@external
def patched_function(arr: DynArray[uint256, 10]):
    # The compiler now re-evaluates the bounds check after the rhs
    # This ensures the write is within bounds
    arr[0] += self.modify_and_return(arr)  # rhs modifies arr, but bounds are re-checked

The patched version ensures that after evaluating the rhs, the bounds check for the target is re-evaluated. This prevents writing to an out-of-bounds location even if the array was modified.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Setup:

1. Install Vyper 0.4.0 (vulnerable): pip install vyper==0.4.0
2. Create a malicious contract:

# exploit.vy
@external
def exploit(target: address):
    # Deploy a contract that calls the vulnerable function
    # with a crafted dynamic array
    pass

3. Deploy the contract to a testnet (e.g., Sepolia) or local Ganache instance.
4. Craft a transaction that triggers the vulnerability.

Exploit Code (Conceptual):

from web3 import Web3
import vyper

# Compile the vulnerable contract
with open('vulnerable.vy', 'r') as f:
    code = f.read()
compiled = vyper.compile_code(code, ['abi', 'bytecode'])

# Deploy
w3 = Web3(Web3.HTTPProvider('http://localhost:8545'))
contract = w3.eth.contract(abi=compiled['abi'], bytecode=compiled['bytecode'])
tx_hash = contract.constructor().transact()
tx_receipt = w3.eth.wait_for_transaction_receipt(tx_hash)

# Call the vulnerable function with a crafted array
arr = [1, 2, 3]  # Initial array
# The rhs function will append to arr, causing memory corruption
tx_hash = contract.functions.vulnerable_function(arr).transact()

Expected Outcome: The transaction may succeed but corrupt the contract's storage, potentially allowing the attacker to overwrite critical variables.

Blue Team (Defense & Detection)

Detection:

1. Static Analysis: Scan Vyper source code for AugAssign operations on DynArray where the rhs is a function call that could modify the array.
2. Runtime Monitoring: Monitor for anomalous storage writes that do not correspond to expected contract logic.
3. Compiler Version Check: Ensure all deployed contracts were compiled with Vyper >= 0.4.1.

Mitigation:

• Upgrade to Vyper 0.4.1 or later.
• Recompile and redeploy all contracts.
• Use a linter or static analyzer to flag dangerous patterns.

Indicators of Compromise (IOCs)

• Contract bytecode hash: Any contract compiled with Vyper < 0.4.1 that uses AugAssign on DynArray.
• Transaction patterns: Transactions that trigger out-of-bounds writes (detectable via EVM traces).
• Storage anomalies: Unexpected changes to storage slots that are not part of normal contract logic.
• Compiler metadata: The vyper version string in the contract metadata (e.g., "vyper": "0.4.0").

Detection Engineering

YARA Rule

rule CVE_2025_27105_Vyper_AugAssign_DynArray {
    meta:
        description = "Detects Vyper bytecode with vulnerable AugAssign on DynArray"
        author = "Valters IT"
        date = "2025-03-01"
        reference = "CVE-2025-27105"
    strings:
        $opcode1 = { 60 00 54 }  // PUSH1 0x00 SLOAD (simplified)
        $opcode2 = { 60 20 51 }  // PUSH1 0x20 MLOAD (simplified)
        $pattern = { 7C }        // AUGASSIGN opcode (hypothetical)
    condition:
        $pattern and ($opcode1 or $opcode2)
}

Sigma Rule

title: CVE-2025-27105 Vyper AugAssign DynArray Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2025-27105 in Vyper smart contracts
author: Valters IT
date: 2025-03-01
logsource:
    category: blockchain
    product: ethereum
detection:
    selection:
        EventName: 'Transaction'
        To: '0x*'  # Contract address
        InputData|contains: '0x'  # Any transaction
    condition: selection
falsepositives:
    - Legitimate use of AugAssign on DynArray in patched versions
level: high

Suricata Rule

alert ip any any -> any any (
    msg:"CVE-2025-27105 Vyper AugAssign DynArray Exploitation";
    content:"|60 00 54|";  // PUSH1 0x00 SLOAD
    content:"|60 20 51|";  // PUSH1 0x20 MLOAD
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Vyper: Run pip install --upgrade vyper to get version 0.4.1 or later.
2. Recompile Contracts: Recompile all smart contracts with the patched compiler.
3. Redeploy: Deploy the new bytecode to the blockchain.
4. Audit: Review all contracts for the vulnerable pattern (AugAssign on DynArray with rhs that modifies the array).

Long-term Measures:

• Implement static analysis in CI/CD pipelines to flag dangerous patterns.
• Use formal verification tools to prove the absence of out-of-bounds writes.
• Monitor the Vyper GitHub repository for security advisories.

Workarounds: None available. The only fix is to upgrade.

Valters IT