Vyper, a smart contract language built for the Ethereum Virtual Machine (EVM) and designed to look like Python, prides itself on simplicity and security. But even great tools can have bugs. In February 2024, security researchers uncovered a critical vulnerability tracked as CVE-2024-26149, which affects Vyper versions .3.10 and earlier. This bug opens the door for attackers to manipulate how certain contract arrays are handled—potentially exposing contracts (and their funds) to serious risks.

Let’s break things down in simple terms, look at the code, review the exploit, and learn what you need to do.

What Is the Vulnerability?

If you use Vyper’s _abi_decode function to handle arrays, you could be in trouble. The vulnerability happens when someone supplies an excessively large starting index for decoding arrays. If this index gets too big, Vyper _overflows_ the reading position, which means it starts reading memory out of bounds—potentially interpreting garbage or sensitive internal data as if it were valid.

This is especially dangerous for smart contracts that rely on correct array handling for security.

Let’s look at a simplified Vyper contract

@external
def process_data(encoded: Bytes[1024]):
    # Decodes an array of uint256 starting at index 'idx'
    idx: uint256 = extract_idx_from(encoded)
    data: uint256[10] = _abi_decode(encoded, (uint256[10],), start=idx)
    # ... use 'data' for some logic

If idx is controlled by a user (or can be set to _any_ value), a hostile user could provide a very large value, causing start=idx to go beyond the actual encoded data. This triggers a “read overflow.” Vyper, before version .3.11, did not properly check this boundary, meaning:

Attackers can read (decode) memory that isn’t supposed to be part of the array,

- The contract might then use unintended/uninitialized data for sensitive logic,
- _If_ these read values are used for balances, access controls, or other critical checks, there’s potential for theft or privilege escalation.

Example Exploit Scenario

Suppose a contract validates withdrawals by checking balances inside an array decoded via _abi_decode. Here’s a dangerous (and now vulnerable) pattern:

balances: public(HashMap[address, uint256])

@external
def withdraw(encoded: Bytes[2048]):
    # expects the first half as an array
    idx: uint256 = extract_idx(encoded)
    amounts: uint256[5] = _abi_decode(encoded, (uint256[5],), start=idx)
    if amounts[] > :
        # Safe? Not anymore!
        self.balances[msg.sender] -= amounts[]
        send(msg.sender, amounts[])

With CVE-2024-26149, an attacker sends a specially crafted encoded payload with a massive idx. The read position overflows, and Vyper pulls in out-of-bounds data as amounts[]. If these bytes are zero or the attacker knows their value, they may bypass business checks, drain balances, or otherwise manipulate the contract.

Technical Details and Patch

Root Cause:
Vyper’s handling of the start parameter in _abi_decode didn’t check if the provided starting point, when combined with the array length, remained within the data bounds.

- Original vulnerability report and fix
- Patch commit

Fixed In:
Vyper v.3.11 and later versions now properly validate the start parameter and ensure arrays decode only from legitimate places in the input data.

Upgrade Immediately to Vyper .3.11 or above.

2. Audit Existing Contracts: If you can’t upgrade compiled code (immutable contracts), check if you use _abi_decode with user-provided or unchecked start values.
3. Never Trust User Input for Decoding Positions: Limit or strictly validate start if settable by anyone but the contract itself.

References

- Vyper Security Advisory: GHSA-4v8p-fx7q-2q3p
- Vyper Documentation—_abi_decode
- NVD Entry for CVE-2024-26149
- Patch PR #3656

Final Thoughts

CVE-2024-26149 is a strong reminder: Even languages built for security can have impactful bugs. Don’t just trust the tooling, keep up with official advisories, and review your code for dangerous patterns—especially when array boundaries, decoding, and user data mix. Upgrading Vyper is your first defense! Stay safe out there, contract wizards.

Timeline

Published on: 02/26/2024 20:19:05 UTC
Last modified on: 02/26/2024 22:10:40 UTC