CVE-2025-27105: Vyperlang Vyper Buffer Overflow Flaw

CVE-2025-27105 Overview

CVE-2025-27105 is an Out-of-Bounds Write vulnerability in Vyper, a Pythonic Smart Contract Language for the Ethereum Virtual Machine (EVM). The vulnerability exists in how Vyper handles AugAssign statements when the target is an access to a DynArray and the right-hand side (rhs) modifies the array. Due to improper caching of the target location, bounds checks are not re-evaluated during the write portion of the statement, potentially allowing out-of-bounds memory writes.

Critical Impact

Smart contracts compiled with vulnerable versions of Vyper may be susceptible to memory corruption attacks through carefully crafted AugAssign operations on dynamic arrays, potentially affecting contract integrity and security.

Affected Products

• Vyperlang Vyper versions prior to 0.4.1

Discovery Timeline

• 2025-02-21 - CVE-2025-27105 published to NVD
• 2025-03-28 - Last updated in NVD database

Technical Details for CVE-2025-27105

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write) and affects the Vyper smart contract compiler's handling of augmented assignment (AugAssign) statements. In Python-like syntax, augmented assignments include operations such as +=, -=, *=, and similar compound assignment operators.

The core issue lies in Vyper's optimization strategy for AugAssign operations. To avoid double evaluation of the target expression, Vyper caches the target location before executing the statement. However, when the target is a dynamic array (DynArray) access and the right-hand side expression modifies that same array, a race condition-like scenario occurs.

The bounds check is performed during the initial caching phase, but when the rhs operation modifies the array (potentially changing its length), the cached location may no longer be valid. Since the bounds check is not re-evaluated during the actual write operation, this can result in an out-of-bounds write condition.

Root Cause

The root cause is the absence of bounds re-validation when writing to a DynArray element after the right-hand side expression has potentially modified the array's structure. The compiler's caching optimization for avoiding double evaluation creates a Time-of-Check to Time-of-Use (TOCTOU) style vulnerability where the array state changes between the initial bounds check and the actual write operation.

Attack Vector

This vulnerability can be exploited through network-accessible smart contract interactions. An attacker could deploy or interact with a smart contract compiled with a vulnerable version of Vyper, using specially crafted inputs that trigger the AugAssign operation on a DynArray where the rhs modifies the same array. The network-based attack vector requires low privileges but involves some level of attack complexity due to the specific conditions needed to trigger the vulnerability.

The vulnerability mechanism involves crafting an augmented assignment where accessing a DynArray element triggers rhs evaluation that modifies the array bounds. Since Vyper caches the target location before evaluating the rhs, the bounds check uses stale information, allowing writes outside the valid array boundaries. For detailed technical analysis, see the GitHub Security Advisory.

Detection Methods for CVE-2025-27105

Indicators of Compromise

• Smart contracts using Vyper versions below 0.4.1 with AugAssign operations on DynArray types
• Unusual memory state changes in deployed contracts that use dynamic arrays with compound assignment operators
• Unexpected contract behavior when processing transactions involving array modifications

Detection Strategies

• Audit smart contract source code for AugAssign patterns where the rhs expression modifies the same DynArray being assigned to
• Review compilation metadata to identify contracts built with vulnerable Vyper versions
• Implement static analysis tools to detect vulnerable patterns in Vyper source code before deployment

Monitoring Recommendations

• Monitor deployed smart contracts for unexpected state changes that could indicate exploitation
• Track Vyper compiler versions used in your development and deployment pipelines
• Implement pre-deployment security scanning for known vulnerable patterns in smart contract code

How to Mitigate CVE-2025-27105

Immediate Actions Required

• Upgrade Vyper to version 0.4.1 or later immediately
• Audit existing smart contracts compiled with vulnerable versions for the affected pattern
• Consider redeploying critical smart contracts compiled with the patched version
• Review and test any contracts using AugAssign operations with DynArray types

Patch Information

The vulnerability has been addressed in Vyper version 0.4.1. All users are strongly advised to upgrade to this version or later. The fix ensures that bounds checks are properly re-evaluated during write operations to DynArray elements in AugAssign statements.

For detailed patch information, refer to the Vyper Security Advisory GHSA-4w26-8p97-f4jp.

Workarounds

• There are no known workarounds for this vulnerability according to the security advisory
• The only recommended mitigation is upgrading to Vyper version 0.4.1 or later
• For contracts that cannot be immediately redeployed, consider implementing additional access controls to limit exposure

# Upgrade Vyper to the patched version
pip install vyper>=0.4.1

# Verify installed version
vyper --version