CVE-2021-48020 in Vyper
Summary
by MITRE • 10/22/2026
Vyper from 0.1.0b4 before 0.3.0 contains a memory corruption vulnerability caused by an incorrect stack tip pointer when internal function calls are performed inside arrays. Contracts placing internal calls in nested array elements, external call arguments, struct literals, or raw_log topics can have memory corrupted, producing incorrect contract behavior.
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Analysis
by VulDB Data Team • 10/22/2026
The Vyper programming language, a Python-like smart contract language for the Ethereum blockchain, suffered from a critical memory corruption vulnerability in versions prior to 0.3.0, specifically affecting releases starting from 0.1.0b4. This flaw stems from an incorrect handling of the stack tip pointer during internal function calls that are embedded within complex data structures such as arrays. The vulnerability is particularly insidious because it does not manifest in simple linear code paths but rather emerges when developers utilize nested array elements, external call arguments passed through these structures, struct literals containing functions, or topics for raw_log events that involve internal method invocations. When the compiler generates bytecode for these specific patterns, it fails to correctly manage the memory stack boundaries, leading to a state where the virtual machine's pointer to the top of the stack is misaligned with actual allocated memory regions.
From a technical perspective, this issue represents a classic case of improper resource management and boundary violation within the execution environment. The Vyper compiler translates high-level Pythonic syntax into EVM bytecode, relying on accurate tracking of variable locations in both memory and storage. In the affected scenarios, the internal function call logic incorrectly calculates or updates the stack pointer, causing subsequent operations to read from or write to unintended memory addresses. This misalignment results in memory corruption, where data belonging to one part of the contract is overwritten by another, or sensitive state variables are inadvertently modified. The consequence is unpredictable and often catastrophic for smart contracts that rely on strict immutability and deterministic execution states.
The operational impact of this vulnerability is severe due to the immutable nature of blockchain deployments. Once a vulnerable contract is deployed on Ethereum, it cannot be patched; users must either migrate funds to a new contract or accept the risk of exploitation. Attackers can leverage this memory corruption to manipulate internal state variables, bypass access controls, or extract unauthorized ether by altering balance mappings and ownership records. Since smart contracts often hold significant financial value, even minor deviations in logic caused by corrupted memory can lead to substantial economic losses for users and protocol stakeholders. The vulnerability affects the integrity of the contract's execution environment, making it impossible to guarantee that the code behaves as intended under all conditions.
This flaw aligns with CWE-120, Buffer Overflow without description of buffer size, or more accurately in this context, CWE-787: Out-of-bounds Write, as the incorrect stack pointer leads to writes outside the allocated memory space for local variables and function arguments. In terms of offensive security frameworks, it relates to MITRE ATT&CK techniques involving exploitation of software vulnerabilities during initial access or lateral movement within decentralized finance ecosystems, specifically targeting logic flaws in smart contract execution environments. The vulnerability highlights the critical importance of rigorous compiler verification and formal methods in blockchain development, where low-level memory management errors can have high-stakes consequences.
Mitigation strategies primarily involve upgrading to Vyper version 0.3.0 or later, which includes fixes for stack pointer handling during internal function calls within complex data structures. Developers should also adopt defensive coding practices by avoiding the placement of internal function calls directly inside array literals or struct definitions whenever possible. Instead, they can refactor code to assign such functions to intermediate variables before passing them as arguments or storing them in arrays. Additionally, employing static analysis tools and formal verification methods specific to Vyper can help identify similar logical errors before deployment. Regular audits by security experts familiar with EVM internals are essential to ensure that complex contract architectures do not introduce subtle memory management flaws that could be exploited maliciously.