CVE-2021-48019 in Vyper
Summary
by MITRE • 10/22/2026
Vyper before 0.2.9 deploys forwarder proxies through the built-in create_forwarder_to function that report a fixed 4096-byte RETURNDATASIZE rather than the actual return data size. Calls through these proxies can yield corrupted data when a function returns more than 4096 bytes without return data sanitation, and fail strict return-size checks such as SafeERC20.safeTransfer.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified in Vyper versions prior to 0.2.9 stems from an implementation flaw within the built-in create_forwarder_to function used for deploying forwarder proxies on Ethereum-compatible blockchains. This specific component is designed to facilitate efficient contract interactions by creating proxy contracts that delegate calls to target implementations, thereby reducing gas costs and enabling modular architecture patterns common in decentralized finance applications. However, the core issue lies in how these proxies handle return data from external call executions. Instead of dynamically determining the size of the returned data buffer based on the actual output of the called function, the proxy logic hardcodes a fixed RETURNDATASIZE value of 4096 bytes. This static allocation fails to account for functions that may legitimately return larger payloads or variable-length dynamic types such as strings and byte arrays exceeding this threshold.
From a technical perspective, this flaw represents a classic case of improper handling of data size constraints, which aligns with CWE-131: Incorrect Calculation of Buffer Size. When a target function returns more than 4096 bytes of data through the compromised forwarder proxy, the EVM execution environment truncates or misinterprets the return buffer because it only reserves space for the first 4096 bytes. This results in corrupted data being passed back to the caller contract. The corruption is not merely a cosmetic issue but can lead to critical logic failures within downstream contracts that rely on accurate data retrieval. For instance, cryptographic hashes or encoded function signatures contained within large return values may be truncated, leading to invalid state transitions or failed verification steps.
The operational impact of this vulnerability is particularly severe in environments where strict input and output validation are enforced by standard library functions. A prominent example involves the SafeERC20.safeTransfer utility from OpenZeppelin, which includes rigorous checks on return data size to ensure compliance with ERC-20 standards that mandate a boolean success flag as part of the return value for certain token implementations. If a vulnerable forwarder proxy is used in conjunction with such strict checking mechanisms, calls may fail unexpectedly even when the underlying transfer operation succeeds. This discrepancy can cause legitimate transactions to revert, leading to user frustration, failed financial operations, and potential loss of funds if users are forced to retry transactions under changing network conditions or gas prices. Furthermore, attackers could potentially exploit this behavior in more complex scenarios where data corruption leads to incorrect decision-making logic within the consuming contract, although the primary immediate risk is functional failure rather than direct exploitation for theft.
This issue maps directly to MITRE ATT&CK techniques related to evasion and integrity violation, specifically those involving manipulation of system resources or execution flow through malformed inputs. While this specific instance does not allow arbitrary code execution, it undermines the reliability and predictability of smart contract interactions, which is fundamental to trustless systems. The vulnerability highlights the importance of dynamic buffer management in low-level EVM operations and serves as a cautionary example for developers relying on internal compiler utilities without fully understanding their limitations regarding variable-length data handling.
To mitigate this risk, organizations and developers must immediately upgrade Vyper compilers to version 0.2.9 or later, where the create_forwarder_to function has been patched to correctly report the actual return data size rather than a fixed constant. For projects already deployed with vulnerable proxies, migration strategies should be considered, including deploying new forwarders using updated compiler versions and transferring state or assets from old proxy addresses to new ones if feasible. Additionally, developers should audit their codebases for any dependencies on large return values through proxy patterns and implement alternative communication mechanisms such as events or off-chain indexing where high-volume data retrieval is required. Regular security audits focusing on low-level EVM interactions and adherence to established standards like CWE-131 are essential to prevent similar issues in future development cycles, ensuring that smart contracts remain robust against both accidental failures and potential adversarial exploitation of size-related edge cases.