CVE-2025-27104 in Vyper
Summary
by MITRE • 02/22/2025
vyper is a Pythonic Smart Contract Language for the EVM. Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. `for s: uint256 in ([read(), read()] if True else [])`) may interleave reads with writes in the loop body. Vyper for loops allow two kinds of iterator targets, namely the `range()` builtin and an iterable type, like SArray and DArray. During codegen, iterable lists are required to not produce any side-effects (in the following code, `range_scope` forces `iter_list` to be parsed in a constant context, which is checked against `is_constant`). However, this does not prevent the iterator from consuming side effects provided by the body of the loop. For SArrays on the other hand, `iter_list` is instantiated in the body of a `repeat` ir, so it can be evaluated several times. This issue is being addressed and is expected to be available in version 0.4.1. Users are advised to upgrade as soon as the patched release is available. There are no known workarounds for this vulnerability.
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Analysis
by VulDB Data Team • 02/22/2025
The vulnerability described in CVE-2025-27104 affects the Vyper smart contract language implementation for the Ethereum Virtual Machine, specifically targeting the handling of for loop iterators and their potential side effect interactions. This issue stems from the improper evaluation of iterator expressions within for loop constructs, where the same expression may be evaluated multiple times during loop execution. The vulnerability represents a sophisticated edge case in smart contract compilation where the language's code generation process fails to properly isolate the evaluation context of loop iterators from the loop body's side effects, creating opportunities for unpredictable program behavior that could potentially be exploited by malicious actors.
The technical flaw manifests when processing for loop iterators that contain expressions capable of producing side effects, particularly in scenarios involving conditional expressions like ifexp constructs. During code generation, the system employs different approaches for handling different iterator types - range() builtins are processed in a constant context through range_scope which enforces is_constant checks, while SArrays are processed within repeat IR constructs where iter_list can be evaluated multiple times. This differential handling creates a vulnerability where iterator expressions that read storage variables updated within the loop body can observe inconsistent states due to the interleaving of reads and writes. The vulnerability specifically affects the evaluation of expressions like `for s: uint256 in ([read(), read()] if True else [])` where the conditional expression allows for multiple reads that can interact with writes occurring in the loop body. This behavior violates fundamental expectations of deterministic execution that smart contracts require, creating potential for unpredictable outcomes that could compromise contract integrity.
The operational impact of this vulnerability extends beyond simple behavioral inconsistencies to potentially serious security implications for smart contract deployments. When iterator expressions consume side effects from the loop body, they can observe storage states that are inconsistent with the expected execution flow, leading to potential data corruption or unexpected control flow. The vulnerability particularly affects SArrays where the iter_list instantiation occurs within repeat IR constructs, allowing multiple evaluations that can produce different results based on the evolving state of the contract. This creates a scenario where a single iterator expression can produce different values across multiple iterations, fundamentally undermining the deterministic nature of smart contract execution. The vulnerability is classified under CWE-691 as "Insufficient Control Flow Management" and aligns with ATT&CK technique T1059.006 for "Command and Scripting Interpreter: Python" in the context of smart contract compilation and execution environments. The potential for exploitation could allow attackers to manipulate contract state in unexpected ways, particularly in scenarios where the iterator behavior directly influences subsequent contract operations or access control decisions.
Mitigation strategies for this vulnerability require immediate action from users and developers, as no effective workarounds exist for the identified issue. The recommended approach is to upgrade to Vyper version 0.4.1 or later where the vulnerability has been addressed through enhanced code generation processes that properly isolate iterator evaluation contexts from loop body side effects. Organizations should conduct thorough testing of existing smart contracts to identify any potential reliance on the problematic iterator behavior before upgrading to ensure continued contract functionality. The fix implemented in the patched version addresses the core issue by ensuring that iterator expressions are evaluated in proper isolation contexts, preventing the interleaving of reads and writes that could produce inconsistent results. Additionally, developers should review their contract code to avoid patterns that might rely on the vulnerable behavior, particularly when using SArrays in for loop constructs. Security teams should monitor for any potential exploitation attempts that might leverage this vulnerability, as the complexity of the issue makes it potentially difficult to detect through standard static analysis approaches. The vulnerability demonstrates the importance of careful attention to edge cases in smart contract languages, where seemingly minor implementation details can have significant security implications for deployed contracts.