Bug 2366634 (CVE-2025-47285)

Summary: CVE-2025-47285 vyper: Vyper: Zero-Length Bytestring Side-Effect Skipping
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A vulnerability has been identified in the Vyper programming language. This flaw can lead to the unintended omission of side effects during string concatenation operations when zero-length bytestring arguments are used. This could result in unexpected or incorrect program behavior, particularly in smart contract logic where side effects are critical for maintaining state and executing intended actions.
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Bug Depends On: 2366742, 2366743    
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Description OSIDB Bzimport 2025-05-15 18:01:35 UTC 
Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b""`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b"" if self.do_some_side_effect() else b""`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.