CVE-2025-47285

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.

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.

Impact

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: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562

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 following example demonstrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[256]:
    a: Bytes[256] = concat(b"" if self.sideeffect() else b"", b"aaaa")
    return a

def sideeffect() -> bool:
    self.counter += 1
    return True

the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.

Patches

fix is tracked in https://github.com/vyperlang/vyper/pull/4644

Workarounds

don't have side effects in expressions which construct zero-length bytestrings.

References

Are there any links users can visit to find out more?

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.

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.

Impact

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: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562

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 following example demonstrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[256]:
    a: Bytes[256] = concat(b"" if self.sideeffect() else b"", b"aaaa")
    return a

def sideeffect() -> bool:
    self.counter += 1
    return True

the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.

Patches

fix is tracked in https://github.com/vyperlang/vyper/pull/4644

Workarounds

don't have side effects in expressions which construct zero-length bytestrings.

References

Are there any links users can visit to find out more?

Impact

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: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562

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 following example demonstrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[256]:
    a: Bytes[256] = concat(b"" if self.sideeffect() else b"", b"aaaa")
    return a

def sideeffect() -> bool:
    self.counter += 1
    return True

the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.

Patches

fix is tracked in https://github.com/vyperlang/vyper/pull/4644

Workarounds

don't have side effects in expressions which construct zero-length bytestrings.

References

Are there any links users can visit to find out more?