CVE-2025-47774

Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). The reason is that for these source locations, the check that length >= 1 is skipped. The result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0. The impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0). The fix in pull request 4645 disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory. The fix is expected to be part of version 0.4.2.

Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). The reason is that for these source locations, the check that length >= 1 is skipped. The result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0. The impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0). The fix in pull request 4645 disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory. The fix is expected to be part of version 0.4.2.

Impact

the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). the reason is that for these source locations, the check that length >= 1 is skipped: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319

the result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191

the impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0).

the following example illustrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[10]:
    b: Bytes[10] = slice(msg.data, self.side_effect(), 0)
    return b

def side_effect() -> uint256:
    self.counter += 1
    return 0

the severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.

Patches

the fix is tracked in https://github.com/vyperlang/vyper/pull/4645, which disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory.

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

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, the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). The reason is that for these source locations, the check that length >= 1 is skipped. The result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0. The impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0). The fix in pull request 4645 disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory. The fix is expected to be part of version 0.4.2.

Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). The reason is that for these source locations, the check that length >= 1 is skipped. The result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0. The impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0). The fix in pull request 4645 disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory. The fix is expected to be part of version 0.4.2.

Impact

the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). the reason is that for these source locations, the check that length >= 1 is skipped: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319

the result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191

the impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0).

the following example illustrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[10]:
    b: Bytes[10] = slice(msg.data, self.side_effect(), 0)
    return b

def side_effect() -> uint256:
    self.counter += 1
    return 0

the severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.

Patches

the fix is tracked in https://github.com/vyperlang/vyper/pull/4645, which disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory.

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

References

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

Impact

the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). the reason is that for these source locations, the check that length >= 1 is skipped: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319

the result is that a 0-length bytestring constructed with slice can be passed to make_byte_array_copier, which elides evaluation of its source argument when the max length is 0: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191

the impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.do_side_effect(), 0).

the following example illustrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[10]:
    b: Bytes[10] = slice(msg.data, self.side_effect(), 0)
    return b

def side_effect() -> uint256:
    self.counter += 1
    return 0

the severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.

Patches

the fix is tracked in https://github.com/vyperlang/vyper/pull/4645, which disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory.

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

References

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