Headline
GHSA-3vcg-j39x-cwfm: Vyper's `slice()` may elide side-effects when output length is 0
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 vyperlang/vyper#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?
References
- GHSA-3vcg-j39x-cwfm
- https://nvd.nist.gov/vuln/detail/CVE-2025-47774
- vyperlang/vyper#4645
- https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319
- https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191