{"id":"CVE-2025-47774","aliases":["GHSA-3vcg-j39x-cwfm","PYSEC-2026-2028"],"title":"Vyper's `slice()` may elide side-effects when output length is 0","summary":"Vyper's `slice()` may elide side-effects when output length is 0","severity":"low","vendor":"vyper","product":"vyper","ecosystem":"pip","affected":["vyper <= 0.4.2rc1"],"published":"2025-05-16","updated":"2026-07-07","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-3vcg-j39x-cwfm","references":[{"url":"https://github.com/vyperlang/vyper/security/advisories/GHSA-3vcg-j39x-cwfm"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2025-47774"},{"url":"https://github.com/vyperlang/vyper/pull/4645"},{"url":"https://github.com/vyperlang/vyper"},{"url":"https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319"},{"url":"https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191"}],"tags":["osv","pip"],"epss":0.00459,"epssPercentile":0.391,"ingestedAt":"2026-07-08T18:25:44.961Z","slug":"CVE-2025-47774","body":"## Overview\n\n### Impact\nthe `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:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319\n\nthe 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:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191\n\nthe 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)`.\n\nthe following example illustrates how the issue would look in user code\n```vyper\ncounter: public(uint256)\n\n@external\ndef test() -> Bytes[10]:\n    b: Bytes[10] = slice(msg.data, self.side_effect(), 0)\n    return b\n\ndef side_effect() -> uint256:\n    self.counter += 1\n    return 0\n```\n\nthe severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.\n\n### Patches\n\nthe 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.\n\n### Workarounds\n_Is there a way for users to fix or remediate the vulnerability without upgrading?_\n\n### References\n_Are there any links users can visit to find out more?_\n\n## Affected packages\n\n- `vyper <= 0.4.2rc1`\n\n## Remediation\n\nRefer to the advisory for the patched release.","depth":"sunlit","depthScore":14,"depthScoreParts":{"impact":13.8,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}