{"id":"PYSEC-2026-2028","summary":"Vyper's `slice()` may elide side-effects when output length is 0","details":"### Impact\nthe `slice()` builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (`msg.data` or `\u003caddress\u003e.code`). the reason is that for these source locations, the check that `length \u003e= 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() -\u003e Bytes[10]:\n    b: Bytes[10] = slice(msg.data, self.side_effect(), 0)\n    return b\n\ndef side_effect() -\u003e 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?_","aliases":["CVE-2025-47774","GHSA-3vcg-j39x-cwfm"],"modified":"2026-07-07T17:47:20.257466071Z","published":"2026-07-07T16:02:52.607813Z","references":[{"type":"WEB","url":"https://github.com/vyperlang/vyper/security/advisories/GHSA-3vcg-j39x-cwfm"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-47774"},{"type":"WEB","url":"https://github.com/vyperlang/vyper/pull/4645"},{"type":"PACKAGE","url":"https://github.com/vyperlang/vyper"},{"type":"WEB","url":"https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319"},{"type":"WEB","url":"https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191"},{"type":"PACKAGE","url":"https://pypi.org/project/vyper"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-3vcg-j39x-cwfm"}],"affected":[{"package":{"name":"vyper","ecosystem":"PyPI","purl":"pkg:pypi/vyper"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"0.4.2rc1"}]}],"versions":["0.1.0b1","0.1.0b10","0.1.0b11","0.1.0b12","0.1.0b13","0.1.0b14","0.1.0b15","0.1.0b16","0.1.0b17","0.1.0b2","0.1.0b3","0.1.0b4","0.1.0b5","0.1.0b6","0.1.0b7","0.1.0b8","0.1.0b9","0.2.1","0.2.10","0.2.11","0.2.12","0.2.13","0.2.14","0.2.15","0.2.16","0.2.2","0.2.3","0.2.4","0.2.5","0.2.6","0.2.7","0.2.8","0.2.9","0.3.0","0.3.1","0.3.10","0.3.10rc1","0.3.10rc2","0.3.10rc3","0.3.10rc4","0.3.10rc5","0.3.2","0.3.3","0.3.4","0.3.5","0.3.6","0.3.7","0.3.8","0.3.9","0.4.0","0.4.0b1","0.4.0b2","0.4.0b3","0.4.0b4","0.4.0b5","0.4.0b6","0.4.0rc1","0.4.0rc2","0.4.0rc3","0.4.0rc4","0.4.0rc5","0.4.0rc6","0.4.1","0.4.1b1","0.4.1b2","0.4.1b3","0.4.1b4","0.4.1rc1","0.4.1rc2","0.4.1rc3","0.4.2rc1"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/vyper/PYSEC-2026-2028.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P"}]}