GHSA-3VCG-J39X-CWFM

Vulnerability from github – Published: 2025-05-16 14:13 – Updated: 2025-05-16 14:13
VLAI?
Summary
Vyper's `slice()` may elide side-effects when output length is 0
Details

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?

Severity ?
2.9 (Low)
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
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "vyper"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "0.4.2rc1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-47774"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-691"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-05-16T14:13:29Z",
    "nvd_published_at": "2025-05-15T18:15:38Z",
    "severity": "LOW"
  },
  "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?_",
  "id": "GHSA-3vcg-j39x-cwfm",
  "modified": "2025-05-16T14:13:29Z",
  "published": "2025-05-16T14:13:29Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Vyper\u0027s `slice()` may elide side-effects when output length is 0"
}


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