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PYSEC-2026-2028

Vulnerability from pysec - Published: 2026-07-07 16:02 - Updated: 2026-07-07 17:25
VLAI
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
Impacted products
Name purl
vyper pkg:pypi/vyper
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "vyper",
        "purl": "pkg:pypi/vyper"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "0.4.2rc1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    }
  ],
  "aliases": [
    "CVE-2025-47774",
    "GHSA-3vcg-j39x-cwfm"
  ],
  "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": "PYSEC-2026-2028",
  "modified": "2026-07-07T17:25:48.713294Z",
  "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"
    }
  ],
  "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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