GHSA-QHR6-MGQR-MCHM

Vulnerability from github – Published: 2025-05-16 14:10 – Updated: 2025-05-16 14:10
VLAI?
Summary
Vyper's `concat()` builtin may elide side-effects for zero-length arguments
Details

Impact

concat() may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562

in practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal b""; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. b"" if self.do_some_side_effect() else b"".

the following example demonstrates how the issue would look in user code

counter: public(uint256)

@external
def test() -> Bytes[256]:
    a: Bytes[256] = concat(b"" if self.sideeffect() else b"", b"aaaa")
    return a

def sideeffect() -> bool:
    self.counter += 1
    return True

the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.

Patches

fix is tracked in https://github.com/vyperlang/vyper/pull/4644

Workarounds

don't have side effects in expressions which construct zero-length bytestrings.

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-2025-47285"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-691"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-05-16T14:10:25Z",
    "nvd_published_at": "2025-05-15T18:15:38Z",
    "severity": "LOW"
  },
  "details": "### Impact\n`concat()` may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562\n\nin practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`.\n\nthe following example demonstrates how the issue would look in user code\n```vyper\ncounter: public(uint256)\n\n@external\ndef test() -\u003e Bytes[256]:\n    a: Bytes[256] = concat(b\"\" if self.sideeffect() else b\"\", b\"aaaa\")\n    return a\n\ndef sideeffect() -\u003e bool:\n    self.counter += 1\n    return True\n```\n\nthe severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.\n\n### Patches\n\nfix is tracked in https://github.com/vyperlang/vyper/pull/4644\n\n### Workarounds\ndon\u0027t have side effects in expressions which construct zero-length bytestrings.\n\n### References\n_Are there any links users can visit to find out more?_",
  "id": "GHSA-qhr6-mgqr-mchm",
  "modified": "2025-05-16T14:10:25Z",
  "published": "2025-05-16T14:10:25Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/security/advisories/GHSA-qhr6-mgqr-mchm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-47285"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/pull/4644"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/vyperlang/vyper"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562"
    }
  ],
  "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 `concat()` builtin may elide side-effects for zero-length arguments"
}


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