{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.4.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "vyper"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.4.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-27104"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": true,
    "github_reviewed_at": "2025-02-21T22:43:36Z",
    "nvd_published_at": null,
    "severity": "LOW"
  },
  "details": "Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. `for s: uint256 in ([read(), read()] if True else [])`) may interleave reads with writes in the loop body.\n\nThe fix is tracked in https://github.com/vyperlang/vyper/pull/4488.\n\n### Vulnerability Details\n\nVyper for loops allow two kinds of iterator targets, namely the `range()` builtin and an iterable type, like SArray and DArray. \n\nDuring codegen, iterable lists are required to not produce any side-effects (in the following code, `range_scope` forces `iter_list` to be parsed in a constant context, which is checked against `is_constant`).\n\n```python\ndef _parse_For_list(self):\n    with self.context.range_scope():\n        iter_list = Expr(self.stmt.iter, self.context).ir_node\n    ...\n\ndef range_scope(self):\n    prev_value = self.in_range_expr\n    self.in_range_expr = True\n    yield\n    self.in_range_expr = prev_value\n\ndef is_constant(self):\n    return self.constancy is Constancy.Constant or self.in_range_expr\n```\n\nHowever, this does not prevent the iterator from consuming side effects provided by the body of the loop. For dynamic arrays, the compiler simply panics:\n```vyper\nx: DynArray[uint256, 3]\n\n@external\ndef test():\n    for i: uint256 in (self.usesideeffect() if True else self.usesideeffect()):\n        pass\n\n@view\ndef usesideeffect() -\u003e DynArray[uint256, 3]:\n    return self.x\n```\n\nFor SArrays on the other hand, `iter_list` is instantiated in the body of a `repeat` ir, so it can be evaluated several times.\n\nHere are three illustrating examples. In the first example, the following test case pre-evaluates the iter list and stores the result to a temporary list in memory. So the list is only evaluated once, before entry into the loop body, and the log output will be 0, 0, 0.\n```vyper\nevent I:\n    i: uint256\n\nx: uint256\n\n@deploy\ndef __init__():\n    self.x = 0\n\n@external\ndef test():\n    for i: uint256 in [self.usesideeffect(), self.usesideeffect(), self.usesideeffect()]:\n        self.x += 1\n        log I(i)\n\n@view\ndef usesideeffect() -\u003e uint256:\n    return self.x\n```\n\nHowever, in the next two examples, because the iterator target is not a list literal, it will be evaluated in the loop body. In the second example, `iter_list` is an ifexp, thus it will be evaluated lazily in the loop body. The log output will be 0, 1, 2 due to consumption of side effects.\n\n```vyper\nevent I:\n    i: uint256\n\nx: uint256\n\n@deploy\ndef __init__():\n    self.x = 0\n\n@external\ndef test():\n    for i: uint256 in ([self.usesideeffect(), self.usesideeffect(), self.usesideeffect()] if True else self.otherclause()):\n        self.x += 1\n        log I(i)\n\n@view\ndef usesideeffect() -\u003e uint256:\n    return self.x\n\n@view\ndef otherclause() -\u003e uint256[3]:\n    return [0, 0, 0]\n```\n\nIn the third example, `iter_list` is also an ifexp, thus it will only be evaluated in the loop body. The log output will be 0, 1, 2 due to consumption of side effects.\n\n```vyper\nevent I:\n    i: uint256\n\nx: uint256[3]\n\n@deploy\ndef __init__():\n    self.x = [0, 0, 0]\n\n@external\ndef test():\n    for i: uint256 in (self.usesideeffect() if True else self.otherclause()):\n        self.x[0] += 1\n        self.x[1] += 1\n        self.x[2] += 1\n        log I(i)\n\n@view\ndef usesideeffect() -\u003e uint256[3]:\n    return self.x\n\n@view\ndef otherclause() -\u003e uint256[3]:\n    return [0, 0, 0]\n```",
  "id": "GHSA-h33q-mhmp-8p67",
  "modified": "2025-02-21T22:43:36Z",
  "published": "2025-02-21T22:43:36Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/security/advisories/GHSA-h33q-mhmp-8p67"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/pull/4488"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/vyperlang/vyper"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [],
  "summary": "Vyper has a double eval in For List Iter"
}

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "vyper is a Pythonic Smart Contract Language for the EVM. Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. `for s: uint256 in ([read(), read()] if True else [])`) may interleave reads with writes in the loop body. Vyper for loops allow two kinds of iterator targets, namely the `range()` builtin and an iterable type, like SArray and DArray. During codegen, iterable lists are required to not produce any side-effects (in the following code, `range_scope` forces `iter_list` to be parsed in a constant context, which is checked against `is_constant`). However, this does not prevent the iterator from consuming side effects provided by the body of the loop. For SArrays on the other hand, `iter_list` is instantiated in the body of a `repeat` ir, so it can be evaluated several times. This issue is being addressed and is expected to be available in version 0.4.1. Users are advised to upgrade as soon as the patched release is available. There are no known workarounds for this vulnerability."
    }
  ],
  "id": "CVE-2025-27104",
  "lastModified": "2025-02-21T22:15:13.773",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackRequirements": "PRESENT",
          "attackVector": "NETWORK",
          "automatable": "NOT_DEFINED",
          "availabilityRequirements": "NOT_DEFINED",
          "baseScore": 2.3,
          "baseSeverity": "LOW",
          "confidentialityRequirements": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirements": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubsequentSystemAvailability": "NOT_DEFINED",
          "modifiedSubsequentSystemConfidentiality": "NOT_DEFINED",
          "modifiedSubsequentSystemIntegrity": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnerableSystemAvailability": "NOT_DEFINED",
          "modifiedVulnerableSystemConfidentiality": "NOT_DEFINED",
          "modifiedVulnerableSystemIntegrity": "NOT_DEFINED",
          "privilegesRequired": "LOW",
          "providerUrgency": "NOT_DEFINED",
          "recovery": "NOT_DEFINED",
          "safety": "NOT_DEFINED",
          "subsequentSystemAvailability": "NONE",
          "subsequentSystemConfidentiality": "NONE",
          "subsequentSystemIntegrity": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnerabilityResponseEffort": "NOT_DEFINED",
          "vulnerableSystemAvailability": "NONE",
          "vulnerableSystemConfidentiality": "LOW",
          "vulnerableSystemIntegrity": "LOW"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2025-02-21T22:15:13.773",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/vyperlang/vyper/pull/4488"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/vyperlang/vyper/security/advisories/GHSA-h33q-mhmp-8p67"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-662"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}