FKIE_CVE-2026-72711

Vulnerability from fkie_nvd - Published: 2026-08-24 20:17 - Updated: 2026-10-12 15:10
Severity
6.3 (Medium) - CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N
Summary
The Lean 4 kernel does not check that the body of an opaque declaration is closed. environment::add_opaque omits the check_no_metavar_no_fvar call that the definition and theorem paths perform, so a value containing a free variable that is absent from the local context is not rejected outright. A metaprogram can first cause the kernel to create a temporary local of type False and record its type in the type checker's inference cache, then restore the local context while that cache entry persists on the same type checker instance, and finally submit an opaque declaration whose value is the now-unbound variable. The cache lookup answers before the branch that would test membership of the local context, so the kernel infers the cached type and admits an opaque constant of type False, from which any proposition follows. The declaration is accepted through the ordinary checked path at maximum kernel checking, without sorry, unsafeCast, debug.skipKernelTC, addDeclWithoutChecking, foreign code or a modified .olean file, and the result carries no axioms. Fixed in 4.32.2 by adding the missing closure check.
References
disclosure@vulncheck.comhttps://github.com/endrazine/lean-cve-poc-14484
disclosure@vulncheck.comhttps://github.com/leanprover/lean4
disclosure@vulncheck.comhttps://github.com/leanprover/lean4/issues/14484
disclosure@vulncheck.comhttps://github.com/leanprover/lean4/pull/14498
disclosure@vulncheck.comhttps://www.vulncheck.com/advisories/lean-4-before-kernel-accepts-opaque-declaration-with-an-unbound-free-variable
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "packageURL": "pkg:github/leanprover/lean4",
          "product": "lean4",
          "repo": "https://github.com/leanprover/lean4",
          "vendor": "leanprover",
          "versions": [
            {
              "lessThan": "4.32.2",
              "status": "affected",
              "version": "0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "source": "disclosure@vulncheck.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "The Lean 4 kernel does not check that the body of an opaque declaration is closed. environment::add_opaque omits the check_no_metavar_no_fvar call that the definition and theorem paths perform, so a value containing a free variable that is absent from the local context is not rejected outright. A metaprogram can first cause the kernel to create a temporary local of type False and record its type in the type checker\u0027s inference cache, then restore the local context while that cache entry persists on the same type checker instance, and finally submit an opaque declaration whose value is the now-unbound variable. The cache lookup answers before the branch that would test membership of the local context, so the kernel infers the cached type and admits an opaque constant of type False, from which any proposition follows. The declaration is accepted through the ordinary checked path at maximum kernel checking, without sorry, unsafeCast, debug.skipKernelTC, addDeclWithoutChecking, foreign code or a modified .olean file, and the result carries no axioms. Fixed in 4.32.2 by adding the missing closure check."
    },
    {
      "lang": "es",
      "value": "El kernel de Lean 4 no verifica que el cuerpo de una declaraci\u00f3n opaca est\u00e9 cerrado. environment::add_opaque omite la llamada a check_no_metavar_no_fvar que realizan las rutas de definici\u00f3n y teorema, por lo que un valor que contiene una variable libre que est\u00e1 ausente del contexto local no es rechazado de inmediato. Un metaprograma puede primero hacer que el kernel cree una variable local temporal de tipo False y registre su tipo en la cach\u00e9 de inferencia del verificador de tipos, luego restaurar el contexto local mientras esa entrada de cach\u00e9 persiste en la misma instancia del verificador de tipos, y finalmente enviar una declaraci\u00f3n opaca cuyo valor es la variable ahora desvinculada. La b\u00fasqueda en cach\u00e9 responde antes de la rama que probar\u00eda la pertenencia al contexto local, por lo que el kernel infiere el tipo en cach\u00e9 y admite una constante opaca de tipo False, de la cual se sigue cualquier proposici\u00f3n. La declaraci\u00f3n es aceptada a trav\u00e9s de la ruta verificada ordinaria con la m\u00e1xima verificaci\u00f3n del kernel, sin sorry, unsafeCast, debug.skipKernelTC, addDeclWithoutChecking, c\u00f3digo externo o un archivo .olean modificado, y el resultado no conlleva axiomas. Corregido en 4.32.2 al a\u00f1adir la verificaci\u00f3n de cierre faltante."
    }
  ],
  "id": "CVE-2026-72711",
  "lastModified": "2026-10-12T15:10:00.207",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "NONE",
          "baseScore": 6.3,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "scope": "CHANGED",
          "userInteraction": "REQUIRED",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 1.8,
        "impactScore": 4.0,
        "source": "disclosure@vulncheck.com",
        "type": "Secondary"
      }
    ],
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "NONE",
          "attackVector": "LOCAL",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 6.8,
          "baseSeverity": "MEDIUM",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "PASSIVE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/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",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "HIGH",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "disclosure@vulncheck.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-72711",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-08-24T20:14:31.474084Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-08-24T20:17:18.657",
  "references": [
    {
      "source": "disclosure@vulncheck.com",
      "url": "https://github.com/endrazine/lean-cve-poc-14484"
    },
    {
      "source": "disclosure@vulncheck.com",
      "url": "https://github.com/leanprover/lean4"
    },
    {
      "source": "disclosure@vulncheck.com",
      "url": "https://github.com/leanprover/lean4/issues/14484"
    },
    {
      "source": "disclosure@vulncheck.com",
      "url": "https://github.com/leanprover/lean4/pull/14498"
    },
    {
      "source": "disclosure@vulncheck.com",
      "url": "https://www.vulncheck.com/advisories/lean-4-before-kernel-accepts-opaque-declaration-with-an-unbound-free-variable"
    }
  ],
  "sourceIdentifier": "disclosure@vulncheck.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-20"
        }
      ],
      "source": "disclosure@vulncheck.com",
      "type": "Secondary"
    }
  ]
}



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