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CVE-2018-20840

Status: ModifiedHigh (8.6)—

An unhandled exception vulnerability exists during Google Sign-In with Google API C++ Client before 2019-04-10. It potentially causes an outage of third-party services that were not designed to recover from exceptions. On the client, ID token handling can cause an unhandled exception because of misinterpretation of an integer as a string, resulting in denial-of-service and then other users can no longer login/sign-in to the affected third-party service. Once this third-party service uses Google Sign-In with google-api-cpp-client, a malicious user can trigger this client/auth/oauth2_authorization.cc vulnerability by requesting the client to receive the ID token from a Google authentication server.

CVSS

Exploitation probability (EPSS)

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

Affected technologies (1)

Google — API C++ Client

CWEs

References

Raw JSON (NVD)

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{
  "id": "CVE-2018-20840",
  "cveTags": [],
  "metrics": {
    "cvssMetricV2": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "version": "2.0",
          "baseScore": 5,
          "accessVector": "NETWORK",
          "vectorString": "AV:N/AC:L/Au:N/C:N/I:N/A:P",
          "authentication": "NONE",
          "integrityImpact": "NONE",
          "accessComplexity": "LOW",
          "availabilityImpact": "PARTIAL",
          "confidentialityImpact": "NONE"
        },
        "acInsufInfo": false,
        "impactScore": 2.9,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 10,
        "obtainUserPrivilege": false,
        "obtainOtherPrivilege": false,
        "userInteractionRequired": false
      }
    ],
    "cvssMetricV30": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "scope": "CHANGED",
          "version": "3.0",
          "baseScore": 8.6,
          "attackVector": "NETWORK",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H",
          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "NONE"
        },
        "impactScore": 4,
        "exploitabilityScore": 3.9
      }
    ]
  },
  "affected": [
    {
      "source": "cve@mitre.org",
      "affectedData": [
        {
          "vendor": "n/a",
          "product": "n/a",
          "versions": [
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              "status": "affected",
              "version": "n/a"
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  "published": "2019-05-30T16:29:01.323",
  "references": [
    {
      "url": "https://github.com/google/google-api-cpp-client/issues/57",
      "tags": [
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        "Issue Tracking",
        "Third Party Advisory"
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      "tags": [
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      "tags": [
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      "source": "af854a3a-2127-422b-91ae-364da2661108"
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      "tags": [
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        "Patch",
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      "source": "af854a3a-2127-422b-91ae-364da2661108"
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  ],
  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-754"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "An unhandled exception vulnerability exists during Google Sign-In with Google API C++ Client before 2019-04-10. It potentially causes an outage of third-party services that were not designed to recover from exceptions. On the client, ID token handling can cause an unhandled exception because of misinterpretation of an integer as a string, resulting in denial-of-service and then other users can no longer login/sign-in to the affected third-party service. Once this third-party service uses Google Sign-In with google-api-cpp-client, a malicious user can trigger this client/auth/oauth2_authorization.cc vulnerability by requesting the client to receive the ID token from a Google authentication server."
    },
    {
      "lang": "es",
      "value": "Se presenta una vulnerabilidad de excepción no controlada durante el Registro (Sign-In)  de Google con Google API C++ Client anterior a la 10-04-2019. Es posible que genere una interrupción de los servicios de terceros que no fueron creados para recuperarse de las excepciones. En el cliente, el manejo del token ID puede producir una excepción no controlada debido a una inapropiada interpretación de un entero como una cadena, lo que conlleva una Denegación de Servicio y entonces otros usuarios ya no pueden iniciar sesión (login) o registrarse (sign-in) en el servicio de terceros impactado. Una vez que este servicio de terceros utiliza el registro (sign-in) de Google con google-api-cpp-client, un usuario malicioso puede activar la vulnerabilidad de client/auth/oauth2_authorization.cc mediante una petición al cliente que reciba el token de ID desde un servidor de autenticación de Google."
    }
  ],
  "lastModified": "2026-06-17T01:53:34.400",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
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              "criteria": "cpe:2.3:a:google:api_c\\+\\+_client:*:*:*:*:*:*:*:*",
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              "versionEndExcluding": "2019-04-10"
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  "sourceIdentifier": "cve@mitre.org"
}