CVE-2018-20852

Status: ModifiedMedium (5.3)—
http.cookiejar.DefaultPolicy.domain_return_ok in Lib/http/cookiejar.py in Python before 3.7.3 does not correctly validate the domain: it can be tricked into sending existing cookies to the wrong server. An attacker may abuse this flaw by using a server with a hostname that has another valid hostname as a suffix (e.g., pythonicexample.com to steal cookies for example.com). When a program uses http.cookiejar.DefaultPolicy and tries to do an HTTP connection to an attacker-controlled server, existing cookies can be leaked to the attacker. This affects 2.x through 2.7.16, 3.x before 3.4.10, 3.5.x before 3.5.7, 3.6.x before 3.6.9, and 3.7.x before 3.7.3.
CVSS

Version: 3.0
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Base score: 5.3
Exploitation probability (EPSS)

Probability of exploitation in the next 30 days: 3.88%
Percentile among all scored CVEs: 90
Score date: 10/5/2026
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)

Python — Python
CWEs

CWE-20
References

Raw JSON (NVD)

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{
  "id": "CVE-2018-20852",
  "cveTags": [],
  "metrics": {
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          "vectorString": "AV:N/AC:L/Au:N/C:P/I:N/A:N",
          "authentication": "NONE",
          "integrityImpact": "NONE",
          "accessComplexity": "LOW",
          "availabilityImpact": "NONE",
          "confidentialityImpact": "PARTIAL"
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        "acInsufInfo": false,
        "impactScore": 2.9,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 10,
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          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "LOW"
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  "published": "2019-07-13T21:15:10.377",
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    {
      "url": "https://python-security.readthedocs.io/vuln/cookie-domain-check.html",
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  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-20"
        }
      ]
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  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "http.cookiejar.DefaultPolicy.domain_return_ok in Lib/http/cookiejar.py in Python before 3.7.3 does not correctly validate the domain: it can be tricked into sending existing cookies to the wrong server. An attacker may abuse this flaw by using a server with a hostname that has another valid hostname as a suffix (e.g., pythonicexample.com to steal cookies for example.com). When a program uses http.cookiejar.DefaultPolicy and tries to do an HTTP connection to an attacker-controlled server, existing cookies can be leaked to the attacker. This affects 2.x through 2.7.16, 3.x before 3.4.10, 3.5.x before 3.5.7, 3.6.x before 3.6.9, and 3.7.x before 3.7.3."
    },
    {
      "lang": "es",
      "value": "http.cookiejar.DefaultPolicy.domain_return_ok en Lib / http / cookiejar.py en Python en versiones anteriores a la 3.7.3 no valida correctamente el dominio: se puede engañar para que envíe las cookies existentes al servidor incorrecto. Un atacante puede abusar de este fallo al usar un servidor con un nombre de host que tiene otro nombre de host válido como sufijo (por ejemplo, pythonicexample.com para robar cookies para example.com). Cuando un programa utiliza http.cookiejar.DefaultPolicy e intenta hacer una conexión HTTP a un servidor controlado por un atacante, las cookies existentes pueden ser filtradas al atacante. Esto afecta a la versión 2.x hasta la versión 2.7.16, versión 3.x en versiones anteriores a la 3.4.10, versión 3.5.x en versiones anteriores a la 3.5.7, versión 3.6.x en versiones anteriores a la 3.6.9 y versión 3.7.x en versiones anteriores a la 3.7.3."
    }
  ],
  "lastModified": "2026-06-17T01:53:35.820",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
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              "criteria": "cpe:2.3:a:python:python:*:*:*:*:*:*:*:*",
              "vulnerable": true,
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            {
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  "sourceIdentifier": "cve@mitre.org"
}