--- id: CVE-2018-20852 title: >- 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 summary: >- 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… severity: medium cvss: 4.3 cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N' cwe: - CWE-20 vendor: python product: python affected: - 'python >= 2.0, <= 2.7.16' - 'python >= 3.0.0, < 3.4.10' - 'python >= 3.5.0, < 3.5.7' - 'python >= 3.6.0, < 3.6.9' - 'python >= 3.7.0, < 3.7.3' patched: - python 3.7.3 published: '2019-07-13' updated: '2026-10-07' sourceUpdated: '2026-10-07T19:17:11.553' source: NVD sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2018-20852' references: - url: 'http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00071.html' label: cve@mitre.org - url: 'http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00074.html' label: cve@mitre.org - url: 'http://lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html' label: cve@mitre.org - url: 'https://access.redhat.com/errata/RHSA-2019:3725' label: cve@mitre.org - url: 'https://access.redhat.com/errata/RHSA-2019:3948' label: cve@mitre.org - url: 'https://bugs.python.org/issue35121' label: cve@mitre.org - url: 'https://lists.debian.org/debian-lts-announce/2019/08/msg00022.html' label: cve@mitre.org - url: 'https://lists.debian.org/debian-lts-announce/2019/08/msg00040.html' label: cve@mitre.org - url: 'https://lists.debian.org/debian-lts-announce/2020/07/msg00011.html' label: cve@mitre.org - url: 'https://lists.debian.org/debian-lts-announce/2020/08/msg00034.html' label: cve@mitre.org - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/COATURTCY7G67AYI6UDV5B2JZTBCKIDX/ label: cve@mitre.org - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/K7HNVIFMETMFWWWUNTB72KYJYXCZOS5V/ label: cve@mitre.org - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZBTGPBUABGXZ7WH7677OEM3NSP6ZEA76/ label: cve@mitre.org - url: 'https://python-security.readthedocs.io/vuln/cookie-domain-check.html' label: cve@mitre.org - url: 'https://security.gentoo.org/glsa/202003-26' label: cve@mitre.org - url: 'https://usn.ubuntu.com/4127-1/' label: cve@mitre.org - url: 'https://usn.ubuntu.com/4127-2/' label: cve@mitre.org - url: 'https://www.oracle.com/security-alerts/cpuapr2020.html' label: cve@mitre.org - url: 'http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00071.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00074.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'http://lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://access.redhat.com/errata/RHSA-2019:3725' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://access.redhat.com/errata/RHSA-2019:3948' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://bugs.python.org/issue35121' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://lists.debian.org/debian-lts-announce/2019/08/msg00022.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://lists.debian.org/debian-lts-announce/2019/08/msg00040.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://lists.debian.org/debian-lts-announce/2020/07/msg00011.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://lists.debian.org/debian-lts-announce/2020/08/msg00034.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/COATURTCY7G67AYI6UDV5B2JZTBCKIDX/ label: af854a3a-2127-422b-91ae-364da2661108 - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/K7HNVIFMETMFWWWUNTB72KYJYXCZOS5V/ label: af854a3a-2127-422b-91ae-364da2661108 - url: >- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZBTGPBUABGXZ7WH7677OEM3NSP6ZEA76/ label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://python-security.readthedocs.io/vuln/cookie-domain-check.html' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://security.gentoo.org/glsa/202003-26' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://usn.ubuntu.com/4127-1/' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://usn.ubuntu.com/4127-2/' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://www.oracle.com/security-alerts/cpuapr2020.html' label: af854a3a-2127-422b-91ae-364da2661108 tags: - nvd - cve.org epss: 0.0388 epssPercentile: 0.8993 ssvc: exploitation: none automatable: 'no' technicalImpact: partial timestamp: '2026-10-07T18:24:08.871365Z' ingestedAt: '2026-10-07T18:42:20.896Z' --- ## Overview 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. ## Affected - `python >= 2.0, <= 2.7.16` - `python >= 3.0.0, < 3.4.10` - `python >= 3.5.0, < 3.5.7` - `python >= 3.6.0, < 3.6.9` - `python >= 3.7.0, < 3.7.3` ## Remediation Upgrade past the affected range: - `python 3.7.3`