---
id: CVE-2026-19553
title: |-
  ssl.SSLContext.wrap_bio() didn't require the server_hostname argument
  to not be None if ssl.SSLContext.check_hostname was set
summary: |-
  ssl.SSLContext.wrap_bio() didn't require the server_hostname argument
  to not be None if ssl.SSLContext.check_hostname was set. Due to a
  missing parameter check in SSLObject, if the server_hostname argument
  isn't supplied then hostname ve…
severity: high
cvss: 7.6
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N'
cwe:
  - CWE-297
vendor: Python Software Foundation
product: CPython
affected:
  - CPython < 3.16.0
published: '2026-09-30'
updated: '2026-09-30'
sourceUpdated: '2026-09-30T19:16:40.923'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-19553'
references:
  - url: 'https://github.com/python/cpython/issues/156793'
    label: cna@python.org
  - url: 'https://github.com/python/cpython/pull/158503'
    label: cna@python.org
  - url: >-
      https://mail.python.org/archives/list/security-announce@python.org/thread/QNZRG3YOAMTHDCMVCICXGY6YEFPY2VDL/
    label: cna@python.org
  - url: 'http://www.openwall.com/lists/oss-security/2026/09/30/16'
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-09-30T17:13:20.841Z'
---

## Overview

ssl.SSLContext.wrap_bio() didn't require the server_hostname argument
to not be None if ssl.SSLContext.check_hostname was set. Due to a
missing parameter check in SSLObject, if the server_hostname argument
isn't supplied then hostname verification would be silently skipped.


This defect could lead to programs where certificate hostname verification
*appeared* to be succeeding with SSLContext.check_hostname = True and no
ValueError being raised due to misconfiguration.


If the program passes a server_hostname value that isn't an empty string
or None to any of these APIs then certificate hostname verification
proceeds as expected and the program is not affected by this vulnerability.


Mitigating this vulnerability doesn't require updating Python or applying
the patch. To mitigate, pass a valid non-None and non-empty
server_hostname value to SSLContext.wrap_bio(),
asyncio.create_connection(), or asyncio.loop.start_tls() and
certificate hostname verification will proceed as expected. Upgrading to
the latest version of Python or applying the patch only changes the
behavior from silently skipping hostname verification to raising a
ValueError, similar to SSLContext.wrap_socket(), when server_hostname
isn't supplied.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.