CVE-2026-19553 SSLContext.wrap_bio() missing validation of server_hostname parameter
Attackers can bypass SSL hostname verification in Python applications if server_hostname is omitted, allowing man-in-the-middle attacks despite enabled checks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-19553?
CVE-2026-19553 is a high-severity vulnerability in CPython, published on 30 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Weakness: CWE-297
Which versions are affected by CVE-2026-19553?
3 open source packages ship an affected version, including libpython, python and python-min.
| Package | Affected | Fixed in |
|---|---|---|
libpython Bitnami | All versions | 3.10.22, 3.11.17, 3.12.15, 3.13.16, 3.14.8 |
python Bitnami | All versions | 3.10.22, 3.11.17, 3.12.15, 3.13.16, 3.14.8 |
python-min Bitnami | All versions | 3.10.22, 3.11.17, 3.12.15, 3.13.16, 3.14.8 |
Linux distribution packages (37)
python-unversioned-command Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3 Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-debug Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-devel Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-idle Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-libs Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-test Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3-tkinter Red Hat:hummingbird:1 | All versions | 0:3.14.7-1.3.hum1 |
python3.10 Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-debug Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-devel Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-idle Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-libs Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-test Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.10-tkinter Red Hat:hummingbird:1 | All versions | 0:3.10.21-1.5.hum1 |
python3.11 Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-debug Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-devel Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-idle Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-libs Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-test Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.11-tkinter Red Hat:hummingbird:1 | All versions | 0:3.11.16-1.5.hum1 |
python3.12 Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-debug Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-devel Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-idle Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-libs Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-test Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.12-tkinter Red Hat:hummingbird:1 | All versions | 0:3.12.14-1.4.hum1 |
python3.13 Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-debug Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-devel Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-freethreading Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-freethreading-debug Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-idle Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-libs Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
python3.13-test Red Hat:hummingbird:1 | All versions | 0:3.13.15-1.4.hum1 |
How do I fix CVE-2026-19553?
Upgrade libpython to a fixed version for your release line: 3.10.22, 3.11.17, 3.12.15 or 3.13.16.
Is CVE-2026-19553 being exploited?
There is no known exploitation of CVE-2026-19553 in the wild and no public exploit code.
Public exploit code: 1 proofs of concept on GitHub.
Sources
Also known as ALPINE-CVE-2026-19553, AZL-105687, AZL-105918, BELL-CVE-2026-19553, BIT-libpython-2026-19553, BIT-python-2026-19553 and 22 more.