SSL Certificate Loading

If using the ssl Python module (e.g. as part of making connections to https:// URLs), Python in its default configuration will want to obtain a list of trusted X.509 / SSL certificates for verifying connections.

If a list of trusted certificates cannot be found, you may encounter errors like ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate.

How Python Looks for Certificates

By default, Python will likely call ssl.SSLContext.load_default_certs() to load the default certificates.

On Windows, Python automatically loads certificates from the Windows certificate store. This should just work with PyOxidizer.

On all platforms, Python attempts to load certificates from the default locations compiled into the OpenSSL library that is being used. With PyOxidizer, the OpenSSL (or LibreSSL) library is part of the Python distribution used to produce a binary.

The OpenSSL library hard codes default certificate search paths. For PyOxidizer’s Python distributions, the paths are:

  • (Windows) C:\Program Files\Common Files\SSL\cert.pem (file) and C:\Program Files\Common Files\SSL\certs (directory).

  • (non-Windows) /etc/ssl/cert.pem (file) and /etc/ssl/certs (directory).

In addition, OpenSSL (but not LibreSSL) will look for path overrides in the SSL_CERT_FILE and SSL_CERT_DIR environment variables.

You can verify all of this behavior by calling ssl.get_default_verify_paths():

$ python3.9
Python 3.9.5 (default, Apr 16 2021, 08:56:35)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ssl
>>> ssl.get_default_verify_paths()
DefaultVerifyPaths(cafile=None, capath='/etc/ssl/certs', openssl_cafile_env='SSL_CERT_FILE', openssl_cafile='/etc/ssl/cert.pem', openssl_capath_env='SSL_CERT_DIR', openssl_capath='/etc/ssl/certs')

On macOS, /etc/ssl should exist, as it is part of the standard macOS install. So OpenSSL / Python should find certificates automatically.

On Windows, the default certificate path won’t exist unless something that isn’t PyOxidizer materializes the aforementioned files/directories. However, since Python loads certificates from the Windows certificate store automatically, OpenSSL / Python should be able to load certificates from PyOxidizer applications without issue.

On Linux, things are more complicated. The /etc/ssl directory is common, but not ubiquitous. This directory likely exists on all Debian based distributions, like Ubuntu. If the directory does not exist, OpenSSL / Python will likely fail to find certificates and summarily fail to verify connections against them.

Using Alternative Certificate Paths

PyOxidizer doesn’t yet have a built-in mechanism for automatically registering additional certificates or certificate paths at run-time. Therefore, if OpenSSL / Python is unable to locate certificates, you will need to add custom logic to your application to have it look for additional certificates.


The certifi Python package provides access to a copy of Mozilla’s trusted certificates list. Using certifi enables you to have access to a known trusted certificates list without dependence on certificates present in the run-time environment / operating system.

Because certifi and its certificate list is distributed with your application, it is guaranteed to be present and certificate loading should just work.

To use certifi with PyOxidizer, you can install it as an additional package. From your Starlark configuration file:

def make_exe():
    dist = default_python_distribution()
    exe = dist.to_python_executable(name="myapp")

    # Check for newer versions at

    return exe

Then from your application’s Python code:

import certifi
import ssl

# Obtain a default ssl.SSLContext but with certifi's certificate data loaded.
ctx = ssl.create_default_context(cadata=certifi.contents())

# Or if you already have an ssl.SSLContext instance and want to load
# certifi's data in it:

# Various APIs that create connections also accept a `cadata` argument.
# Under the hood they pass this argument to construct the ssl.SSLContext.
# e.g. urllib.request.urlopen().
import urllib.request
urllib.request.urlopen(url, cadata=certifi.contents())

Manually Specifying Paths to Certificates

If you know the paths to certificates to use, you can specify those paths via various ssl APIs, often through the cafile and capath arguments. e.g.

import ssl

ctx = ssl.create_default_context(capath="/path/to/ssl/certs")

import urllib.request
urllib.request.urlopen(url, capath="/path/to/ssl/certs")

Using Environment Variables

OpenSSL (but not LibreSSL) will look for the SSL_CERT_FILE and SSL_CERT_DIR environment variables to automatically set the CA file and directory, respectively.

You can set these within your process to point to alternative paths. e.g.

import os

os.environ["SSL_CERT_DIR"] = "/path/to/ssl/certs"