requests 2.32 SSLContext regression: the wrong mTLS client certificate

In May 2024, psf/requests#6667 changed how Requests verifies TLS certificates. Instead of loading the default CA bundle for each new connection, it loads the bundle once, at import, into a single SSLContext, and hands that object to every connection made with verify=True. The goal was sound: reloading root certificates is slow, especially on Windows with OpenSSL 3. The diff was small, one file, and it shipped in Requests 2.32.0.

On 2026-10-07 we replayed the pull request through Warpway as it stood before merge, without the review discussion or anything that happened later. In that replay, Warpway's Correctness lens reports two high-severity problems. Users reported both after release, and a year later the change was reverted.

The change

_preloaded_ssl_context = create_urllib3_context()
_preloaded_ssl_context.load_verify_locations(
    extract_zipped_paths(DEFAULT_CA_BUNDLE_PATH)
)

# when building the connection pool for a request
    elif verify is True:
        pool_kwargs["ssl_context"] = _preloaded_ssl_context

How the bug works

An SSLContext is not just configuration that gets read. urllib3 also writes to it: when a request passes cert=(certificate, key) for mutual TLS, urllib3 loads that client certificate into the context it was given. Before 2.32.0, each connection pool built its own context, so that write stayed local. From 2.32.0, every verify=True connection in the process receives the same object. A certificate loaded for one request is now present for others: requests from another Session, to another host, or with no cert= at all.

With concurrent requests using different client certificates, as in #6726, the shared context is modified while other connections are using it. The reporter's reproducer fails with an exception, and they first noticed the problem as the wrong certificates being presented.

The second finding has the same root. Loading the bundle moved from the verified-request path to module import, so anything wrong with the default bundle now breaks import requests for everyone, including programs that only make plain HTTP requests or pass their own CA bundle.

Are you affected?

You are affected if you run Requests 2.32.0 to 2.32.4 and any of these apply:

• You use client certificates (cert=) with verify=True, especially several different certificates in one process or across threads.
• You verify servers signed by a private or corporate root CA that is not in the default bundle.
• Your default CA bundle is extracted to a shared location that other users cannot read.

Fix or workaround

Upgrade to Requests 2.32.5 or later, which removes the shared default context.

If you cannot upgrade yet, pass a CA bundle path instead of True. With a string, Requests 2.32.x builds the connection from ca_certs and never uses the shared context:

import certifi
import requests

session = requests.Session()
session.cert = ("client.pem", "client.key")
session.verify = certifi.where()  # a path, not True