[Robbie-Microsoft]

Scope

Follow-up to #938. Adds opt-in custom HTTP-pipeline support for the mTLS token endpoint. The default certificate-bearing transport and ordinary non-mTLS custom HTTP behavior remain unchanged.

Developer experience

A custom http_client opts in by adding create_mtls_client(binding_context) alongside its existing ordinary get/post methods. The argument is the public msal.MtlsBindingContext, not a token result. There is no marker class or new application constructor parameter.

def create_mtls_client(self, binding_context):
    session = self.new_policy_session()  # Fresh session with existing middleware/policy
    session.mount("https://", binding_context.create_requests_adapter(
        max_retries=self.retries,
        pool_connections=self.pool_connections,
        pool_maxsize=self.pool_maxsize,
        pool_block=True,
    ))
    self.mtls_sessions.append(session)  # Factory owns retention and cleanup
    return session

MtlsBindingContext.create_requests_adapter(**HTTPAdapter_options) reuses MSAL's certificate-bearing Requests transport, CA selection, proxy handling, and Requests-version compatibility. Other HTTP implementations can use create_ssl_context(verify=...) after resolving their effective verification policy.

sample/confidential_client_mtls_http_client_sample.py is a runnable example retaining middleware, CA/proxy/environment settings, timeout, retry, and pooling policy. It owns and closes both ordinary and derived pipelines.

Isolation and ownership

• MSAL lazily creates and reuses one derived token pipeline per application/certificate. Creation is serialized within an application; different applications receive separate factory calls.
• Factories must return a fresh client with callable post(url, **kwargs), isolated connection pools and TLS sessions, and the required networking/middleware policy. The factory owns cleanup, including partial constructions; MSAL does not close caller-owned clients.
• Missing capabilities, malformed results, returning the ordinary client, and directly reusing another live application's client fail explicitly. Factory/network errors propagate without fallback.
• Default and custom mTLS token paths enforce request-override and redirect protections before cache lookup. Ordinary HTTP behavior is unchanged.
• This is a trusted implementer contract: MSAL cannot prove that arbitrary custom middleware uses the certificate or preserves networking policy.
• binding_certificate is returned only for a final mtls_pop result, never for ordinary Bearer. Downgraded responses remain rejected and uncached.

Validation

Unit and integration coverage includes factory validation, default/ordinary compatibility, concurrent construction, client reuse, lifecycle ownership, redirects, request overrides, and token/HTTP cache isolation.

Loopback TLS tests verify the selected peer certificate, ordinary/discovery traffic without a client certificate, middleware and timeout policy, CA/proxy settings, isolated connection pools, certificate rotation, and failure without fallback.

[Copilot]

🟡 Changes recommended

The endpoint guard raises an exception for otherwise compatible custom responses whose URL metadata is a URL object.

1 open issue

• Normalize response URL before parsing · New

What changed in this PR

Adds certificate-aware custom HTTP pipelines to the existing mTLS token flow while preserving the default transport and ordinary HTTP behavior.

Changes:

• Adds an opt-in client factory and a public Requests adapter helper.
• Enforces pipeline isolation, request restrictions, and endpoint checks.
• Documents ownership requirements and adds a runnable sample with unit and local TLS coverage.

File	Description
tests/​test_mtls_http_client.py	Tests custom factories, isolation, lifecycle, and local TLS behavior.
tests/​test_application.py	Updates redirect tests and transport cleanup.
sample/​confidential_client_mtls_pop_sample.py	Updates comments to reference the custom pipeline sample.
sample/​confidential_client_mtls_http_client_sample.py	Demonstrates policy-preserving pipelines and explicit cleanup.
msal/​mtls.py	Adds adapter customization, ownership tracking, and endpoint guards.
msal/​application.py	Integrates lazy custom pipeline creation and request validation.
docs/​index.rst	Documents factory usage, trust boundaries, and lifecycle responsibilities.

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

Custom certificate-bearing authentication pipelines introduce security-sensitive isolation, redirect, and lifecycle contracts that warrant final human review.

No open issues 🎉

1 resolved since last review

• Normalize response URL before parsing

_{Review effort: Balanced}

[Gladwin Johnson VR (gladjohn)]

I do not see a support for non-exportable CNGs. This is the key scenario.