Draft PEP: Enabling certificate verification by default for stdlib mail modules

.github/CODEOWNERS

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peps/pep-0738.rst

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+PEP: 738
+Title: Enabling certificate verification by default for stdlib mail modules
+Author: Martin Schobert <[email protected]>
+Status: Draft
+Type: Standards Track
+Created: 17-Nov-2023
+
+Abstract
+=========
+
+The Python mail libraries such as ``smtplib``, ``imaplib`` and ``pop3lib``
+do not verify server certificates per default, when a client based on these
+mail libraries connects to a server via TLS. Any server certificate is
+accepted per default. This means, a client can't ensure that it connects
+to the server to which the connection was intended.
+
+This allows an active attacker in a machine-in-the-middle postion to intercept
+communication, read mail contents, credentials or may abuse an SMTP server
+for spaming.
+
+Motivation
+===========
+
+This PEP proposes to enable verification of X509 certificates for Python's
+mail clients by default, subject to opt-out on a per-call basis. This change
+would be applied to all maintained Python versions.
+
+Rationale
+=========
+
+It can be expected that a Python mail client, written in the most simple way, is
+secure by default. If progam code wants to insecurely connect to a server, this
+action should require and explicit confirmation. Not verifying a server certificate
+and accepting it violates PEP 20's principle "errors should never pass silently."
+
+It can also be expected that Python standard libraries behave in a consitent way.
+While Python's HTTP stdlib modules verify server certificates per default after
+implementing the change request from PEP 476, developers may expect Python mail
+libraries to behave similar.
+
+The failure of various applications to note Python's negligence in this matter
+is a source of vulnerabilities [#]_ [#]_ [#]_ [#]_.
+
+.. [#] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-39441
+.. [#] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-38686
+.. [#] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-39441
+.. [#] https://www.pentagrid.ch/en/blog/python-mail-libraries-certificate-verification/
+
+Specification
+=================
+
+Python would use the system provided certificate database on all platforms.
+Failure to locate such a database would be an error, and users would need to
+explicitly specify a location to fix it.
+
+Python will change the ``_create_stdlib_context`` as default context in the
+mail libraries with ``ssl.create_default_context``.
+
+If a developer wants to disable certificate verification, an explicitly created
+SSL/TLS context may be passed that has been created with ``_create_unverified_context``.
+
+Trust database
+--------------
+
+This PEP proposes using the system-provided certificate database.
+
+
+Opting out
+----------
+
+For users who wish to opt out of certificate verification on a single
+connection, they can achieve this by providing the ``ssl_context``
+for ``imaplib.IMAP4_SSL`` respectively ``context`` for ``smtplib.SMTP_SSL`` and
+``poplib.POP3_SSL``.
+
+
+Backwards compatibility
+=======================
+
+This change may result in failed connections, when servers do not use
+valid and trusted certificates. With invalid or untrusted certificates, the Python
+TLS library will raise an exception during TLS handshake.
+
+Nevertheless, this will only happen with invalid mail server certificates or
+server certificates that do not rely on a trust anchor known to Python's
+runtime environment. In these cases it is necessary that the system admin
+either installs a valid mail server certificate or the client ensures
+there is a trust anchor defined (a valid certificate chain leading from a CA
+file to the server certificate). As an unrecommended alternative, the client
+code may use an SSL context that does not verify certificates as descibed
+in the next section.
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.