.. bpo: 44022
.. date: 2021-05-05-17-37-04
.. nonce: bS3XJ9
.. release date: 2021-06-28
.. section: Security

:mod:`http.client` now avoids infinitely reading potential HTTP headers after
a ``100 Continue`` status response from the server.

..

.. bpo: 43882
.. date: 2021-04-25-07-46-37
.. nonce: Jpwx85
.. section: Security

The presence of newline or tab characters in parts of a URL could allow some
forms of attacks.

Following the controlling specification for URLs defined by WHATWG
:func:`urllib.parse` now removes ASCII newlines and tabs from URLs,
preventing such attacks.

..

.. bpo: 42988
.. date: 2021-03-24-14-16-56
.. nonce: P2aNco
.. section: Security

CVE-2021-3426: Remove the ``getfile`` feature of the :mod:`pydoc` module
which could be abused to read arbitrary files on the disk (directory
traversal vulnerability). Moreover, even source code of Python modules can
contain sensitive data like passwords. Vulnerability reported by David
Schwörer.

..

.. bpo: 43285
.. date: 2021-03-13-03-48-14
.. nonce: g-Hah3
.. section: Security

:mod:`ftplib` no longer trusts the IP address value returned from the server
in response to the PASV command by default.  This prevents a malicious FTP
server from using the response to probe IPv4 address and port combinations
on the client network.

Code that requires the former vulnerable behavior may set a
``trust_server_pasv_ipv4_address`` attribute on their :class:`ftplib.FTP`
instances to ``True`` to re-enable it.

..

.. bpo: 43075
.. date: 2021-01-31-05-28-14
.. nonce: DoAXqO
.. section: Security

Fix Regular Expression Denial of Service (ReDoS) vulnerability in
:class:`urllib.request.AbstractBasicAuthHandler`.  The ReDoS-vulnerable
regex has quadratic worst-case complexity and it allows cause a denial of
service when identifying crafted invalid RFCs. This ReDoS issue is on the
client side and needs remote attackers to control the HTTP server.

..

.. bpo: 43660
.. date: 2021-03-29-19-50-34
.. nonce: scTgag
.. section: Core and Builtins

Fix crash that happens when replacing ``sys.stderr`` with a callable that
can remove the object while an exception is being printed. Patch by Pablo
Galindo.

..

.. bpo: 41561
.. date: 2021-03-18-10-34-42
.. nonce: pDg4w-
.. section: Tests

Add workaround for Ubuntu's custom OpenSSL security level policy.
