Our vendored copy of Modules/expat/ should be updated to Expat 2.4.1 to retrieve the fix for the security vulnerabily CVE-2013-0340 "Billion Laughs":
https://blog.hartwork.org/posts/cve-2013-0340-billion-laughs-fixed-in-expat-2-4-0/

The table of vulnerabilities in Python XML parsers should be updated as well:
https://docs.python.org/dev/library/xml.html#xml-vulnerabilities

My outdated notes on Modules/expat/: copy of libexpat

* ./configure --with-system-expat
* Rationale: https://mail.python.org/pipermail/python-dev/2017-June/148287.html
* Used on Windows and macOS, Linux distributions use system libexpat
* Version: search for XML_MAJOR_VERSION in Modules/expat/expat.h
* Script to update it: see attached script to https://bugs.python.org/issue30947
* Recent update: https://bugs.python.org/issue30947
* Python 2.7, 3.3-3.6 use libexpat 2.2.1

https://pythondev.readthedocs.io/files.html

(From PSRT list, Sebastian:)

 Please note that the vulnerability fix also added two new functions to
the API that would be great to have xml.parsers.expat expose to the
users for full control.  These are:

- XML_SetBillionLaughsAttackProtectionMaximumAmplification and
- XML_SetBillionLaughsAttackProtectionActivationThreshold

Module xml.parsers.expat.errors and its docs also needs 6 new error code
entries to be complete:

  /* Added in 2.0. */
  38 XML_ERROR_RESERVED_PREFIX_XML
  39 XML_ERROR_RESERVED_PREFIX_XMLNS
  40 XML_ERROR_RESERVED_NAMESPACE_URI

  /* Added in 2.2.1. */
  41 XML_ERROR_INVALID_ARGUMENT

  /* Added in 2.3.0. */
  42 XML_ERROR_NO_BUFFER

  /* Added in 2.4.0. */
  43 XML_ERROR_AMPLIFICATION_LIMIT_BREACH

With regard to the table of vulnerabilities mentioned in the ticket,
please note that vulnerability "quadratic blowup" is also fixed by
>=2.4.0.  Personally, I consider it a flavor of Billion Laughs and all
know variations are covered, including that one.

FTR that^^ Sebastian is me :)

Attached cpython_rebuild_expat_dir.sh script updates Modules/expat/ to our libexpat copy to 2.4.1. I used it to create attached PR 26945.

New changeset 3fc5d84046ddbd66abac5b598956ea34605a4e5d by Victor Stinner in branch 'main':
bpo-44394: Update libexpat copy to 2.4.1 (GH-26945)
https://github.com/python/cpython/commit/3fc5d84046ddbd66abac5b598956ea34605a4e5d

New changeset c9c2a0bc9820f93f1020f3498f6893a3544c9b76 by Miss Islington (bot) in branch '3.8':
bpo-44394: Update libexpat copy to 2.4.1 (GH-26945) (GH-28033)
https://github.com/python/cpython/commit/c9c2a0bc9820f93f1020f3498f6893a3544c9b76

New changeset 270678564c16452614a8acd93763bdf64fb4d286 by Miss Islington (bot) in branch '3.10':
bpo-44394: Update libexpat copy to 2.4.1 (GH-26945)
https://github.com/python/cpython/commit/270678564c16452614a8acd93763bdf64fb4d286

New changeset 007221a43e566db08c0c5c00756d80dfd9dccafe by Miss Islington (bot) in branch '3.9':
bpo-44394: Update libexpat copy to 2.4.1 (GH-26945) (GH-28032)
https://github.com/python/cpython/commit/007221a43e566db08c0c5c00756d80dfd9dccafe

3.6 will need a separate backport because it's using expat 2.2.6 at the moment (from b2260e59ff1eaf20de4738099005ddf507b7b27d).

3.7 conflicted since it didn't include local changes to the vendored 2.2.8 that were introduced in 3.8+. I fixed that, the backport is up.

I created https://python-security.readthedocs.io/vuln/expat-billion-laughs.html to track this vulnerability.

New changeset 79101b890ee021a901a8b6837a3a320d57adb725 by Łukasz Langa in branch '3.7':
[3.7] bpo-44394: Update libexpat copy to 2.4.1 (GH-26945) (GH-28042)
https://github.com/python/cpython/commit/79101b890ee021a901a8b6837a3a320d57adb725

New changeset 910886a6448e4bf1edf49eeace4aa240b6403772 by Ned Deily in branch '3.6':
[3.6] bpo-44394: Update libexpat copy to 2.4.1 (GH-26945) (GH-28042) (GH-28080)
https://github.com/python/cpython/commit/910886a6448e4bf1edf49eeace4aa240b6403772

PRs merged in 3.7 branch for release in 3.7.12 and in 3.6 branch for release in 3.6.15.