cpython bundles expat in Modules/expat/ and needs to be updated to expat-2.2.8 to security vulnerability CVE-2019-15903.

From Sebastian Pipping on XML-DEV ML:

Expat 2.2.8 [1] has been released yesterday.  This release fixes a
security issue — a heap buffer over-read known as CVE-2019-15903 [2]
reported by Joonun Jang resulting in Denial of Service —, starts using
the rand_s function on Windows and MinGW (ending the previous
LoadLibrary hack), includes non-security bugfixes, many build system
fixes and improvements, improvements to xmlwf usability, and more.

For more details regarding the latest release, please check out the
changelog [3].

If you maintain Expat packaging or a bundled copy of Expat or a pinned
version of Expat somewhere, please update to 2.2.8.  Thank you!

[1] https://github.com/libexpat/libexpat/releases/tag/R_2_2_8
[2] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-15903
[3] https://github.com/libexpat/libexpat/blob/R_2_2_8/expat/Changes