From the announcement:

Expat 2.2.1 has been released.  The change log has more details [2] than this mail, including commit SHA1s. For a quick overview of the security fixes and CVEs, we have:

   CVE-2017-9233  External entity infinite loop DoS [1]
  (CVE-2016-9063) Integer overflow (re-fix)
             n/a  More integer overflow fixes
  (CVE-2016-0718) Fix regression bugs from 2.2.0's fix to CVE-2016-0718
  (CVE-2016-5300) Use os-specific entropy sources like getrandom
             n/a  No longer leak parser pointer information
             n/a  Prevent use of uninitialised variables
             n/a  Add missing API parameter validation (NULL, len<0)
  (CVE-2012-0876) Counter hash flooding with SipHash

https://github.com/libexpat/libexpat/blob/R_2_2_1/expat/Changes

https://libexpat.github.io/doc/cve-2017-9233/