CiviCRM versions 5.3.0 and 4.6.37 (and earlier)
CiviCRM version 5.3.1 and 4.6.38 (and later)
When retrieving cases via AJAX, some parameters were not properly validated. This allowed for SQL injection.
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Patrick Figel of Greenpeace for reporting the issue.
Coleman Watts of CiviCRM Core Team for fixing the issue.