An issue was discovered in QPDF before 7.0.0. There is an infinite loop due to looping xref tables in QPDF.cc.
https://usn.ubuntu.com/3638-1/
https://github.com/qpdf/qpdf/issues/149
https://github.com/qpdf/qpdf/commit/85f05cc57ffa0a863d9d9b23e73acea9410b2937