The tokenizer in QPDF 6.0.0 and 7.0.b1 is recursive for arrays and dictionaries, which allows remote attackers to cause a denial of service (stack consumption and segmentation fault) or possibly have unspecified other impact via a PDF document with a deep data structure, as demonstrated by a crash in QPDFObjectHandle::parseInternal in libqpdf/QPDFObjectHandle.cc.
https://usn.ubuntu.com/3638-1/
https://github.com/qpdf/qpdf/issues/146
https://github.com/qpdf/qpdf/commit/ad527a64f93dca12f6aabab2ca99ae5eb352ab4b