An issue was discovered in QPDF before 7.0.0. There is a large heap-based out-of-bounds read in the Pl_Buffer::write function in Pl_Buffer.cc. It is caused by an integer overflow in the PNG filter.
https://usn.ubuntu.com/3638-1/
https://github.com/qpdf/qpdf/issues/150
https://github.com/qpdf/qpdf/commit/ec7d74a386c0b2f38990079c3b0d2a2b30be0e71