An issue was discovered in QPDF before 7.0.0. There is a stack-based out-of-bounds read in the function iterate_rc4 in QPDF_encryption.cc.
https://usn.ubuntu.com/3638-1/
https://github.com/qpdf/qpdf/issues/147
https://github.com/qpdf/qpdf/commit/dea704f0ab7f625e1e7b3f9a1110b45b63157317