Null pointer dereference was found in upx PackLinuxElf::canUnpack() in p_lx_elf.cpp,in version UPX 4.0.0. That allow attackers to execute arbitrary code and cause a denial of service via a crafted file.
https://github.com/upx/upx/issues/485
https://github.com/upx/upx/commit/90279abdfcd235172eab99651043051188938dcc