An assertion abort was found in upx MemBuffer::alloc() in mem.cpp, in version UPX 4.0.0. The flow allows attackers to cause a denial of service (abort) via a crafted file.
https://github.com/upx/upx/pull/487
https://github.com/upx/upx/issues/486
https://github.com/upx/upx/commit/28e761cd42211dfe0124b7a29b2f74730f453e46