Heap-based buffer overflow in the pefromupx function in libclamav/upx.c in Clam AntiVirus (ClamAV) 0.81 through 0.88.3 allows remote attackers to execute arbitrary code via a crafted UPX packed file containing sections with large rsize values.
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.