Comodo Antivirus includes a full x86 emulator that is used to unpack executables that are being scanned. Files read from disk or received over the network, including email, browser cache and so on can all trigger emulation. The emulator itself uses a sequence of nested lookup tables to translate opcodes to the routines that emulate them. The xmm/ymm registers are used like a union in C. For example, the registers can be treated as 4 floats, 2 doubles, 2 dwords, 8 shorts and so on – whatever is appropriate. The comodo emulator uses a union to represent these registers, and then each emulated instruction uses whichever union member matches it’s function. For example, PUNPCKLBW would use regs->words, PSRLQ would use regs->qwords and so on. The code for PSUBUSB incorrectly uses the wrong union member (words instead of bytes), meaning it will clobber double the space allocated by CPU::MMX_OPCODE(). The fix for this vulnerability is to use the bytes member of the union instead.