The UIL and XPM image encoder do not validate the pixel index value returned by GetPixelIndex() before using it as an array subscript. In HDRI builds, Quantum is a floating-point type, so pixel index values can be negative. An attacker can craft an image with negative pixel index values to trigger a global buffer overflow read during conversion, leading to information disclosure or a process crash.

READ of size 1 at 0x55a8823a776e thread T0
  #0 0x55a880d01e85 in WriteUILImage coders/uil.c:355

READ of size 1 at 0x55fa1c04c66e thread T0
  #0 0x55fa1a9ee415 in WriteXPMImage coders/xpm.c:1135