Tk GIF processing buffer overflow

Buffer overflow in the FileReadGIF function in tkImgGIF.c for Tk Toolkit 8.4.12 and earlier, and 8.3.5 and earlier, allows user-assisted attackers to cause a denial of service (segmentation fault) via an animated GIF in which the first subimage is smaller than a subsequent subimage, which triggers the overflow in the ReadImage function, a different vulnerability than CVE-2007-5137.