Stack buffer overflow in image_load_bmp()

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In image_load_bmp(), the colors_used variable is read from the BMP file header and directly used to read into a fixed-size buffer.

static int          /* O - 0 = success, -1 = fail */
image_load_bmp(image_t *img,    /* I - Image to load into */
               FILE    *fp, /* I - File to read from */
           int     gray,    /* I - Grayscale image? */
               int     load_data)/* I - 1 = load image data, 0 = just info */
{
  ...
  uchar     colormap[256][4];
  ...
  colors_used      = read_dword(fp);
  ...
  fread(colormap, colors_used, 4, fp);
  ...
}

A maliciously crafted BMP file could set the colors_used variable to a number big enough to overflow the stack and thus the return address. I am attaching a proof of concept below. It can be tested with: ./htmldoc --webpage -f out.pdf ./poc.html

poc.zip

[michaelrsweet]

[master 27d0898] Fix potential BMP stack overflow (Issue #453)

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I have requested a CVE for this issue.

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This issue has been assigned CVE-2021-43579.