Stack out-of-bounds read in gif_get_code()

[Jorgecmartins]

In gif_get_code(), in image.cxx, there is a stack out-of-bounds read in the following code:

267    curbit    = (curbit - lastbit) + 8 * last_byte;
268    last_byte += (unsigned)count;
269    lastbit   = last_byte * 8;
270  }
271
272  for (ret = 0, i = curbit + (unsigned)code_size - 1, j = (unsigned)code_size;
273       j > 0;
274       i --, j --)
275    ret = (ret << 1) | ((buf[i / 8] & bits[i & 7]) != 0);

The expression curbit - lastbit, line 267, can result in an integer overflow when lastbit > curbit, updating curbit to a large number since it is unsigned. Later on line 272 the variable i is set to number less than code_size, since curbit + (unsigned)code_size - 1 overflows, which results after a few iterations in a stack out of bounds read in buf[i/8].

I've attached poc.zip that contains a malicious gif and a html file and triggers the out of bounds read resulting in a segmentation fault.

Steps to reproduce

The following should result in a segmentation fault:

$ unzip poc.zip
$ htmldoc --webpage -f output.pdf crash.html

Steps to analyse the crash on gdb

gdb --args htmldoc --webpage -f output.pdf ./crash.html
# set a breakpoint on gif_get_code
run
continue 3
# reached the gif_get_code that will crash

[michaelrsweet]

@Jorgecmartins I've tried this on macOS and Linux (Ubuntu 20.04), but only Linux reproduces.

[michaelrsweet]

[master 776cf0f] Fix potential stack overflow with GIF images (Issue #463)

[Jorgecmartins]

@Jorgecmartins I've tried this on macOS and Linux (Ubuntu 20.04), but only Linux reproduces.

@michaelrsweet I was also able to reproduce it on macOS.

Jorge@MacBook-Pro-de-Jorge htmldoc % ./htmldoc --webpage -f output.pdf ./crash.html
ERR005: Unable to open psglyphs data file!
ERR005: Unable to open character set file iso-8859-1!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Times-Roman.afm!
ERR005: Unable to open psglyphs data file!
ERR005: Unable to open character set file iso-8859-1!
ERR005: Unable to open psglyphs data file!
ERR005: Unable to open character set file iso-8859-1!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
ERR005: Unable to open font width file /usr/local/share/htmldoc/fonts/Helvetica.afm!
PAGES: 2
ERR005: Unable to open font file /usr/local/share/htmldoc/fonts/Helvetica.pfa!
zsh: segmentation fault  ./htmldoc --webpage -f output.pdf ./crash.html

macOS version: 11.6

Jorge@MacBook-Pro-de-Jorge htmldoc % uname -a
Darwin MacBook-Pro-de-Jorge.local 20.6.0 Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64 x86_64

[michaelrsweet]

@Jorgecmartins I added another layer of protection here:

[master 312f0f9] Block GIF images with a code size > 12 (Issue #463)

[Jorgecmartins]

@Jorgecmartins I added another layer of protection here:

[master 312f0f9] Block GIF images with a code size > 12 (Issue #463)

@michaelrsweet The extra protection fixed the issue.