JLSEC-2026-928

Summary

A single root cause in the CLAHE implementation — tile width/height becoming zero — produces two distinct but related unsafe behaviors. Vulnerabilities exists in the CLAHEImage() function of ImageMagick’s MagickCore/enhance.c.

1. Unsigned integer underflow → out-of-bounds pointer arithmetic (OOB): when tile_info.height == 0, the expression tile_info.height - 1 (unsigned) wraps to a very large value; using that value in pointer arithmetic yields a huge offset and OOB memory access (leading to memory corruption, SIGSEGV, or resource exhaustion).
2. Division/modulus by zero: where code performs ... / tile_info.width or ... % tile_info.height without re-checking for zero, causing immediate division-by-zero crashes under sanitizers or abort at runtime.

Both behaviors are triggered by the same invalid tile condition (e.g., CLI exact -clahe 0x0! or automatic tile derivation dim >> 3 == 0 for very small images).

Details

Unsigned underflow(can lea to OOB)

• Location: MagickCore/enhance.c, around line 609

• Version tested: 7.1.2-8 (local ASan(undefined). /UBSan build)

• Vulnerable code

  enhance.c: 609

  p += (ptrdiff_t) clahe_info->width * (tile.height - 1);
  

• Root Cause

  • If tile.height == 0, then (tile.height - 1) underflows to UINT_MAX.
  • Multiplication with clahe_info->width yields a huge value close to SIZE_MAX.
  • Adding this to p causes pointer arithmetic underflow.

Division-by-zero

• File / Location: MagickCore/enhance.c, around line 669

• Version tested: 7.1.2-8 (local ASan(undefined). /UBSan build)

• vulnerable code

  enhance.c: 669-673

   if ((image->columns % tile_info.width) != 0)
      tile_info.x=(ssize_t) (tile_info.width-(image->columns % tile_info.width));
    tile_info.y=0;
    if ((image->rows % tile_info.height) != 0)
      tile_info.y=(ssize_t) (tile_info.height-(image->rows % tile_info.height));
  

• Root cause

  Missing input validation / bounds checks after computing default tile dimensions:

  If either tile_info.width or tile_info.height is 0, this triggers a division by zero. Zeros can reach this point through:

  1. Exact tiles: CLI clahe 0x0! (the ! forces zero to be used verbatim).
  2. Auto tiles on tiny images: When a requested tile is 0 (no !), the code derives a default from the image size (e.g., dim >> 3). For images with dim < 8, this result is 0 unless clamped.

Reproduction

Unsigned underflow

Environment

Built with AddressSanitizer and UndefinedBehaviorSanitizer enabled.

export UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1
export ASAN_OPTIONS=abort_on_error=1:allocator_may_return_null=1:detect_leaks=0

Command

./magick xc:black -clahe 0x0 null:

Output

MagickCore/enhance.c:609:6: runtime error: addition of unsigned offset overflowed
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior MagickCore/enhance.c:609:6 in CLAHEImage

./magick -size 10x10 xc:black -clahe 0x0 null:

Command

./magick -size 16x2 gradient: -type TrueColor -depth 8 -clahe 0x0! null:

Output

Suggested concrete patch snippets

Apply in CLAHEImage() after tile_info is computed but before any division/modulus/pointer arithmetic:

if (exact_tiles_requested && (tile_info.width == 0 || tile_info.height == 0)) {
  ThrowMagickException(exception, GetMagickModule(), OptionError,
                       "CLAHEInvalidTile", "%lux%lu",
                       (unsigned long) tile_info.width,
                       (unsigned long) tile_info.height);
  return (Image *) NULL;
}

if (!exact_tiles_requested) {
  tile_info.width  = (tile_info.width  == 0) ? MagickMax((size_t)1, image->columns >> 3) : tile_info.width;
  tile_info.height = (tile_info.height == 0) ? MagickMax((size_t)1, image->rows    >> 3) : tile_info.height;
}

if (tile_info.width == 0 || tile_info.height == 0) {
  ThrowMagickException(exception, GetMagickModule(), OptionError,
                       "CLAHEInvalidTile", "%lux%lu",
                       (unsigned long) tile_info.width,
                       (unsigned long) tile_info.height);
  return (Image *) NULL;
}

ssize_t tile_h_minus1 = (ssize_t)tile_info.height - 1;
if (tile_h_minus1 < 0) {
  ThrowMagickException(exception, GetMagickModule(), OptionError,
                       "CLAHEInvalidTile", "%lux%lu",
                       (unsigned long) tile_info.width,
                       (unsigned long) tile_info.height);
  return (Image *) NULL;
}
p += (ptrdiff_t) clahe_info->width * tile_h_minus1;

Notes about exact_tiles_requested: if the CLI/Wand parser already exposes whether ! was present, use it. If not, add a parse-time flag so CLAHEImage can know whether 0 is literal or auto.

Credit

Team Whys

Bug Hunting Master Program, HSpace/Findthegap

Youngmin Kim kunshim@naver.com

Woojin Park

@jin-156 1203kids@gmail.com

Youngin Won

@amethyst0225 youngin04@korea.ac.kr

Siyeon Han

@hanbunny kokosyeon@gmail.com

Shinyoung Won

@yosiimich yosimich123@gmail.com