"details": "## Summary\n\nA single root cause in the CLAHE implementation — tile width/height becoming zero — produces two distinct but related unsafe behaviors.\nVulnerabilities exists in the `CLAHEImage()` function of ImageMagick’s `MagickCore/enhance.c`.\n\n1. 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).\n2. **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.\n\nBoth 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). \n\n---\n\n## Details\n\n### **Unsigned underflow(can lea to OOB)**\n\n- Location: `MagickCore/enhance.c`, around line 609\n- Version tested: 7.1.2-8 (local ASan(undefined). /UBSan build)\n- Vulnerable code\n \n enhance.c: 609\n \n ```c\n p += (ptrdiff_t) clahe_info->width * (tile.height - 1);\n ```\n \n- Root Cause\n - If `tile.height == 0`, then `(tile.height - 1)` underflows to `UINT_MAX`.\n - Multiplication with `clahe_info->width` yields a huge value close to `SIZE_MAX`.\n - Adding this to `p` causes pointer arithmetic underflow.\n\n### **Division-by-zero**\n\n- File / Location: `MagickCore/enhance.c`, around line 669\n- Version tested: 7.1.2-8 (local ASan(undefined). /UBSan build)\n- vulnerable code\n \n enhance.c: 669-673\n \n ```c\n if ((image->columns % tile_info.width) != 0)\n tile_info.x=(ssize_t) (tile_info.width-(image->columns % tile_info.width));\n tile_info.y=0;\n if ((image->rows % tile_info.height) != 0)\n tile_info.y=(ssize_t) (tile_info.height-(image->rows % tile_info.height));\n ```\n \n- Root cause\n \n Missing input validation / bounds checks after computing default tile dimensions:\n \n If either `tile_info.width` or `tile_info.height` is 0, this triggers a division by zero. Zeros can reach this point through:\n \n 1. Exact tiles: CLI `clahe 0x0!` (the `!` forces zero to be used verbatim).\n 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.\n\n---\n\n## Reproduction\n\n### **Unsigned underflow**\n\n**Environment**\n\nBuilt with AddressSanitizer and UndefinedBehaviorSanitizer enabled.\n\n```c\nexport UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1\nexport ASAN_OPTIONS=abort_on_error=1:allocator_may_return_null=1:detect_leaks=0\n```\n\n**Command**\n\n```bash\n./magick xc:black -clahe 0x0 null:\n```\n\n**Output**\n\n```\nMagickCore/enhance.c:609:6: runtime error: addition of unsigned offset overflowed\nSUMMARY: UndefinedBehaviorSanitizer: undefined-behavior MagickCore/enhance.c:609:6 in CLAHEImage\n```\n\n`./magick -size 10x10 xc:black -clahe 0x0 null:`\n\n<img width=\"1068\" height=\"64\" alt=\"image\" src=\"https://github.com/user-attachments/assets/cd9637ee-1d03-4066-834d-fda22410dd8b\" />\n\nmemory region corruption.\n\n`./magick -size 2000x2000 xc:black -clahe 0x0 null:`\n\n<img width=\"1069\" height=\"70\" alt=\"image\" src=\"https://github.com/user-attachments/assets/ecbab79c-a3c2-4e8c-96c9-8e2aa8f0d2b2\" />\n\n→ Significant memory consumption and evidence of memory region corruption.\n\n`./magick -size 4000x4000 xc:black -clahe 0x0 null:`\n\n<img width=\"776\" height=\"49\" alt=\"image\" src=\"https://github.com/user-attachments/assets/63a7cec5-616b-4aa5-87f3-a546a87e6625\" />\n\n→ Much larger memory usage; process appears to be aggressively consuming cache and address space.\n\n`./magick -size 8000x8000 xc:black -clahe 0x0 null:`\n\n<img width=\"748\" height=\"46\" alt=\"image\" src=\"https://github.com/user-attachments/assets/48b3aac8-98b3-4fbb-a5ca-4e7936bca44b\" />\n\n→ Memory usage escalates further and begins exhausting available cache. If left running, the process is likely to crash (DoS) after sustained allocation attempts.\n\n### **Division-by-zero**\n\n**Environment:** ASan/UBSan-enabled build.\n\n```c\nexport UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1\nexport ASAN_OPTIONS=abort_on_error=1:allocator_may_return_null=1:detect_leaks=0\n```\n\n**Command**\n\n```bash\n./magick -size 16x2 gradient: -type TrueColor -depth 8 -clahe 0x0! null:\n```\n\n**Output**\n\n<img width=\"1915\" height=\"818\" alt=\"image\" src=\"https://github.com/user-attachments/assets/cfe44432-b429-49e4-8673-2ed55ba9a961\" />\n\n**Notes:** Without sanitizers, the process may terminate with just `Aborted` (still DoS).\n\n---\n\n## Impact\n\n- Primary: Denial-of-Service — crash or sustained resource exhaustion (memory/cache thrash) when processing crafted parameters or small images via CLI or API. Attackers can trivially trigger via `clahe 0x0!` or by uploading very small images to services using ImageMagick.\n- Secondary (theoretical): OOB memory accesses and memory corruption could potentially be combined with other vulnerabilities to achieve more severe outcomes; however, no reliable code execution was demonstrated from these PoCs alone.\n\n---\n\n## Suggested concrete patch snippets\n\nApply in `CLAHEImage()` after `tile_info` is computed but **before** any division/modulus/pointer arithmetic:\n\n```c\nif (exact_tiles_requested && (tile_info.width == 0 || tile_info.height == 0)) {\n ThrowMagickException(exception, GetMagickModule(), OptionError,\n \"CLAHEInvalidTile\", \"%lux%lu\",\n (unsigned long) tile_info.width,\n (unsigned long) tile_info.height);\n return (Image *) NULL;\n}\n\nif (!exact_tiles_requested) {\n tile_info.width = (tile_info.width == 0) ? MagickMax((size_t)1, image->columns >> 3) : tile_info.width;\n tile_info.height = (tile_info.height == 0) ? MagickMax((size_t)1, image->rows >> 3) : tile_info.height;\n}\n\nif (tile_info.width == 0 || tile_info.height == 0) {\n ThrowMagickException(exception, GetMagickModule(), OptionError,\n \"CLAHEInvalidTile\", \"%lux%lu\",\n (unsigned long) tile_info.width,\n (unsigned long) tile_info.height);\n return (Image *) NULL;\n}\n\nssize_t tile_h_minus1 = (ssize_t)tile_info.height - 1;\nif (tile_h_minus1 < 0) {\n ThrowMagickException(exception, GetMagickModule(), OptionError,\n \"CLAHEInvalidTile\", \"%lux%lu\",\n (unsigned long) tile_info.width,\n (unsigned long) tile_info.height);\n return (Image *) NULL;\n}\np += (ptrdiff_t) clahe_info->width * tile_h_minus1;\n```\n\nNotes 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.\n\n---\n\n## Credit\n\n### Team Whys \n\n**Bug Hunting Master Program, HSpace/Findthegap**\n\n**Youngmin Kim** \
[email protected]\n\n**Woojin Park**\n\n[@jin-156](https://github.com/jin-156)\n[
[email protected]](mailto:
[email protected])\n\n**Youngin Won**\n\n[@amethyst0225](https://github.com/amethyst0225)\n[
[email protected]](mailto:
[email protected])\n\n**Siyeon Han**\n\n[@hanbunny](https://github.com/hanbunny)\n[
[email protected]](mailto:
[email protected])\n\n**Shinyoung Won**\n\n[@yosiimich](
[email protected])\n[
[email protected]](mailto:
[email protected])",
0 commit comments