GHSA-wpp4-vqfq-v4hp

## 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).

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## 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 ```c 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 ```c 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.

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## Reproduction

### **Unsigned underflow**

**Environment**

Built with AddressSanitizer and UndefinedBehaviorSanitizer enabled.

```c 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**

```bash ./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:`

<img width="1068" height="64" alt="image" src="https://github.com/user-attachments/assets/cd9637ee-1d03-4066-834d-fda22410dd8b" />

memory region corruption.

`./magick -size 2000x2000 xc:black -clahe 0x0 null:`

<img width="1069" height="70" alt="image" src="https://github.com/user-attachments/assets/ecbab79c-a3c2-4e8c-96c9-8e2aa8f0d2b2" />

→ Significant memory consumption and evidence of memory region corruption.

`./magick -size 4000x4000 xc:black -clahe 0x0 null:`

<img width="776" height="49" alt="image" src="https://github.com/user-attachments/assets/63a7cec5-616b-4aa5-87f3-a546a87e6625" />

→ Much larger memory usage; process appears to be aggressively consuming cache and address space.

`./magick -size 8000x8000 xc:black -clahe 0x0 null:`

<img width="748" height="46" alt="image" src="https://github.com/user-attachments/assets/48b3aac8-98b3-4fbb-a5ca-4e7936bca44b" />

→ Memory usage escalates further and begins exhausting available cache. If left running, the process is likely to crash (DoS) after sustained allocation attempts.

### **Division-by-zero**

**Environment:** ASan/UBSan-enabled build.

```c 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**

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

**Output**

<img width="1915" height="818" alt="image" src="https://github.com/user-attachments/assets/cfe44432-b429-49e4-8673-2ed55ba9a961" />

**Notes:** Without sanitizers, the process may terminate with just `Aborted` (still DoS).

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## Impact

- 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. - 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.

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## Suggested concrete patch snippets

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

```c 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](https://github.com/jin-156) [1203kids@gmail.com](mailto:1203kids@gmail.com)

**Youngin Won**

[@amethyst0225](https://github.com/amethyst0225) [youngin04@korea.ac.kr](mailto:youngin04@korea.ac.kr)

**Siyeon Han**

[@hanbunny](https://github.com/hanbunny) [kokosyeon@gmail.com](mailto:kokosyeon@gmail.com)

**Shinyoung Won**

[@yosiimich](yosimich123@gmail.com) [yosimich123@gmail.com](mailto:yosimich123@gmail.com)