CVE-2025-62594

MEDIUM4.7EPSS 0.02%

ImageMagick CLAHE : Unsigned underflow and division-by-zero lead to OOB pointer arithmetic and process crash (DoS)

Published: 10/27/2025Modified: 5/9/2026

Description

## 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 ```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. --- ## 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). --- ## 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. --- ## 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** [email protected] **Woojin Park** [@jin-156](https://github.com/jin-156) [[email protected]](mailto:[email protected]) **Youngin Won** [@amethyst0225](https://github.com/amethyst0225) [[email protected]](mailto:[email protected]) **Siyeon Han** [@hanbunny](https://github.com/hanbunny) [[email protected]](mailto:[email protected]) **Shinyoung Won** [@yosiimich]([email protected]) [[email protected]](mailto:[email protected])

Affected packages (13)

CVSS scores

SourceVersionSeverityVector
osvCVSS 3.1MEDIUM4.7CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H

References (5)