Summary

• Target: ImageMagick (commit ecc9a5eb456747374bae8e07038ba10b3d8821b3)
• Type: Undefined Behavior (function-type-mismatch) in splay tree cloning callback
• Impact: Deterministic abort under UBSan (DoS in sanitizer builds). No crash in a non-sanitized build; likely low security impact.
• Trigger: Minimal 2-byte input parsed via MagickWand, then coalescing.

Environment

OS: macOS (Apple Silicon/arm64) Homebrew clang version 20.1.8 Target: arm64-apple-darwin24.5.0 Thread model: posix InstalledDir: /opt/homebrew/Cellar/llvm/20.1.8/bin Configuration file: /opt/homebrew/etc/clang/arm64-apple-darwin24.cfg Homebrew ImageMagick: magick -version → ImageMagick 7.1.2-0 Q16-HDRI aarch64 pkg-config: MagickWand-7.Q16HDRI version 7.1.2 Library configure flags (capsule build): ./configure --disable-shared --enable-static --without-modules --without-magick-plus-plus --disable-openmp --without-perl --without-x --with-png=yes --without-jpeg --without-tiff --without-xml --without-lqr --without-gslib Harness compile flags: -fsanitize=fuzzer,address,undefined -fno-omit-frame-pointer pkg-config cflags/libs supplied: -I<...>/include/ImageMagick-7 -DMAGICKCOREHDRIENABLE=1 -DMAGICKCOREQUANTUMDEPTH=16 -DMAGICKCORECHANNELMASK_DEPTH=32 and linked against MagickWand-7.Q16HDRI and MagickCore-7.Q16HDRI Sanitizer runtime: ASan+UBSan defaults. Repro also with UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1

PoC

• Bytes (hex): 1c 02
• Base64: HAI=
• sha256 (optional): <fill in>

Reproduction

Create PoC:

printf '\x1c\x02' > poc.bin

Option A: libFuzzer harness

• Run once: ./harness_ImageMagick_... -runs=1 ./poc.bin
• Expected: UBSan aborts with function-type-mismatch at MagickCore/splay-tree.c:372:43.

Option B: standalone reproducer (C)

• Compile (ensure PKG_CONFIG_PATH points to your ImageMagick if needed):

/opt/homebrew/opt/llvm/bin/clang -g -O1 -fsanitize=address,undefined $(/opt/homebrew/bin/pkg-config --cflags MagickWand-7.Q16HDRI) repro.c -o repro $(/opt/homebrew/bin/pkg-config --libs MagickWand-7.Q16HDRI)

• Run:

UBSANOPTIONS=printstacktrace=1:haltonerror=1 ./repro ./poc.bin Observed output (excerpt) MagickCore/splay-tree.c:372:43: runtime error: call to function ConstantString through pointer to incorrect function type 'void ()(void *)' string.c:680: note: ConstantString defined here

0 CloneSplayTree splay-tree.c:372

1 CloneImageProfiles profile.c:159

2 CloneImage image.c:832

3 CoalesceImages layer.c:269

4 MagickCoalesceImages magick-image.c:1665

5 main repro.c:XX

Root cause The splay tree clone callback expects a function pointer of type void *(*)(void *). ConstantString has a different signature (char *ConstantString(const char *)). Calling through the mismatched function type is undefined behavior in C and triggers UBSan’s function-type-mismatch. The path is exercised during coalescing: CloneImage → CloneImageProfiles → CloneSplayTree. Scope Reproduces with a minimal, sanitizer-instrumented, PNG-enabled build and delegates disabled (policy.xml), suggesting the issue is in MagickCore rather than external delegates. Suggested fix (sketch) Use a wrapper that matches the expected callback prototype, or adjust the splay-tree callback typedef for const-correctness. For example: static void *CloneStringShim(const void *p) { return (void *) ConstantString((const char *) p); }

/* When setting splay-tree clone_value, use CloneStringShim instead of ConstantString. */

Alternatively, update the clone callback typedefs to use const void* consistently (and return void*) and ensure callers pass a correctly typed wrapper.

Artifacts Minimised PoC: attached (poc.bin, 2 bytes; base64 HAI=) Harness source and exact build command (attached) Full UBSan trace (attached) Commit SHA and configure flags (above) Credits Discovered by: Lumina Mescuwa Method: libFuzzer + UBSan Verification

• UBSan build: Reproduces with halt_on_error=1; aborts at MagickCore/splay-tree.c:372.
• Non-sanitized Homebrew build (macOS arm64, clang 20.1.8): No crash; repro completes silently.