ImageMagick has a NULL pointer dereference in MSL parser via <comment> tag before image load

Summary

NULL pointer dereference in MSL (Magick Scripting Language) parser when processing <comment> tag before any image is loaded.

Version

• ImageMagick 7.x (tested on current main branch)
• Commit: HEAD

Steps to Reproduce

Method 1: Using ImageMagick directly

magick MSL:poc.msl out.png

Method 2: Using OSS-Fuzz reproduce

python3 infra/helper.py build_fuzzers imagemagick
python3 infra/helper.py reproduce imagemagick msl_fuzzer poc.msl

Or run the fuzzer directly:

./msl_fuzzer poc.msl

Expected Behavior

ImageMagick should handle the malformed MSL gracefully and return an error message.

Actual Behavior

convert: MagickCore/property.c:297: MagickBooleanType DeleteImageProperty(Image *, const char *): Assertion `image != (Image *) NULL' failed.
Aborted

Root Cause Analysis

In coders/msl.c:7091, MSLEndElement() calls DeleteImageProperty() on msl_info->image[n] when handling the </comment> end tag without checking if the image is NULL:

if (LocaleCompare((const char *) tag,"comment") == 0 )
  {
    (void) DeleteImageProperty(msl_info->image[n],"comment");  // No NULL check
    ...
  }

When <comment> appears before any <read> operation, msl_info->image[n] is NULL, causing the assertion failure in DeleteImageProperty() at property.c:297.

Impact

• DoS: Crash via assertion failure (debug builds) or NULL pointer dereference (release builds)
• Affected: Any application using ImageMagick to process user-supplied MSL files

Fuzzer

This issue was discovered using a custom MSL fuzzer:

#include <cstdint>
#include <Magick++/Blob.h>
#include <Magick++/Image.h>
#include "utils.cc"

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size)
{
  if (IsInvalidSize(Size))
    return(0);
  try
  {
    const Magick::Blob blob(Data, Size);
    Magick::Image image;
    image.magick("MSL");
    image.fileName("MSL:");
    image.read(blob);
  }
  catch (Magick::Exception)
  {
  }
  return(0);
}

This issue was found by Team FuzzingBrain @ Texas A&M University