diff --git a/HEIF_IMPLEMENTATION_PLAN.md b/HEIF_IMPLEMENTATION_PLAN.md
index 443eeb6891..11327e4d5c 100644
--- a/HEIF_IMPLEMENTATION_PLAN.md
+++ b/HEIF_IMPLEMENTATION_PLAN.md
@@ -62,7 +62,7 @@ This snapshot pins or classifies the available references and failures; it does
 | `Av1FrameInfo`, `Av1TileReader`, and `Av1BlockDecoder` transform/coefficient storage | AV1 section 5.11.39 coefficient syntax and section 7.11.2 reconstruction | libaom `av1/decoder/decodetxb.c` and `av1/decoder/decoder.h` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Preserve separate luma and chroma transform coefficients at monotonically advancing per-plane offsets within each superblock so reconstruction consumes the same transform-block order produced by tile parsing. |
 | `Av1InverseQuantizer` and `Av1InverseQuantizationLookup` | AV1 section 7.12.3 inverse quantization | libaom `aom_dsp/aom_dsp_common.h`, `av1/common/quant_common.c`, and `av1/decoder/decodetxb.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Select the per-segment matrix level, alias 64-pixel transform dimensions to their adjusted matrices, retain a flat level-15 matrix, and apply the five-bit inverse-matrix weight scale. The large managed lookup remains a single process-wide table. |
 | `Av1Inverse2dTransformer` and `Av1InverseTransformerFactory` | AV1 section 7.11.2 inverse transform and reconstruction | libaom `av1/common/av1_inv_txfm1d.c`, `av1/common/av1_inv_txfm2d.c`, and `av1/common/idct.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Scalar transform oracle for coefficient-row traversal, intermediate layout, stage ranges, clipping, and high-bit-depth sample addition. The managed 16-bit overload is also used as a parity oracle for the byte overload. |
-| `HeifDecoderCore` and `HeifEncoderCore` item-property containers and associations | ISO/IEC 14496-12 section 8.11.14 item properties and `ipma` syntax | libavif `src/read.c` and `src/write.c` at `092276ce89098ead06db80975173191e5fee1826` | Preserve the position of every property in `ipco`, associate properties by item ID, and read or write the essential bit plus one-based 7-bit or 15-bit property index according to the full-box flags. Independent HEIC, HIF, and AVIF fixtures provide the reader oracle; container-level identification of encoded output guards the writer independently of pixel roundtripping. |
+| `HeifDecoderCore` box extension handling and `HeifDecoderCore`/`HeifEncoderCore` item-property associations | ISO/IEC 14496-12 box extensibility and section 8.11.14 item properties and `ipma` syntax | libavif `src/read.c` and `src/write.c` at `092276ce89098ead06db80975173191e5fee1826` | Skip unrecognized top-level and metadata child boxes, preserve the position of every property in `ipco`, reject an unrecognized property only when its item association marks it essential, associate properties by item ID, and read or write the essential bit plus one-based 7-bit or 15-bit property index according to the full-box flags. Independent HEIC, HIF, and AVIF fixtures provide the reader oracle; container-level identification of encoded output guards the writer independently of pixel roundtripping. |
 
 This table is intentionally incomplete. Add a row before each additional AV1 or HEVC algorithm is ported or materially reshaped.
 
diff --git a/src/ImageSharp/Formats/Heif/HeifDecoderCore.cs b/src/ImageSharp/Formats/Heif/HeifDecoderCore.cs
index 642811bc1a..7f881520a8 100644
--- a/src/ImageSharp/Formats/Heif/HeifDecoderCore.cs
+++ b/src/ImageSharp/Formats/Heif/HeifDecoderCore.cs
@@ -19,6 +19,8 @@ namespace SixLabors.ImageSharp.Formats.Heif;
 /// </summary>
 internal sealed class HeifDecoderCore : ImageDecoderCore
 {
+    private static readonly object UnknownProperty = new();
+
     /// <summary>
     /// The general configuration.
     /// </summary>
@@ -81,7 +83,8 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
                     stream.Skip((int)(stream.Length - stream.Position));
                     break;
                 default:
-                    throw new ImageFormatException($"Unknown box type of '{PrettyPrint(boxType)}'");
+                    SkipBox(stream, boxLength);
+                    break;
             }
         }
 
@@ -245,7 +248,8 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
                     SkipBox(stream, length);
                     break;
                 default:
-                    throw new ImageFormatException($"Unknown metadata box type of '{PrettyPrint(boxType)}'");
+                    SkipBox(stream, length);
+                    break;
             }
         }
     }
@@ -484,7 +488,9 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
                     properties.Add(new KeyValuePair<Heif4CharCode, object>(itemType, new object()));
                     break;
                 default:
-                    throw new ImageFormatException($"Unknown item type in property box of '{PrettyPrint(itemType)}'");
+                    // Unknown properties still occupy an ipco index and become an error only when marked essential.
+                    properties.Add(new KeyValuePair<Heif4CharCode, object>(itemType, UnknownProperty));
+                    break;
             }
         }
     }
@@ -546,6 +552,11 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
                 }
 
                 KeyValuePair<Heif4CharCode, object> prop = properties[(int)propertyIndex];
+                if (essential && ReferenceEquals(prop.Value, UnknownProperty))
+                {
+                    throw new InvalidImageContentException($"Item {itemId} associates unknown essential property '{PrettyPrint(prop.Key)}'.");
+                }
+
                 switch (prop.Key)
                 {
                     case Heif4CharCode.Ispe:
diff --git a/tests/ImageSharp.Tests/Formats/Heif/HeifDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Heif/HeifDecoderTests.cs
index 1675801bc3..0aca10f07e 100644
--- a/tests/ImageSharp.Tests/Formats/Heif/HeifDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Heif/HeifDecoderTests.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using System.Buffers.Binary;
 using SixLabors.ImageSharp.Formats.Heif;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -10,6 +11,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif;
 [ValidateDisposedMemoryAllocations]
 public class HeifDecoderTests
 {
+    private const uint UnknownBoxType = 0x74657374U;
+
     [Theory]
     [InlineData(TestImages.Heif.Image1, HeifCompressionMethod.Hevc, 3992, 2992)]
     [InlineData(TestImages.Heif.Sample640x427, HeifCompressionMethod.Hevc, 640, 428)]
@@ -42,4 +45,128 @@ public class HeifDecoderTests
         image.CompareToReferenceOutput(provider);
         Assert.Equal(HeifCompressionMethod.LegacyJpeg, heicMetadata.CompressionMethod);
     }
+
+    [Fact]
+    public void DecodeIgnoresUnknownTopLevelBox()
+    {
+        byte[] data = CreateEncodedContainer();
+        data = InsertBytes(data, data.Length, CreateUnknownBox());
+
+        using Image<Rgba32> image = Image.Load<Rgba32>(data);
+
+        Assert.Equal(new Size(2, 3), image.Size);
+    }
+
+    [Fact]
+    public void IdentifyIgnoresUnknownMetadataBox()
+    {
+        byte[] data = CreateEncodedContainer();
+        int metaOffset = FindBoxOffset(data, Heif4CharCode.Meta, 0, data.Length);
+        int metaSize = (int)BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(metaOffset));
+        data = InsertBytes(data, metaOffset + metaSize, CreateUnknownBox());
+        IncrementBoxSize(data, metaOffset, 8);
+
+        ImageInfo imageInfo = Image.Identify(data);
+
+        Assert.Equal(new Size(2, 3), imageInfo.Size);
+    }
+
+    [Fact]
+    public void IdentifyIgnoresUnknownNonEssentialProperty()
+    {
+        byte[] data = CreateContainerWithUnknownProperty(false);
+
+        ImageInfo imageInfo = Image.Identify(data);
+
+        Assert.Equal(new Size(2, 3), imageInfo.Size);
+    }
+
+    [Fact]
+    public void IdentifyRejectsUnknownEssentialProperty()
+    {
+        byte[] data = CreateContainerWithUnknownProperty(true);
+
+        InvalidImageContentException exception = Assert.Throws<InvalidImageContentException>(() => Image.Identify(data));
+
+        Assert.Contains("essential", exception.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    private static byte[] CreateEncodedContainer()
+    {
+        using Image<Rgba32> image = new(2, 3);
+        using MemoryStream stream = new();
+        image.Save(stream, new HeifEncoder());
+        return stream.ToArray();
+    }
+
+    private static byte[] CreateContainerWithUnknownProperty(bool essential)
+    {
+        byte[] data = CreateEncodedContainer();
+        int metaOffset = FindBoxOffset(data, Heif4CharCode.Meta, 0, data.Length);
+        int metaSize = (int)BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(metaOffset));
+        int iprpOffset = FindBoxOffset(data, Heif4CharCode.Iprp, metaOffset + 12, metaSize - 12);
+        int iprpSize = (int)BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(iprpOffset));
+        int ipcoOffset = FindBoxOffset(data, Heif4CharCode.Ipco, iprpOffset + 8, iprpSize - 8);
+        int ipcoSize = (int)BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(ipcoOffset));
+        int ipmaOffset = FindBoxOffset(data, Heif4CharCode.Ipma, iprpOffset + 8, iprpSize - 8);
+
+        // Insert the property before ipma so its one-based index is 2 and all parent box sizes remain explicit.
+        data = InsertBytes(data, ipcoOffset + ipcoSize, CreateUnknownBox());
+        IncrementBoxSize(data, metaOffset, 8);
+        IncrementBoxSize(data, iprpOffset, 8);
+        IncrementBoxSize(data, ipcoOffset, 8);
+        ipmaOffset += 8;
+
+        // The generated container has one item with one property association; append the unknown property to that entry.
+        int associationCountOffset = ipmaOffset + 18;
+        data[associationCountOffset]++;
+        int associationOffset = ipmaOffset + (int)BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(ipmaOffset));
+        byte association = (byte)(2 | (essential ? 0x80 : 0));
+        data = InsertBytes(data, associationOffset, new byte[] { association });
+        IncrementBoxSize(data, metaOffset, 1);
+        IncrementBoxSize(data, iprpOffset, 1);
+        IncrementBoxSize(data, ipmaOffset, 1);
+        return data;
+    }
+
+    private static byte[] CreateUnknownBox()
+    {
+        byte[] box = new byte[8];
+        BinaryPrimitives.WriteUInt32BigEndian(box, (uint)box.Length);
+        BinaryPrimitives.WriteUInt32BigEndian(box.AsSpan(4), UnknownBoxType);
+        return box;
+    }
+
+    private static int FindBoxOffset(ReadOnlySpan<byte> data, Heif4CharCode type, int offset, int length)
+    {
+        int endOffset = offset + length;
+        while (offset < endOffset)
+        {
+            int boxSize = (int)BinaryPrimitives.ReadUInt32BigEndian(data[offset..]);
+            Heif4CharCode boxType = (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(data[(offset + 4)..]);
+            if (boxType == type)
+            {
+                return offset;
+            }
+
+            offset += boxSize;
+        }
+
+        return -1;
+    }
+
+    private static byte[] InsertBytes(byte[] data, int offset, ReadOnlySpan<byte> inserted)
+    {
+        byte[] result = new byte[data.Length + inserted.Length];
+        data.AsSpan(0, offset).CopyTo(result);
+        inserted.CopyTo(result.AsSpan(offset));
+        data.AsSpan(offset).CopyTo(result.AsSpan(offset + inserted.Length));
+        return result;
+    }
+
+    private static void IncrementBoxSize(byte[] data, int offset, int increment)
+    {
+        uint size = BinaryPrimitives.ReadUInt32BigEndian(data.AsSpan(offset));
+        BinaryPrimitives.WriteUInt32BigEndian(data.AsSpan(offset), size + (uint)increment);
+    }
 }