Defensive code when downloaded image stream is broken #201

Author: onizet

src/Html2OpenXml/IO/ImageHeader.cs

@@ -119,6 +119,12 @@ public static Size KeepAspectRatio(Size actualSize, Size preferredSize)
     private static FileType DetectFileType (SequentialBinaryReader reader)
     {
         byte[] magicBytes = new byte[MaxMagicBytesLength];
+        var availableBytes = reader.BaseStream.Length - reader.BaseStream.Position;
+        // reasonably, we can assume that if we are at the end of the stream and we read the header,
+        // the image content must be invalid or truncated.
+        if (availableBytes < MaxMagicBytesLength)
+            return FileType.Unrecognized;
+
         reader.Read(magicBytes, 0, MaxMagicBytesLength);
 
         var headerSpan = magicBytes.AsSpan();

test/HtmlToOpenXml.Tests/ImageFormats/ImageHeaderTests.cs

@@ -63,5 +63,15 @@ public ImageHeader.FileType GuessFormat_ReturnsFileType(string resourceName)
             Assert.That(success, Is.EqualTo(true));
             return guessType;
         }
+
+        [Test(ExpectedResult = ImageHeader.FileType.Unrecognized)]
+        public ImageHeader.FileType GuessFormat_WithEmpty_ReturnsFileType()
+        {
+            using var memoryStream = new MemoryStream();
+            bool success = ImageHeader.TryDetectFileType(memoryStream, out var guessType);
+
+            Assert.That(success, Is.EqualTo(false));
+            return guessType;
+        }
     }
 }