Add check for valid tga image type in the format detector

src/ImageSharp/Formats/Tga/TgaConstants.cs

@@ -16,5 +16,10 @@ namespace SixLabors.ImageSharp.Formats.Tga
         /// The list of file extensions that equate to a targa file.
         /// </summary>
         public static readonly IEnumerable<string> FileExtensions = new[] { "tga", "vda", "icb", "vst" };
+
+        /// <summary>
+        /// The file header length of a tga image in bytes.
+        /// </summary>
+        public const int FileHeaderLength = 18;
     }
 }

src/ImageSharp/Formats/Tga/TgaFileHeader.cs

@@ -17,7 +17,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
         /// <summary>
         /// Defines the size of the data structure in the targa file.
         /// </summary>
-        public const int Size = 18;
+        public const int Size = TgaConstants.FileHeaderLength;
 
         public TgaFileHeader(
             byte idLength,

src/ImageSharp/Formats/Tga/TgaImageFormatDetector.cs

@@ -11,7 +11,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
     public sealed class TgaImageFormatDetector : IImageFormatDetector
     {
         /// <inheritdoc/>
-        public int HeaderSize => 18;
+        public int HeaderSize => TgaConstants.FileHeaderLength;
 
         /// <inheritdoc/>
         public IImageFormat DetectFormat(ReadOnlySpan<byte> header)
@@ -21,7 +21,14 @@ namespace SixLabors.ImageSharp.Formats.Tga
 
         private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
         {
-            return header.Length >= this.HeaderSize;
+            if (header.Length >= this.HeaderSize)
+            {
+                // There is no magick bytes in a tga file, so at least the image type in the header will be checked for a valid value.
+                var imageType = (TgaImageType)header[2];
+                return imageType.IsValid();
+            }
+
+            return true;
         }
     }
 }

src/ImageSharp/Formats/Tga/TgaImageTypeExtensions.cs

@@ -22,5 +22,30 @@ namespace SixLabors.ImageSharp.Formats.Tga
 
             return false;
         }
+
+        /// <summary>
+        /// Checks, if the image type has valid value.
+        /// </summary>
+        /// <param name="imageType">The image type.</param>
+        /// <returns>true, if its a valid tga image type.</returns>
+        public static bool IsValid(this TgaImageType imageType)
+        {
+            byte imageTypeVal = (byte)imageType;
+
+            switch (imageTypeVal)
+            {
+                case 0:
+                case 1:
+                case 2:
+                case 3:
+                case 9:
+                case 10:
+                case 11:
+                    return true;
+
+                default:
+                    return false;
+            }
+        }
     }
 }