What causes "Invalid gif colormap size '{result.GlobalColorTableSize}'" in SixLabors/ImageSharp?

Re-obtain the GIF from a known-good source; the header packed field is corrupt

SixLabors/ImageSharp · error · ImageFormatException

Invalid gif colormap size

Error message

Invalid gif colormap size '{result.GlobalColorTableSize}'

What it means

Thrown when parsing the GIF Logical Screen Descriptor if the computed global color table size exceeds the maximum the format and this decoder allow (255 * 4 entries). A larger value indicates corrupt packed-field bits or a non-conformant file. Raised as ImageFormatException during header parsing.

Solutions

Re-obtain the GIF from a known-good source; the header packed field is corrupt
Validate the file (e.g. magick identify) before processing to reject malformed inputs early
Catch ImageFormatException at your ingest boundary and quarantine the file
If files come from an untrusted pipeline, sanitize/re-encode them before ImageSharp decode

Example fix

// before
var image = Image.Load(stream);
// after
try { var image = Image.Load(stream); }
catch (ImageFormatException ex) { quarantine(path, ex); }

Defensive patterns

Strategy: validation

Validate before calling

// Check the global color table size bits in the packed field before decode
if (stream.CanSeek)
{
    var head = new byte[13]; long pos = stream.Position; stream.Read(head, 0, 13); stream.Position = pos;
    if ((head[10] & 0x80) != 0 && ((head[10] & 0x07) > 7)) throw new InvalidDataException("Malformed GIF color table size");
}

Try / catch

try { return Image.Load(stream); }
catch (ImageFormatException ex) when (ex.Message.StartsWith("Invalid gif colormap size")) { Quarantine(path, ex); return null; }

Prevention

Quarantine files failing header validation instead of crashing batch jobs
Re-encode untrusted GIFs before decoding
Validate uploads with magick identify or Image.Identify first
Treat this as corruption — always re-fetch from source

When it happens

Trigger: Image.Load/Decode on a GIF whose packed field's global-color-table-size bits decode to a value greater than 255*4 — a corrupt or maliciously malformed header.

Common situations: Bit-flipped/corrupt headers from damaged storage, hand-crafted fuzzed GIFs, or files passed through transformers that mangle the packed byte.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

AI-assisted analysis of SixLabors/ImageSharp@59ce6af6fc (2026-09-13). Data as JSON: /api/errors/afaff05c7b6201fc. Report an issue: GitHub.

Appendix: source

Thrown at src/ImageSharp/Formats/Gif/Sections/GifLogicalScreenDescriptor.cs:98

    /// Gets the color depth, in number of bits per pixel.
    /// The lowest 3 packed bits represent the bit depth minus 1.
    /// </summary>
    public int BitsPerPixel => (this.Packed & 0x07) + 1;

    public void WriteTo(Span<byte> buffer)
    {
        ref GifLogicalScreenDescriptor dest = ref Unsafe.As<byte, GifLogicalScreenDescriptor>(ref MemoryMarshal.GetReference(buffer));

        dest = this;
    }

    public static GifLogicalScreenDescriptor Parse(ReadOnlySpan<byte> buffer)
    {
        GifLogicalScreenDescriptor result = MemoryMarshal.Cast<byte, GifLogicalScreenDescriptor>(buffer)[0];

        if (result.GlobalColorTableSize > 255 * 4)
        {
            throw new ImageFormatException($"Invalid gif colormap size '{result.GlobalColorTableSize}'");
        }

        return result;
    }

    public static byte GetPackedValue(bool globalColorTableFlag, int colorResolution, bool sortFlag, int globalColorTableSize)
    {
        /*
        Global Color Table Flag    | 1 Bit
        Color Resolution           | 3 Bits
        Sort Flag                  | 1 Bit
        Size of Global Color Table | 3 Bits
        */

        byte value = 0;

        if (globalColorTableFlag)
        {

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