SixLabors/ImageSharp · error · ImageFormatException

Invalid Webp data, could not read chunk size.

Error message

Invalid Webp data, could not read chunk size.

What it means

WebpChunkParsingUtils.ReadPaddedChunkSize throws ImageFormatException when it cannot read the 4-byte chunk size field and the chunk is marked required. RIFF/WebP chunk sizes are padded to even lengths; a short read here means the stream ends before the size field completes, i.e. truncated or malformed WebP data.

Solutions

1. Validate the file is complete (byte length vs expected) and re-download if truncated.
2. Wrap Image.Load/Identify in try/catch (ImageFormatException) to handle corrupt input gracefully.
3. Ensure the stream position is at a valid chunk boundary (chunk type + size are 8-byte aligned units).
4. For network sources, fully buffer before decoding.

Example fix

// before
using var image = Image.Identify(partialStream); // throws on truncated chunk
// after
try { var info = Image.Identify(partialStream); }
catch (ImageFormatException ex) { log.Warn($"WebP truncated/corrupt: {ex.Message}"); }

// require a plausible minimum size and RIFF signature
byte[] head = new byte[12];
stream.ReadExactly(head);
bool ok = head[0..4] is "RIFF" && head[8..12] is "WEBP";
stream.Position = 0;

try { var info = await Image.IdentifyAsync(stream); }
catch (ImageFormatException ex) { log.Warn(ex, "WebP chunk size unreadable (truncated?)"); }

When it happens

Trigger: Decoding/Identify on a WebP stream that ends within a chunk header where required=true; stream.Read returns fewer than 4 bytes for the size.

Common situations: Partially downloaded WebP files; upload truncation; seeking to a wrong offset so the reader lands near end-of-stream; corrupted storage objects.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

• Invalid Webp data, could not read chunk type.
• Invalid Webp data, could not read unsigned 24 bit integer.
• The ANI file does not contain any frame resources.
• The stream does not contain an ANI RIFF container.
• The ANI RIFF container size is invalid.

AI-assisted analysis of SixLabors/ImageSharp@59ce6af6fc (2026-09-13). Data as JSON: /api/errors/ac69ce594102be31. Report an issue: GitHub.

Appendix: source

Thrown at src/ImageSharp/Formats/Webp/WebpChunkParsingUtils.cs:340

    /// <summary>
    /// Reads a chunk's complete padded extent without wrapping a uint-sized payload length.
    /// </summary>
    /// <param name="stream">The input stream.</param>
    /// <param name="buffer">The four-byte size buffer.</param>
    /// <param name="required">Whether an incomplete size field is an error.</param>
    /// <returns>The padded extent, or remaining bytes when an optional size field is incomplete.</returns>
    private static ulong ReadPaddedChunkSize(BufferedReadStream stream, Span<byte> buffer, bool required)
    {
        if (stream.Read(buffer) is 4)
        {
            uint chunkSize = BinaryPrimitives.ReadUInt32LittleEndian(buffer);
            return (ulong)chunkSize + (chunkSize & 1);
        }

        if (required)
        {
            throw new ImageFormatException("Invalid Webp data, could not read chunk size.");
        }

        // Return the size of the remaining data in the stream.
        return (ulong)stream.RemainingBytes;
    }

    /// <summary>
    /// Identifies the chunk type from the chunk.
    /// </summary>
    /// <param name="stream">The stream to read the data from.</param>
    /// <param name="buffer">Buffer to store the data read from the stream.</param>
    /// <exception cref="ImageFormatException">
    /// Thrown if the input stream is not valid.
    /// </exception>
    public static WebpChunkType ReadChunkType(BufferedReadStream stream, Span<byte> buffer)
    {
        if (stream.Read(buffer) == 4)
        {

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