SixLabors/ImageSharp · error · ImageFormatException

Invalid window size for ZLIB header: cinfo=

Error message

Invalid window size for ZLIB header: cinfo={cinfo}

What it means

The ZLIB header's CINFO field encodes the LZ77 window size (2^(CINFO+8)); RFC 1950 only defines values up to 7 (32 KiB). When inflating a critical chunk's ZLIB stream, a CINFO above 7 throws an ImageFormatException; for non-critical chunks the decoder just skips the chunk instead.

Solutions

1. Re-encode/re-save the PNG with a standard deflate encoder (zlib window size <= 32 KiB).
2. Recover the image bytes from a clean source; treat the file as corrupt.
3. Catch ImageFormatException and skip the image or fall back to another decoder.

// PNG chunk-level header check is not practical pre-call; validate file integrity instead:
// verify signature + chunk CRCs with a PNG validator before decoding.

try { using var img = Image.Load(path); }
catch (ImageFormatException ex) when (ex.Message.Contains("ZLIB header"))
{ logger.LogWarning(ex, "Corrupt zlib stream in {Path}", path); }

When it happens

Trigger: Decoding a PNG whose critical chunk (IDAT) carries a ZLIB stream with a corrupt or non-deflate compression method window size, e.g. manually re-compressed or maliciously crafted files.

Common situations: Files rewritten by broken encoders; data truncated/corrupted in transit so the header bytes are garbage; fuzzed or adversarial PNG inputs.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• Bad method for ZLIB header: cmf=
• Invalid compressed PNG metadata.
• Unknown filter type.
• The ANI sequence references a missing frame resource.
• The ANI file does not contain any decodable animation steps.

AI-assisted analysis of SixLabors/ImageSharp@59ce6af6fc (2026-09-13). Data as JSON: /api/errors/a32ee9621b15e8a0. Report an issue: GitHub.

Appendix: source

Thrown at src/ImageSharp/Compression/Zlib/ZlibInflateReader.cs:87

        int cmf = this.segmentStream.ReadByte();
        int flag = this.segmentStream.ReadByte();
        if (cmf == -1 || flag == -1)
        {
            return false;
        }

        if ((cmf & 0x0F) == 8)
        {
            // CINFO is the base-2 logarithm of the LZ77 window size, minus eight.
            int cinfo = (cmf & 0xF0) >> 4;

            if (cinfo > 7)
            {
                if (isCriticalChunk)
                {
                    // Values of CINFO above 7 are not allowed in RFC1950.
                    // CINFO is not defined in this specification for CM not equal to 8.
                    throw new ImageFormatException($"Invalid window size for ZLIB header: cinfo={cinfo}");
                }

                return false;
            }
        }
        else if (isCriticalChunk)
        {
            throw new ImageFormatException($"Bad method for ZLIB header: cmf={cmf}");
        }
        else
        {
            return false;
        }

        // The preset dictionary.
        bool fdict = (flag & 32) != 0;
        if (fdict)
        {

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