gchq/CyberChef · error · OperationError

Unrecognised input format.

Error message

Unrecognised input format.

What it means

The Parse IPv4 Header operation accepts only 'Hex' or 'Raw' as the input format argument. This defensive else-branch fires for any other value. In the UI, the dropdown prevents invalid selection; this only triggers with programmatic or hand-edited recipe configurations.

Source

Thrown at src/core/operations/ParseIPv4Header.mjs:62

    }

    /**
     * @param {string} input
     * @param {Object[]} args
     * @returns {html}
     */
    run(input, args) {
        const format = args[0];
        const outputFormat = args[1];

        let output;

        if (format === "Hex") {
            input = fromHex(input);
        } else if (format === "Raw") {
            input = new Uint8Array(Utils.strToArrayBuffer(input));
        } else {
            throw new OperationError("Unrecognised input format.");
        }

        let ihl = input[0] & 0x0f;
        const dscp = (input[1] >>> 2) & 0x3f,
            ecn = input[1] & 0x03,
            length = input[2] << 8 | input[3],
            identification = input[4] << 8 | input[5],
            flags = (input[6] >>> 5) & 0x07,
            fragOffset = (input[6] & 0x1f) << 8 | input[7],
            ttl = input[8],
            protocol = input[9],
            checksum = input[10] << 8 | input[11],
            srcIP = input[12] << 24 | input[13] << 16 | input[14] << 8 | input[15],
            dstIP = input[16] << 24 | input[17] << 16 | input[18] << 8 | input[19],
            checksumHeader = [...input.slice(0, 10), 0, 0, ...input.slice(12, 20)];
        let version = (input[0] >>> 4) & 0x0f,
            options = [];

View on GitHub (pinned to 4290ea7539)

Solutions

1. Set the 'Input format' argument to either 'Hex' or 'Raw'
2. Validate format values programmatically before recipe execution

const VALID_FORMATS = ["Hex", "Raw"];
if (!VALID_FORMATS.includes(args[0])) {
  throw new Error(`Input format must be one of: ${VALID_FORMATS.join(", ")}`);
}

function isIPv4InputFormat(v) {
  return v === "Hex" || v === "Raw";
}

When it happens

Trigger: args[0] is a string other than 'Hex' or 'Raw'. Occurs when constructing recipes programmatically with an invalid format value, or hand-editing recipe JSON.

Common situations: Programmatically building a recipe with a typo in the format argument. Recipe JSON edited by hand with an incorrect value. Recipe copied between CyberChef versions with different option sets.

Related errors

• Invalid input format selected.
• Unrecognised input format.
• CIDR must be less than 32 for IPv4 or 128 for IPv6
• Invalid input format.
• IPv4 CIDR must be less than 32

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/09da07814a45d1c9. Report an issue: GitHub.