gchq/CyberChef · error · OperationError

Badly formatted IPv6 address.

Error message

Badly formatted IPv6 address.

What it means

Thrown by strToIpv6's inner parseBlocks when `ipStr.split(":")` yields fewer than 3 or more than 8 parts. Valid IPv6 text (including a single '::' compression) splits to 3-8 colon-separated groups; anything outside that is malformed. OperationError.

Source

Thrown at src/core/lib/IP.mjs:370

        ipv6 = new Array(8);

    for (let i = 0; i < 8; i++) {
        if (isNaN(numBlocks[j])) {
            ipv6[i] = 0;
            if (i === (8-numBlocks.slice(j).length)) j++;
        } else {
            ipv6[i] = numBlocks[j];
            j++;
        }
    }
    return ipv6;

    /**
     * Converts a list of 3-8 numeric hex strings in the range 0-65535 to a list of numbers.
     */
    function parseBlocks(blocks) {
        if (blocks.length < 3 || blocks.length > 8)
            throw new OperationError("Badly formatted IPv6 address.");
        const numBlocks = [];
        for (let i = 0; i < blocks.length; i++) {
            numBlocks[i] = parseInt(blocks[i], 16);
            if (numBlocks[i] < 0 || numBlocks[i] > 65535)
                throw new OperationError("Block out of range.");
        }
        return numBlocks;
    }
}

/**
 * Converts an IPv6 address from numerical array format to string format.
 *
 * @param {number[]} ipv6
 * @param {boolean} compact - Whether or not to return the address in shorthand or not
 * @returns {string}
 *
 * @example

View on GitHub (pinned to 4290ea7539)

Solutions

1. Validate the string against an IPv6 regex before calling strToIpv6.
2. Ensure 3-8 colon-separated groups after splitting.
3. Route IPv4 input to strToIpv4.

Example fix

// before
strToIpv6("ff00");

// after
strToIpv6("ff00::");

function assertIpv6Shape(ipStr) {
  const blocks = ipStr.split(":");
  if (blocks.length < 3 || blocks.length > 8) {
    throw new Error(`IPv6 must split into 3-8 colon groups, got ${blocks.length}`);
  }
}
assertIpv6Shape(ipStr);
strToIpv6(ipStr);

const isIpv6Shape = ipStr => {
  const n = ipStr.split(":").length;
  return n >= 3 && n <= 8;
};

try {
  strToIpv6(ipStr);
} catch (err) {
  if (err instanceof OperationError && /Badly formatted IPv6 address/.test(err.message)) {
    // wrong number of colon groups; correct the input
  } else throw err;
}

When it happens

Trigger: Calling strToIpv6('ff00') (1 block), '1:2:3:4:5:6:7:8:9' (9 blocks), or an IPv4 address mistakenly passed in. Note 'ff00::' splits to ['ff00','',''] = 3 and is accepted.

Common situations: Missing colons; too many groups (>8); IPv4 address routed to the IPv6 parser; an address missing the '::' compression entirely.

Related errors

• IPv6 CIDR must be less than 128
• More than 4 blocks.
• Invalid input. Enter either a CIDR range (e.g. 10.0.0.0/24)
• Invalid IPv6 address
• Byte length must be a positive integer

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/1b4efeefd52c18a3. Report an issue: GitHub.