Your Signup Form Probably Rejects ITIN Holders. Here's How to Fix It

If your product asks for a U.S. tax ID (payroll, lending, banking, tax prep, marketplaces paying out sellers), there is a good chance your form quietly rejects a whole group of real customers: people who file taxes with an ITIN instead of a Social Security Number.

An ITIN (Individual Taxpayer Identification Number) is issued by the IRS to people who must file U.S. taxes but can't get an SSN. It looks exactly like an SSN, which is precisely why so many validators get it wrong.

This post is a checklist of the bugs I see most often, with code you can drop in.

1. Your SSN validator is rejecting every ITIN

ITINs are nine digits, formatted like an SSN (9XX-XX-XXXX), and always start with 9. SSNs, on the other hand, are never issued with an area number from 900 to 999. So the classic SSN rule "reject anything starting with 9" is correct for SSNs and fatal for ITINs.

The fix is to validate a taxpayer ID, not an SSN, and to classify it:

import re

TIN = re.compile(r"^(\d{3})-?(\d{2})-?(\d{4})$")
ITIN_GROUPS = [range(50, 66), range(70, 89), range(90, 93), range(94, 100)]


def classify_tin(value: str) -> str | None:
    """Return "SSN", "ITIN", or None if the number cannot be valid."""
    match = TIN.match(value.strip())
    if not match:
        return None
    area, group, serial = match.groups()
    if area.startswith("9"):
        return "ITIN" if any(int(group) in r for r in ITIN_GROUPS) else None
    if area in ("000", "666") or group == "00" or serial == "0000":
        return None
    return "SSN"

The ITIN check uses the 4th and 5th digits: the IRS issues ITINs with those digits in the ranges 50 to 65, 70 to 88, 90 to 92, and 94 to 99. Keep that list in one place so it is easy to update if the IRS expands it again.

The same logic on the client, so people get instant feedback:

const ITIN_GROUPS = [[50, 65], [70, 88], [90, 92], [94, 99]];

export function classifyTin(value) {
  const m = value.trim().match(/^(\d{3})-?(\d{2})-?(\d{4})$/);
  if (!m) return null;
  const [, area, group, serial] = m;
  if (area.startsWith("9")) {
    const g = Number(group);
    return ITIN_GROUPS.some(([lo, hi]) => g >= lo && g <= hi) ? "ITIN" : null;
  }
  if (area === "000" || area === "666" || group === "00" || serial === "0000") return null;
  return "SSN";
}

Always repeat the check on the server. Client-side validation is a convenience, not a control.

Test the edges

A handful of unit tests catches most regressions. Make sure your suite covers at least:

• A number starting with 9 whose 4th and 5th digits are in an ITIN range is accepted as an ITIN
• A number starting with 9 whose 4th and 5th digits are outside those ranges (for example 49 or 93) is rejected
• 000, 666, a 00 group and a 0000 serial are rejected as SSNs
• The same number is accepted with and without dashes, and with surrounding spaces

2. Your label tells people they don't belong

A field labeled "Social Security Number" tells an ITIN holder to leave. Even if your backend accepts ITINs, many people will abandon the form at that label.

• Label the field "SSN or ITIN" or "Taxpayer ID (SSN or ITIN)"
• Add one line of help text: "We accept either a Social Security Number or an ITIN."
• Don't make people pick the ID type from a dropdown if you can detect it from the number
• Use inputmode="numeric" and autocomplete="off", and accept the number with or without dashes

3. Your name and address fields assume one culture

Many ITIN holders have names that break naive validation: two surnames, accents, apostrophes, or a single name.

• Store names as Unicode and never strip accents ("María José García López" is one valid name)
• Prefer a single full name field plus an optional "what should we call you?" field
• Don't require a "last name" or reject hyphens and apostrophes
• If you need a mailing address, allow non-U.S. addresses and don't force a U.S. state

4. Your business logic treats an ITIN like an SSN

They look the same, but they are not interchangeable, and the differences matter for your product rules:

• An ITIN is not work authorization. Don't use it for I-9 or employment eligibility checks.
• ITINs expire. An ITIN that isn't used on a federal tax return at least once in three consecutive years expires and has to be renewed. If your product depends on a valid ITIN, plan a friendly "you may need to renew" path instead of a hard failure.
• Some financial products accept ITINs, some don't. Make that a product decision made explicitly, not an accident of your validator.

5. You store it like a username

An ITIN is as sensitive as an SSN. Treat it that way:

1. Encrypt it at rest, separately from the rest of the user record
2. Show only the last four digits after it is saved (***-**-1234)
3. Keep it out of logs, analytics events, URLs, and error trackers
4. Restrict who and what can read the full value, and audit those reads

The short checklist

• Validate a taxpayer ID, and classify it as SSN or ITIN instead of rejecting numbers that start with 9
• Label the field "SSN or ITIN"
• Accept real-world names and non-U.S. addresses
• Model ITIN-specific rules (no work authorization, expiration) explicitly
• Protect it like an SSN

None of this is hard, and it opens your product to millions of taxpayers that a one-line regex was turning away.

If you have hit other ITIN edge cases in production, share them in the comments. I'll fold the good ones into this checklist.