What should be the valid characters in usernames?

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Crafting Secure and Usable Usernames: A Guide to Valid Characters

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Explore the best practices and technical considerations for defining valid characters in usernames, balancing security, usability, and system compatibility.

Defining valid characters for usernames is a critical decision in any application or system design. It directly impacts security, user experience, and compatibility across different platforms and databases. While it might seem straightforward, a well-thought-out character set can prevent numerous issues, from injection vulnerabilities to user frustration. This article delves into the considerations for choosing appropriate username characters, offering guidance on what to allow and what to restrict.

Balancing Security and Usability

The primary challenge in defining username character sets is finding the right balance between security and usability. Overly restrictive rules can frustrate users and lead to forgotten credentials, while overly permissive rules can open doors to security vulnerabilities like SQL injection, cross-site scripting (XSS), or directory traversal. The goal is to allow enough flexibility for users to create memorable usernames without introducing unnecessary risks.

flowchart TD
    A[Start: Define Username Policy] --> B{Security Concerns?}
    B -- Yes --> C[Restrict Special Chars (e.g., <, >, ', ", &)]
    B -- No --> D{Usability Concerns?}
    D -- Yes --> E[Allow Common Special Chars (e.g., ., -, _)]
    D -- No --> F[Allow Alphanumeric Only]
    C --> G[Validate Input on Server-Side]
    E --> G
    F --> G
    G --> H[End: Implement & Test]

Decision flow for defining username character policies.

Generally, usernames should be composed of characters that are widely supported and do not carry special meaning in common programming languages, databases, or file systems. Here's a breakdown of character types:

Alphanumeric Characters

These are almost universally safe and recommended. They form the backbone of most usernames.

  • Uppercase letters: A-Z
  • Lowercase letters: a-z
  • Digits: 0-9

Common Special Characters

Certain special characters are often desired by users for readability and memorability. When allowing these, ensure proper sanitization and validation.

  • Hyphen/Dash: - (e.g., john-doe)
  • Underscore: _ (e.g., john_doe)
  • Period/Dot: . (e.g., john.doe)

Characters to Avoid or Restrict Heavily

These characters often have special meaning in various contexts and can lead to security vulnerabilities if not handled with extreme care. It's generally safer to disallow them.

  • Whitespace: Spaces can cause issues with parsing and command-line tools.
  • Forward Slash: / (often used in file paths)
  • Backslash: \ (often used in file paths and escape sequences)
  • Angle Brackets: < > (potential for XSS)
  • Quotes: ' " (potential for SQL injection)
  • Ampersand: & (HTML entities, command separators)
  • Pipe: | (command separators)
  • Parentheses/Brackets: () [] {} (can have special meaning in regex, programming)
  • Pound/Hash: # (comments, anchors)
  • Percent: % (URL encoding, format strings)
  • Asterisk: * (wildcards)
  • Question Mark: ? (wildcards, query parameters)
  • Exclamation Mark: ! (negation, command modifiers)
  • Colon/Semicolon: : ; (separators, command chaining)
  • At Symbol: @ (email addresses, mentions - if not an email username)

Unicode Characters

While allowing a broader range of Unicode characters can improve inclusivity, it introduces significant complexity:

  • Normalization: Different representations of the same character (e.g., é as a single character vs. e + combining acute accent) can lead to issues.
  • Homoglyph Attacks: Characters that look similar but are different (e.g., Latin a vs. Cyrillic а) can be exploited for phishing.
  • Database Support: Ensure your database and all layers of your application correctly handle the chosen Unicode range.

For most applications, sticking to a limited set of ASCII alphanumeric characters and a few safe symbols is the most robust approach.

Implementation Considerations

Beyond just defining the allowed characters, how you implement and enforce these rules is equally important.

function isValidUsername(username) {
  // Allows alphanumeric, hyphen, underscore, and period
  const regex = /^[a-zA-Z0-9._-]+$/;
  // Enforce length constraints (e.g., 3 to 20 characters)
  const minLength = 3;
  const maxLength = 20;

  if (!username || username.length < minLength || username.length > maxLength) {
    return false;
  }
  return regex.test(username);
}

console.log(isValidUsername("john.doe_123")); // true
console.log(isValidUsername("user-name"));    // true
console.log(isValidUsername("bad username!"));  // false
console.log(isValidUsername("admin'"));        // false

Example JavaScript function for username validation using a regular expression.

import re

def is_valid_username(username):
    # Allows alphanumeric, hyphen, underscore, and period
    regex = r"^[a-zA-Z0-9._-]+$"
    # Enforce length constraints (e.g., 3 to 20 characters)
    min_length = 3
    max_length = 20

    if not username or len(username) < min_length or len(username) > max_length:
        return False
    return bool(re.match(regex, username))

print(is_valid_username("john.doe_123")) # True
print(is_valid_username("user-name"))    # True
print(is_valid_username("bad username!"))  # False
print(is_valid_username("admin'"))        # False

Example Python function for username validation using a regular expression.

Best Practices for Username Policies

To ensure robust and user-friendly username handling, follow these best practices:

1. Define a Clear Character Set

Explicitly list the allowed characters. A common and safe set includes a-z, A-Z, 0-9, _, -, and ..

2. Enforce Length Constraints

Set minimum and maximum lengths (e.g., 3-20 characters) to prevent overly short or excessively long usernames, which can impact security and database performance.

3. Implement Server-Side Validation

Always validate usernames on the server to prevent malicious input, regardless of client-side checks.

4. Provide Clear User Feedback

Inform users about the valid character set and length requirements during registration to minimize frustration.

5. Consider Case Sensitivity

Decide if usernames should be case-sensitive or insensitive. If insensitive, normalize them (e.g., to lowercase) before storage and comparison.

6. Avoid Reserved Words

Prevent users from registering usernames that are reserved keywords in your system (e.g., admin, root, guest).