owasp-amass/amass · error

improper mask used

Error message

improper mask used: %s

What it means

ExpandMask only understands specific mask characters (e.g. 'u' upper, 'd' digit, 'l' letters, 's' special); anything else after '?' hits the default branch and returns this error. It signals that the mask placeholder syntax is not recognized by this implementation.

Solutions

1. Use only the supported placeholders: u, d, l, s
2. Check config/wordlist.go for the exact maskLetters/maskSpecial/upper/digit sets
3. Pre-validate mask strings before expansion

Example fix

// before
words, err := ExpandMask("?a?d")
// after
words, err := ExpandMask("?s?d")

var validPlaceholders = map[byte]bool{'u':true,'d':true,'l':true,'s':true}
func isValidMaskSyntax(w string) bool {
  for i := 0; i < len(w); i++ { if w[i] == '?' && i+1 < len(w) && !validPlaceholders[w[i+1]] { return false } }
  return true
}

func isSupportedPlaceholder(c byte) bool { return c=='u'||c=='d'||c=='l'||c=='s' }

words, err := ExpandMask(m)
if err != nil {
  if strings.HasPrefix(err.Error(), "improper mask used") {
    // report the unsupported placeholder and skip this entry
  }
  return err
}

When it happens

Trigger: Calling ExpandMask with an unsupported placeholder such as ?a, ?h, or ?1 that the hashcat-style subset in this library does not implement.

Common situations: Copying hashcat masks that use the full hashcat charset into this simpler expander; typos like ?L or ?d?; invalid characters in a wordlist entry intended as a mask.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• exceeded maximum mask size (3)
• error gz-reading the file
• error opening the file
• error reading the first 512 bytes from
• error rewinding the file

AI-assisted analysis of owasp-amass/amass@79299dce87 (2026-09-06). Data as JSON: /api/errors/69eca36c5e2be5e3. Report an issue: GitHub.

Appendix: source

Thrown at config/wordlist.go:51

		return expanded, fmt.Errorf("exceeded maximum mask size (3): %s", word)
	}

	parts := strings.SplitN(word, "?", 2)
	if len(parts) > 1 {
		if len(parts[1]) > 0 {
			switch parts[1][0] {
			case 'a':
				chars = maskLetters + maskDigits + maskSpecial
			case 'd':
				chars = maskDigits
			case 'u':
				fallthrough
			case 'l':
				chars = maskLetters
			case 's':
				chars = maskSpecial
			default:
				return expanded, fmt.Errorf("improper mask used: %s", word)
			}
			for _, ch := range chars {
				newWord := parts[0] + string(ch) + parts[1][1:]
				nextRound, err := ExpandMask(newWord)
				if err != nil {
					return expanded, err
				}
				expanded = append(expanded, nextRound...)
			}
		}
	} else {
		expanded = append(expanded, word)
	}
	return expanded, nil
}

// ExpandMaskWordlist performs ExpandMask on a slice of words.
func ExpandMaskWordlist(wordlist []string) ([]string, error) {

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