spring-projects/spring-security · error · IllegalArgumentException

Invalid salt

Error message

Invalid salt

What it means

BCrypt.hashpw() validates that the salt string is at least 28 characters before parsing. A shorter string cannot contain the bcrypt modular format ($2a$NN$ + 22-char salt), so "Invalid salt" is thrown.

Source

Thrown at crypto/src/main/java/org/springframework/security/crypto/bcrypt/BCrypt.java:625

		BCrypt B;
		String real_salt;
		byte saltb[], hashed[];
		char minor = (char) 0;
		int rounds, off;
		StringBuilder rs = new StringBuilder();

		// Enforce max length for new passwords only
		if (!for_check && passwordb.length > 72) {
			throw new IllegalArgumentException("password cannot be more than 72 bytes");
		}
		if (salt == null) {
			throw new IllegalArgumentException("salt cannot be null");
		}

		int saltLength = salt.length();

		if (saltLength < 28) {
			throw new IllegalArgumentException("Invalid salt");
		}

		if (salt.charAt(0) != '$' || salt.charAt(1) != '2') {
			throw new IllegalArgumentException("Invalid salt version");
		}
		if (salt.charAt(2) == '$') {
			off = 3;
		}
		else {
			minor = salt.charAt(2);
			if ((minor != 'a' && minor != 'x' && minor != 'y' && minor != 'b') || salt.charAt(3) != '$') {
				throw new IllegalArgumentException("Invalid salt revision");
			}
			off = 4;
		}

		// Extract number of rounds
		if (salt.charAt(off + 2) > '$') {

View on GitHub (pinned to 96852e8860)

Solutions

1. Pass the FULL stored hash (including $2a$10$ prefix) as the salt when verifying
2. Store hashes in a column of at least 60 chars to avoid truncation
3. Use BCryptPasswordEncoder.matches(rawPassword, storedHash) rather than manual hashpw

Example fix

// before
String hash = BCrypt.hashpw(pw, storedHash.substring(0, 22));
// after
String hash = BCrypt.hashpw(pw, storedHash); // full 60-char hash string

if (salt == null || salt.length() < 28 || !salt.startsWith("$2")) {
    throw new IllegalArgumentException("Not a valid bcrypt salt string");
}

boolean isBcryptFormat(String s) {
    return s != null && s.length() >= 28 && s.startsWith("$2");
}

try {
    hash = BCrypt.hashpw(pw, salt);
} catch (IllegalArgumentException e) {
    if (e.getMessage().equals("Invalid salt")) { /* treat as corrupt record / force reset */ }
}

When it happens

Trigger: Passing a truncated hash, a raw 22-character base64 salt without the $2a$ prefix, an empty string, or a plain-text value to hashpw as the salt parameter.

Common situations: Database column sized too small so stored hashes got truncated; slicing only the salt portion out of a stored hash then passing it back for verification; storing the result of gensalt incorrectly.

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

• Invalid salt version
• Invalid salt revision
• Missing salt rounds
• Bad salt length
• salt cannot be null

AI-assisted analysis of spring-projects/spring-security@96852e8860 (2026-09-10). Data as JSON: /api/errors/860b8e938fa45b53. Report an issue: GitHub.