Peppering user passwords

[Fatih Erikli]

Currently a user's password stored in database in this format:

<algorithm>$<iterations>$<salt>$<hash>

Hash (Last column) is the password hashed with the salt in previous column.

Example, these are two computed passwords, stored on database, for the same password of two users.

pbkdf2_sha256$600000$fb9cUsHWK4EMZ7VWGBAcGD$2uwiVefFwanIhxLrv+/t3sKvP4X6tDKEMw/ysHD5dIc=
pbkdf2_sha256$600000$HgWHWrF2qQD9Owj4XeEkjY$rh0qzfo+/ZCzWbL9ZJa8aKhiO5xoEMfT4EtP/+A+LzI=

The password is 123456.

Imagine I am an attacker, who got the database of different django project, I want to look up the users who have chosen the password 123456.

I have the salts of users stored in database.

make_password('123456', 'fb9cUsHWK4EMZ7VWGBAcGD')
pbkdf2_sha256$600000$fb9cUsHWK4EMZ7VWGBAcGD$2uwiVefFwanIhxLrv+/t3sKvP4X6tDKEMw/ysHD5dIc=

make_password('123456', 'HgWHWrF2qQD9Owj4XeEkjY')
pbkdf2_sha256$600000$HgWHWrF2qQD9Owj4XeEkjY$rh0qzfo+/ZCzWbL9ZJa8aKhiO5xoEMfT4EtP/+A+LzI=

These are correct password combinations. I am able to lookup the users who have their passwords exposed in public.

Password 123456 is not a possible case in Django, since the password fields have a complexity validation. However, the salt is available to the attacker when a database is stolen. Salt could be used

• to hash the raw password pair in a rainbow table.
• to hash the already exposed passwords.

There is one more element needed for hashing the password, pepper, should be project specific. When a database is exposed in public, the attacker will not be able to lookup the passwords, since they don't have the secret pepper key.

I am not sure about the vulnerability enumeration, but CWE-760 seems closer. Salt is not weak, but it is known. The salt is stored next to the hashed password.

This is a case of when a database is stolen, however I think Django, by default, should do everything that could be done at the framework level to keep the user information secured.

[Fatih Erikli]

Here is an example hasher:

# yourapp.hashers.py
class PepperedPBKDF2PasswordHasher(PBKDF2PasswordHasher):
    algorithm = "pepperred_pbkdf2_sha256"

    def encode(self, password, salt, iterations=None):
        iterations = iterations or self.iterations
        hash = pbkdf2(password, salt, iterations, digest=self.digest)
        hash = base64.b64encode(
            hash+settings.PASSWORD_PEPPER).decode('ascii').strip()
        return "%s$%d$%s$%s" % (self.algorithm, iterations, salt, hash)

In settings:

PASSWORD_PEPPER = b'4545randombytes342445'
PASSWORD_HASHERS = [
    "yourapp.hashers.PepperedPBKDF2PasswordHasher",
    "django.contrib.auth.hashers.PBKDF2PasswordHasher",
    "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
    "django.contrib.auth.hashers.Argon2PasswordHasher",
    "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
    "django.contrib.auth.hashers.ScryptPasswordHasher",
]

The passwords will break when a pepper is changed. The affected users should be associated with the pepper in case if it is exposed to the public. They will be needed for logging the users in, however they should be redirected to password change page as soon as they logged in for creating a password with the new pepper.

[Mariusz Felisiak]

Duplicate of #30561.