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. Pepper and the associated users should be archived in case if a pepper secret key is exposed to the public. The previous peppers will be needed for logging the affected users in.

[Mariusz Felisiak]

Duplicate of #30561.