Andrea Cardaci — 22 December 2018
This post is based on the README of the mysql-unsha1 (March 2017) project that also contains the handshake sniffer and a patch for the MySQL client, go here if you just need the tools.
This has also been featured on the SANS Internet Storm Center podcast.
This PoC shows how it is possible to authenticate against a MySQL server under certain circumstances without knowing the cleartext password when the Secure Password Authentication authentication plugin (aka mysql_native_password, the default method) is used.
Preconditions are:
to obtain a read-only access to the mysql.user table in the target database in order to fetch the hashed password for a given user;
to be able to intercept a successful authentication handshake performed by the aforementioned user (i.e., authentication via SSL would nullify this attempt).
The above are quite strong propositions, but this attack could provide an alternative way to obtain database access in a completely stealth way.
By default, passwords are stored in the mysql.user table and are hashed using the PASSWORD function which is just a two-stage SHA1 digest:
mysql> SELECT DISTINCT password FROM mysql.user WHERE user = 'root';
*2470C0C06DEE42FD1618BB99005ADCA2EC9D1E19
mysql> SELECT PASSWORD('password');
*2470C0C06DEE42FD1618BB99005ADCA2EC9D1E19
mysql> SELECT SHA1(UNHEX(SHA1('password')));
2470c0c06dee42fd1618bb99005adca2ec9d1e19
After the TCP connection phase, initiated by the client, the MySQL authentication handshake continues as follows (simplified):
the server sends a Server Greeting packet containing a salt (s);
the client replies with a Login Request packet containing the session password (x), computed as follows:
x := SHA1(password) XOR SHA1(s + SHA1(SHA1(password)))
where password is the cleartext password as provided by the user and + is a mere string concatenation operator;
the server can verify the challenge and authenticate the client if:
SHA1(x XOR SHA1(s + SHA1(SHA1(password)))) = SHA1(SHA1(password))
where SHA1(SHA1(password)) is the two-stage SHA1 digest of the password, stored in the mysql.user table; the server does not know the cleartext password nor its SHA1 digest.
With enough information an attacker is able to obtain SHA1(password) and therefore to solve the server challenge without the knowledge of the cleartext password.
Let:
h be the hashed password obtained from the mysql.user table (i.e., SHA1(SHA1(password)));
s and x be the salt and the session password respectively obtained from the intercepted handshake.
The first-stage SHA1 can be obtained as follows:
SHA1(password) = x XOR SHA1(s + h)
mysql-unsha1 comes with two tools:
a simple sniffer to extract and check the handshake information either live or offline from a PCAP file;
a patch for MySQL client which allows to treat the prompted passwords as SHA1 digests instead of cleartexts.
Refer to the GitHub repository for compilation and usage instructions.