CORE SDI

                                   http://www.core-sdi.com



                   Vulnerability Report for MySQL Authentication

Vulnerability





Date Published: 2000-10-23



Advisory ID: CORE-20001023



Bugtraq ID: 1826



CVE CAN: Not currently assigned.



Title: MySQL Authentication Vulnerability



Class: Design Error



Remotely Exploitable: Yes



Locally Exploitable: No





Vulnerability Description:



 The "MySQL Database Engine" uses an authentication scheme designed

 to prevent the flow of plaintext passwords over the network and

 the storage of them in plaintext. For that purpose a challenge-response

 mechanism for authentication has been implemented on all versions

 of MySQL. Slight variations are to be found between version 3.20

 and 3.21 and above.



 Regrettably, this authentication mechanism is not cryptographically

 strong. Specifically, each time a user executes this mechanism,

 information allowing an attacker to recover this user's password is

 leaked. Using an attack of our design, described in the "Technical

 details" section of this advisory,  an eavesdropper is able to recover

 the user's password after witnessing only a few executions of this

 protocol, and thence is able to authenticate to the database engine

 impersonating a valid user.



Vulnerable Packages/Systems:

  All versions of MySQL



Solution/Vendor Information/Workaround:



 The vendor is aware of the problems described and suggests

 encrypting the traffic between client and server to prevent

 exploitation.

 For further details refer to:



http://www.mysql.com/documentation/mysql/commented/manual.php?section=Securi

ty



 Plans to implement a stronger authentication mechanism are being

 discussed for future versions of MySQL.



 Additionally, advisories and information on security issues

 in MySQL can be obtained from:



        http://www.securityfocus.com/bid/1147

        http://www.securityfocus.com/bid/975

        http://www.securityfocus.com/bid/926



Vendor notified on: October 19th, 2000



Credits:



 These vulnerabilities were found and researched by Ariel "Wata"

 Waissbein, Emiliano Kargieman, Carlos Sarraute, Gerardo Richarte and

 Agustin "Kato" Azubel of CORE SDI, Buenos Aires, Argentina.



 This advisory was drafted with the help of the SecurityFocus.com

 Vulnerability Help Team. For more information or assistance drafting

 advisories please mail vulnhelp@securityfocus.com.



Technical Description - Exploit/Concept Code:



 1. The challenge/response mechanism



 The challenge-response mechanism devised in MySQL does the following:

 From mysql-3.22.32/sql/password.c:



 /***********************************************************************

 The main idea is that no passwords are sent between client & server on

 connection and that no passwords are saved in mysql in a decodable

 form.



 MySQL provides users with two primitives used for authentication: a

 hash function and a (supposedly) random generator. On connection a random

 string is generated by the server and sent to the client. The client,

 using as input the hash value of the random string he has received and

 the hash value of his password, calculates a new string using the

 random generator primitive.

 This 'check' string is sent to the server, where it is compared with a

 string generated from the stored hash_value of the password and the

 random string.



 The password is saved (in user.password) by using the PASSWORD()

 function in mysql.



  Example:

    update user set password=PASSWORD("hello") where user="test"

  This saves a hashed number as a string in the password field.

  **********************************************************************/



 To accomplish that purpose several functions and data structures are

  implemented:



  mysql-3.22.32/include/mysql_com.h:

   struct rand_struct {

    unsigned long seed1,seed2,max_value;

    double max_value_dbl;

   };



  mysql-3.22.32/sql/password.c:

   void randominit(struct rand_struct *rand_st,ulong seed1, ulong seed2)

    Initializes the PRNG, used by versions 3.21 and up



   static void old_randominit(struct rand_struct *rand_st,ulong seed1)

    Initializes the PRNG, used by versions up to 3.20



  double rnd(struct rand_struct *rand_st)

    Provides a random floating point (double) number taken from

    the PRNG between 0 and rand_st->max_value



  void hash_password(ulong *result, const char *password)

    Calculates a hash of a password string and stores it

    in 'result'.



  void make_scrambled_password(char *to,const char *password)

    Hashes and stores the password in a readable form in 'to'



  char *scramble(char *to,const char *message,const char *password,

               my_bool old_ver)

    Genererate a new message based on message and password

    The same thing is done in client and server and the results are

    checked.



  my_bool check_scramble(const char *scrambled, const char *message,

                       ulong *hash_pass, my_bool old_ver)

    Checks if the string generated by the hashed password and the

    message sent matches the string received from the other endpoint.

    This is the check for the challenge-response mechanism.



  The MySQL engine initializes the PRNG upon startup of the server

  as follows:



  mysql-3.22.32/sql/mysqld.cc:main()

  randominit(&sql_rand,(ulong) start_time,(ulong) start_time/2);

    Where start_time is obtained using the seconds since 0:00 Jan 1,

    1970 UTC using time(3) when the server starts. Our first observation

    is that the PRNG is seeded with an easily guessable value. Though,

    this observation has no direct implications in the vulnerability we

    present.



  Upon connection to the server from a client a new thread is created to

  handle it and a random string is calculate and stored in per

  connection structure, this is done in

  mysql-3.22.32/sql/mysqld.cc:create_new_thread():

    ...

    (thread_count-delayed_insert_threads > max_used_connections)

    max_used_connections=thread_count-delayed_insert_threads;

    thd->thread_id=thread_id++;

    for (uint i=0; i < 8 ; i++)         // Generate password teststring

      thd->scramble[i]= (char) (rnd(&sql_rand)*94+33);

    thd->scramble[8]=0;

    thd->rand=sql_rand;

    threads.append(thd);



    /* Start a new thread to handle connection */

    ...

  The challenge/response exchange is performed and checked in

  mysql-3.22.32/sql/sql_parse.cc:check_connections():

    ....

    memcpy(end,thd->scramble,SCRAMBLE_LENGTH+1);

    end+=SCRAMBLE_LENGTH+1;

    ...

    if (net_write_command(net,protocol_version, buff, (uint) (end-buff)) ||

        (pkt_len=my_net_read(net)) == packet_error || pkt_len < 6)

    {

      inc_host_errors(&thd->remote.sin_addr);

      return(ER_HANDSHAKE_ERROR);

    }

    Here the random string has been sent (along with other server

     data) and the response has been read.

    The authentication checks are then perfomed

     ...

     char *passwd= strend((char*) net->read_pos+5)+1;

     if (passwd[0] && strlen(passwd) != SCRAMBLE_LENGTH)

       return ER_HANDSHAKE_ERROR;

      thd->master_access=acl_getroot(thd->host, thd->ip, thd->user,

                                 passwd, thd->scramble, &thd->priv_user,

                                 protocol_version == 9 ||

                                 !(thd->client_capabilities &

                                   CLIENT_LONG_PASSWORD));

     thd->password=test(passwd[0]);

     ...

     acl_getroot() in mysql-3.22.32/sql/sql_acl.cc does the permission

     checks for the username and host the connection comes from and

     calls the check_scramble function described above to verify the

     valid reponse to the challenge sent. If the response is checked

     valid we say this (challenge and response) test was passed.



 2. The problem: Cryptographically weak authentication scheme





  The hash function provided by MySQL outputs eight-bytes strings

  (64 bits), whereas the random number generator outputs five-bytes

  strings (40 bits).

  Notice that as for the authentication mechanism described above, to

  impersonate a user only the hash value of this user's password is

  needed, e.g. not the actual password.



  We now describe why the hash value of the password can be

  efficiently calculated using only a few executions of the challenge-

  and-response mechanism for the same user. In particular, we introduce

  a weakness of this authentication scheme, and deduce that an attack

  more efficient than brute-force attack can be carried out.



  Firstly we describe how the MySQL random generator (PRNG) works.

  Then we proceed to analyse this scheme's security. The algorithm for

  making these calculations will be briefly described in the following

  section.



  Let n := 2^{30}-1 (here n is the max_value used in randominit() and

  old_randoninit() respectively). Fix a user U. And initiate a challenge

  and response. That is, suppose the server has sent a challenge to the

  user U. The hash value of this user's password is 8 bytes long. Denote

  by P1 the first (leftmost) 4 bytes of this hash value and by P2 the

  last 4 bytes (rightmost). Likewise, let C1 denote the first 4 bytes of

  the challenge's hash value and C2 the last 4. Then, the random

  generator works as follows:



  -calculate the values seed1 := P1^C1 and seed2 := P2^C2

  (here ^ denotes the bitwise exclusive or (XOR) function)



  -calculate recursively for 1 =< i =< 8



    seed1 = seed1+(3*seed2)         modulo (n)

    seed2 = seed1+seed2+33          modulo (n)

    r[i] = floor((seed1/n)*31)+64



  -calculate form the preceding values



    seed1 = seed1+(3*seed2)         modulo (n)

    seed2 = seed1+seed2+33          modulo (n)

    r[9] = floor((seed1/n)*31)



  -output the checksum value

   S=(r[1]^r[9] || r[2]^r[9] || ... || r[7]^r[9] || r[8]^r[9])



  It is this checksum that is sent, by U, to the server. The server, who

  has in store the hash value of U's password, recalculates the checksum

  by this same process and succintly verifies the authenticity of the

  value it has received. However it is a small collection of these

  checksums that allows any attacker to obtain P1 and P2 (the hash value

  of the user's password). Hence, it is therefore possible to

  impersonate any user with only the information that travels on the

  wire between server and client (user).



  The reason why the process of producing the checksum out of the hash

  values of both the password and the challenge is insecure is that this

  process can be efficently reversed due to it's rich arithmetic

  properties.

  More specifically, consider the random generator described above as a

  mapping 'f' that takes as input the two values X and Y and produces

  the checksum value f(X,Y)=S (e.g., in our case X:=P1^C1 and Y:=P2^C2).

  Then we can efficiently calculate all of the values X',Y' which map to

  the same checksum value than X,Y, i.e. if f(X,Y)=S, then we calculate

  the set of all the values X',Y' such that f(X',Y')=S. This set is of

  negligible size in comparison to the 2^64 points set of all the

  possible passwords' hashes in which it is contained. Furthermore,

  given a collection of challenges and responses made between the same

  user and the server, it is possible to efficiently calculate the set

  of all (hash values of) passwords passing the given tests.





 3. The attack





  We now give a brief description of the attack we propose. This

  description shall enable readers to verify our assertion that the

  MySQL authentication scheme leaks information. This attack has been

  implemented on Squeak Smalltalk and is now perfectly running. A

  complete description of the attack-algorithm lies beyond the scope of

  this text and will be the matter of future work.



  The attack we designed is mainly divided into two stages. In these two

  stages we respectively use one of our two algorithmic tools:



  Procedure 1 is an algorithmic process which has as input a

  checksum S and the corresponding hash value of the challenge

  C1||C2, and outputs a set consisting of all the pairs X,Y mapping

  through the random generator to the checksum S, i.e. in symbols

  {(X,Y): f(X,Y)=S} (here of course we have 0 <=X,Y< 2^{32}).



  In our attack Procedure 1 is used to cut down the number of possible

  hashed passwords from the brute-force value 2^64 to a much smaller

  cardinality of 2^20. This set is highly efficiently described, e.g.

  less than 1Kb memory.

  For this smaller set, it is feasible to eliminate the invalid (hashed)

  passwords using further challenges and responses by our Procedure 2.



  Procedure 2 is an algorithmic process having as input a set SET of

  possible (hashed) passwords, and a new pair (S,C1||C2) of checksum

  and challenge, and producing as output the subset of SET of all the

  passwords passing this new test.



  The way in which Procedure 2 is used in our algorithm should now be

  clear. We first use Procedure 1 to reduce the set of passwords to the

  announced set consisting of 2^{20} points, using as input only two

  challenge and responses for the same user.

  This set contains all the passwords passing this two tests. Suppose

  now that the attacker has in his possession a new pair (S,C1||C2) of

  challenge and response, then he can use Procedure 2 to produce the

  smaller set of all the passwords passing the first three tests (the

  ones corresponding to the three pairs of challenge and response he has

  used). Notice that this process can be repeated for every new pair of

  challenge and response the attacker gets. With each application of

  this process the set of possible passwords becomes smaller.

  Furthermore, the cardinality of these sets is not only decresing

  but eventually becomes 1. In that case the one element remaining is

  the (hashed) password.





 4. Statistics and Conclusions



  In the examples we tested, about 300 possible passwords were left with

  the use of only 10 pairs of challenge and response. Notice that in a

  plain brute-force attack about 2^{64}-300=18,446,744,073,709,551,316

  would remain as possible passwords. It took about 100 pairs of

  challenge and response to cut the 300 set two a set containing two

  possible passwords (i.e., a fake password and the password indeed).

  Finally it took about 300 pairs of challenge and response to

  get the password.



  We therefore are able to make a variety of attacks depending on the

  amount of pairs of challenge and response we get from the user we want

  to impersonate.

  The two extreme cases being very few pairs of challenge and response

  from the same user, and a lot of pairs of challenge and response. The

  second attack, that of many pairs of challenge and response captured,

  is straight-forward:

  Apply the algorithm described above until the password is found.

  The first case, that of only a few pairs of challenge and response

  captured, is as well easy to carry out: simply apply the algorithm we

  described with all the pairs of challenge and response captured, then

  use any possible password in the set produced by the application of

  the algorithm for authenticating yourself as a user (some of these

  fake passwords will still pass many tests!).





DISCLAIMER:



  The contents of this advisory are copyright (c) 2000 CORE SDI S.A.

  and may be distributed freely provided that no fee is charged for this

  distribution and proper credit is given.



$Id: MySQLauth-advisory.txt,v 1.11 2000/10/23 21:30:57 iarce Exp $



---



"Understanding. A cerebral secretion that enables one having it to know

 a house from a horse by the roof on the house,

 It's nature and laws have been exhaustively expounded by Locke,

 who rode a house, and Kant, who lived in a horse." - Ambrose Bierce





==================[ CORE Seguridad de la Informacion S.A. ]=========

Iván Arce

Presidente

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email   : iarce@core-sdi.com

http://www.core-sdi.com

Florida 141 2do cuerpo Piso 7

C1005AAG Buenos Aires, Argentina.

Tel/Fax : +(54-11) 4331-5402

=====================================================================





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