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MySQL server is affected by a remote DoS attack, which could be exploited by a remote unauthenticated attacker to cause a loss of availability on the targeted service.

The issue has been verified to affect 5.6.X branch up to 5.6.35 and 5.7.X branch up to 5.7.17. It is strongly recommended that MySQL servers are updated to the latest version.

Upon connection from a client, the server sends a greeting message and the client continues the communication by starting the authentication process. The authentication packet sent by the client contains a wealth of information including the client capabilities, username, password, etc. The packet is received by the server, and parsed by parse_client_handshake_packet() function, in /sql/auth/sql_authentication.cc.

In particular, the following code retrieves the password from the packet:


  passwd= get_length_encoded_string(&end, &bytes_remaining_in_packet,
                                    &passwd_len);

get_length_encoded_string in turn calls get_56_lenc_string function.

The password field in the packet is defined by two values: the length of the password (1 byte), followed by the actual password field content. This length is calculated by calling the net_field_length_ll() function.

my_ulonglong net_field_length_ll(uchar **packet)
{
  uchar *pos= *packet;
  if (*pos < 251)
  {
    (*packet)++;
    return (my_ulonglong) *pos;
  }
  if (*pos == 251)
  {
    (*packet)++;
    return (my_ulonglong) NULL_LENGTH;
  }
  if (*pos == 252)
  {
    (*packet)+=3;
    return (my_ulonglong) uint2korr(pos+1);
  }
  if (*pos == 253)
  {
    (*packet)+=4;
    return (my_ulonglong) uint3korr(pos+1);
  }
  (*packet)+=9;					/* Must be 254 when here */
  return (my_ulonglong) uint8korr(pos+1);
}

As shown in the code above, if the buffer sent to the MySQL server points to a length of \xFF, the pointer to the packet would be increased by 9 bytes.

If an attacker sends an authentication packet with a password length of \xFE or \xFF with less than 9 bytes following that value, the net_field_length_ll function will position the pointer to the packet outside the boundaries of the variable.

The get_56_lenc_string function will continue the execution calculating the number of bytes remaining in the packet.

The instruction max_bytes_available -= len_len will provoke an integer overflow where the value of  max_bytes_available will become a very large unsigned integer.

The next time get_string function is called, it will make memchr read out of boundaries and exit with a segmentation fault.

A Proof of Concept code for this vulnerability has been released.

References:

Oracle Critical Patch Update