Techniques Adopted By 'System Crackers' When Attempting To Break Into
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                   Corporate or Sensitive Private Networks.
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   0    Table Of Contents
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   1.   Introduction
   1.1  Just who is vulnerable anyway?
   1.2  Profile of a typical 'system cracker'

   2    Networking
   2.1  Networking methodologies adopted by many companies
   2.2  Understanding vulnerabilities in such networked systems

   3    The attack itself
   3.1  Techniques used to 'cloak' the attackers location
   3.2  Network probing and information gathering
   3.3  Identifying trusted network components
   3.4  Identifying vulnerable network components
   3.5  Taking advantage of vulnerable network components
   3.6  Upon gain access to vulnerable network components

   4    Abusing network access and privileges
   4.1  Downloading sensitive information
   4.2  Cracking other trusted hosts networks
   4.3  Installing backdoors and trojaned files
   4.4  Taking down networks



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   1.0  Introduction
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 This white paper was written to help give systems administrators and network
 operations staff an insight into the tactics and methodologies adopted by
 typical system crackers when targeting large networks.

 This document is not a guide about how to secure your networks, although it
 should help you identify security risks in your networked environment and
 maybe help point out any accidents that are waiting to happen.

 We hope you enjoy reading this paper, and hopefully learn a little about
 how crackers operate in the meantime!



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   1.1  Just who is vulnerable anyway?
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 Networked computer environments are used everyday by corporations and various
 organisations. Networks of computers allow users to share vast amounts of
 data very efficiently.

 Usually corporate networks are not designed and implemented with security
 in mind, merely functionality and efficiency, although this is good from a
 business standpoint in the short-term, security problems usually arise
 later, which can cost millions to solve in larger environments.

 Most corporate and sensitive private networks work on a client-server
 principle, where employees use workstations to connect to servers in order
 to share information. In this paper we will concentrate on server security,
 as most crackers will always target servers first, the server is much like
 a 'hub' where all the information is stored. If a cracker can gain
 unauthorised access to such a server, the rest of his work is easy.

 Vulnerable parties to large-scale network probes usually include :

   Financial institutions and banks
   Internet service providers
   Pharmaceutical companies
   Government and defense agencies
   Contractors to various goverment agencies
   Multinational corporations


 Although many of these attacks take place internally (by users who have
 authorised access to parts of the corporate or sensitive networks already),
 we will be concentrating on the techniques used when breaking into such
 networks entirely from the outside.

 Financial institutions and banks are probed and attacked in attempts to
 commit fraud. Many banks have been targeted in this way, risking vast
 monetary funds. Banks make it policy not to admit to being victims
 of such external attacks because they will certainly lose customers and
 trust if attacks are publically known.

 Internet service providers are a common target by crackers, as ISP servers
 are easily accessible from the internet, and ISP's have access to large
 fibre optic connections which can be used by crackers to move large
 amounts of data across the internet. The larger ISP's also have customer
 databases, which usually contain confidential user information such as
 credit card numbers, names and addresses.

 Pharmaceutical companies are victims of mainly industrial espionage attempts,
 where a team of crackers will be paid large amounts in exchange for stolen
 pharmaceutical data, such drug companies often spend millions on research
 and development, and a lot can be lost as a result of such an attack.

 Over the last 6 years, Government and defence agencies in the United States
 have been victim to literally millions of attacks originating from the
 internet. Due to the low information security budgets and the weak security
 policies of such agencies, information security has become an uphill battle,
 as government and military servers are constantly being probed and attacked
 by crackers.

 Defence contractors, although security conscious, are targets to crackers
 seeking classified or sensitive military data. Such data can then be
 'sold on' by crackers to foreign groups. Although only a handful of these
 cases have been publically known, such activities can occur at an alarming
 rate.

 Multinational corporations are prime examples of victims of industrial
 espionage attempts. Multinational corporations have offices based all around
 the world, and large corporate networks are installed in order for employees
 to be able to share information efficiently. NSS staff have performed
 penetration tests for multinational corporations, and our findings in
 most cases have shown that many can be compromised.

 Like pharmaceutical companies, multinational corporations operating in
 electronics, software or computer-related industries, spend millions on
 research and development of new technologies. It is very tempting for a
 competitor of such a corporation, to employ a team of 'system crackers' to
 steal data from a target corporation. Such data can then be used to quickly
 and easily improve the competitors knowledge of key technologies, and result
 in financial losses of the target corporation.

 Another form of attack adopted by competitors of corporations, is to 'take
 down' a corporate network for a certain amount of time, this results in
 loss of earnings for the target corporation. In most cases it is extremely
 difficult to locate the source of such an attack. Depending on the internal
 network segmentation in place, this kind of attack can be hugely effective
 and result in massive financial losses.

 Such 'foul play' is commonplace in today's networked society, and should be
 taken very seriously.



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   1.2  Profile of a typical 'system cracker'
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 Studies have shown that typical a 'system cracker' is usually male, aged
 between 16 and 25. Such crackers usually become interested in breaking into
 machines and networks in order to improve their cracking skills, or to use
 network resources for their own purposes. Most crackers are quite
 persistent in their attacks, this is due to the amount of spare time an
 average cracker has.


 A high percentage of crackers are opportunists, and run scanners to check
 massive numbers of hosts for remote system vulnerabilities. Upon identifying
 hosts or networks that are vulnerable to remote attacks, the cracker will
 usually gain root access to the host, then install a backdoor and patch the
 host from common remote vulnerabilities, this prevents other crackers from
 being able to use the same popular techniques to gain access to the host.

 Opportunists operate on primarily two domains, the first being the
 internet, the second being telephone networks.

 To scan internet hosts for common remote vulnerabilities, the cracker will
 usually launch a scanning operation from a host that he has access to with
 a fast connection to the internet, usually on a fibre-optic connection.

 To scan for machines operating on telephone networks, being terminal servers,
 bulletin board systems, or voice mail systems. The cracker will use a
 wardialling program, this will automatically scan large amounts of telephone
 numbers for 'carriers', thus identifying such systems.


 A very small percentage of crackers actually define targets and attempt to
 attack them, such crackers are far more skilled, and adopt 'cutting-edge'
 techniques to compromise networks. It is known for these types of crackers
 to attack corporate networks that are firewalled from the internet by
 exploiting non-published vulnerabilities and 'features' in firewalls.

 The networks and hosts targeted by these crackers usually have sensitive
 data contained within them, such as research and development notes,
 or other data that will prove useful to the cracker.

 Such crackers are also known to have access to exploits and tools used by
 security consultants and large security companies, and then use them to
 scan defined targets for all known remote vulnerabilities. Crackers that
 are attacking specific hosts are also usually very patient, and have been
 known to spend many months gathering data before attempting to gain access
 to a host or network.



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   2.1  Networking methodologies adopted by many companies
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 A typical corporation will have an internet presence for the following
 purposes :

   The hosting of corporate webservers
   E-mail and other global communications via. the internet
   To give employees internet access


 Of the corporations NSS has performed network penetration tests from the
 internet for, a networked environment is adopted where the corporate network
 and the internet are seperated by firewalls and application proxies.

 In such environments, the corporate webservers and mailservers are usually
 kept on the 'outside' of the corporate network, and then information is
 passed via. trusted channels onto the corporate network.

 In the case of trust present between external mailservers and hosts on the
 corporate network, a well-thought filtering policy has to be put into
 effect, as usually the external mailservers should only be able to
 connect to port 25 of a single 'secure' mailserver on the corporate
 network, as this will massively minimise the probability of unauthorised
 access, even if the external mailserver is compromised.

 One of the corporate networks NSS has performed penetration test on also had
 a handful of 'dual-homed' hosts, these hosts had network interfaces active
 on both the internet and the corporate network. From a security standpoint,
 such hosts that operate on multiple networks can pose a massive threat to
 network security, as upon compromising a host, it then acts as a simple
 'bridge' between networks.



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   2.2  Understanding vulnerabilities in such networked systems
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 On the internet, a corporation may have 5 external webservers, 2 external
 mailservers, and a firewall or filtering system implemented.

 Webservers are usually not attacked by crackers wanting to gain access to
 the corporate network, unless the firewall is misconfigured in some way
 that will allow the cracker access to the corporate network upon compromising
 the webserver. Although it is always good practise to secure your webservers
 and run TCP wrappers to allow only trusted parties to connect to the telnet
 and ftp ports.

 Mailservers are commonly targeted by crackers wanting to gain access to the
 corporate network, as a mailserver must have access to mailservers on the
 corporate network in order to distribute and exchange mail between the
 internet and the corporate network. Again, depending on the filtering in
 place, this tactic may or may not be effective on the cracker's part.

 Filtering routers are also commonly targeted by crackers with agressive-SNMP
 scanners and community string brute-force programs, if such an attack is
 effective, the router can easily be turned into a bridge, thus allowing
 unauthorised access to the corporate network.


 In this kind of situation the cracker will evaluate exactly which external
 hosts he has access to, and then attempt to identify any kinds of trust
 between the corporate network and the external hosts. Therefore if you
 install TCP wrappers on all your external hosts, which define that only
 trusted parties can connect to the critical ports of your hosts, which
 are usually :

   ftp  (21),   ssh  (22),   telnet (23),   smtp   (25),   named (53),
   pop3 (110),  imap (143),  rsh    (514),  rlogin (513),  lpd   (515).


 SMTP, named and portmapper should be filtered accordingly depending on
 the host's role is on the network.

 Such filtering has been proven to massively reduce the risk of an attack
 on the corporate network.


 In the cases of networks with no clear 'corporate to internet' network
 security policy, multiple-homed hosts and misconfigured routers will exist.
 A lack of internal network segmentation will also usually exist, this
 makes it a lot easier for an cracker based on the internet to gain
 unauthorised access to the corporate network.

 Corporate network mapping can easily occur if external DNS servers are
 misconfigured, as NSS has performed penetration tests where we have been
 able to map the corporate network via. such a misconfigured DNS server,
 because of this, it is very important that DNS doesn't exist between
 hosts on the corporate network and external hosts, it is far safer to
 simply use IP addresses to connect to external machines from the corporate
 network and vice-versa.

 Insecure hosts with network interfaces active on multiple networks can
 be abused to gain access to the corporate network very easily.
 The insecure host doesn't even have to be compromised. It is very easy
 to abuse a finger daemon on such a host that allows forwarding.. as users,
 hosts and other network information can be collected to identify easily
 exploitable hosts on the corporate network, the operating system of a
 host can even be determined in many cases by issuing a finger request
 for root@host, bin@host and daemon@host. 


 Some crackers are now starting to adopt techniques regarding the
 'wardialling' of corporate locations, such as buildings and network
 operation centres.

 If a cracker was to find and then compromise a corporate terminal server,
 he would usually have a degree of access to the corporate network, thus
 totally bypassing any firewalls or filters that seperate the corporate
 network from the internet. It is therefore very important to identify
 and ensure the security of your terminal servers, logging of connections
 to such servers is also strongly advised.


 When trying to understand vulnerabilities in networked systems, a key
 point to remember, is trust between hosts on your network. Either
 through the use of TCP wrappers, hosts.equiv files, .rhosts or .shosts
 files, many larger networks are commonly attacked by exploiting the
 trust between hosts.

 For example, if an attacker uses a CGI exploit to view your hosts.allow
 file, he may find that you all connections to your ftp and telnet ports
 from *.trusted.com. Of course, the attacker can then gain access to any
 host at trusted.com, and gain access to your hosts easily.

 For these reasons, it is always a good idea to ensure that trusted hosts
 are equally secure from remote attack.


 One other attack that should be mentioned, is the installation of trojans
 and backdoors on corporate hosts (such as Windows 95/98 machines), if the
 employees have internet access through using an application proxy and a
 firewall, then they will sometimes visit 'warez' sites to download pirated
 software.

 Such 'warez' sites usually have screesaver software, and other utilities
 on offer, which in some cases contain trojan horse programs, such as the
 Cult of the Dead Cow's 'Back Orifice' trojan. Upon the installation of
 the screensaver, the trojan infests itself within the machine's registry
 and is run every time the machine boots.

 In the case of the BO trojan, plugins can be applied to the trojan to
 make the machine perform certain operations automatically, such as connect
 to IRC servers and join channels, and the like. This can prove very
 dangerous, as a trojaned machine on your corporate network could easily
 be controlled by someone on the internet.

 The BO trojan is infinitely more effective if the cracker already has
 access to the corporate network, either because he is an employee or has
 unauthorised access to corporate hosts. The BO trojan could be installed
 on every single Windows 95/98 machine in a matter of weeks if the cracker
 uses the correct strategy, after which he will have total remote control
 over the machines in question, including being able to manipulate files,
 reboot machines and even format drives, entirely remotely.



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   3.1  Techniques used to 'cloak' the attackers location
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 Typical crackers will usually use the following techniques to hide
 their true IP address :

   - Bouncing through previously compromised hosts via. telnet or rsh.
   - Bouncing through windows hosts via. Wingates.
   - Bouncing through hosts using misconfigured proxies.


 If such a cracker has a pattern of always scanning your hosts from previously
 compromised machines, wingates or proxies, then it is advisable to contact
 the administrator of the machine by telephone, and notify him of the problems
 in hand. Never e-mail an administrator in such a case, because the cracker
 can simply intercept the e-mail beforehand.
 

 The more talented crackers who are skilled in breaking into hosts via.
 telephone exchanges, may use the following techniques :

  - Bouncing through '800-number' private telephone exchanges before
    connecting to an ISP using a 'cracked', 'phished' or 'carded' account.

  - Connecting to a host by telephone, that is in turn connected to the
    internet.



 Crackers adopting the techniques of bouncing through telephone networks
 before connecting to the internet are extremely hard to track down,
 because they could be literally anywhere in the world. If a cracker
 was to use an '800-number' dialup, he could dial into machines globally
 without having to worry about the cost.



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   3.2  Network probing and information gathering
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 Before setting out to attack a corporate network from the internet, a typical
 cracker will perform some preliminary probes of your networks external
 hosts present on the internet. A cracker will attempt to gain external and
 internal hostnames by using the following techniques :

   - Using nslookup to perform 'ls ' requests.
   - View the HTML on your webservers to identify any other hosts.
   - View the documents on your FTP servers.
   - Connect to your mailservers and perform 'expn ' requests.
   - Finger users on your external hosts.


 Crackers usually attempt to gather information about the layout of your
 network itself first as opposed to identifying specific vulnerabilities.

 By looking at results from the queries listed above, it is usually easy
 for a cracker to build a list of hosts and start to understand the
 relationships that exist between them.

 When performing these preliminary probes, a typical cracker will make
 very small mistakes and sometimes use his own IP to connect to ports of
 your machines to check operating system versions and other small details.

 If your hosts are compromised, it is a good idea to check your FTP and
 HTTPD logs for the presence of any strange requests.



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   3.3  Identifying trusted network components
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 Crackers look for trusted network components to attack, a trusted network
 component is usually an administrators machine, or a server that is regarded
 as secure.

 A cracker will start out by checking the NFS exports of any of your machines
 running nfsd or mountd, the case being that critical directories on some
 of your hosts (such as /usr/bin, /etc and /home for example) may be
 mountable by such a trusted host.

 The finger daemon is often abused to identify trusted hosts and users,
 being users who often log into the machine from specific hosts.

 The cracker will then check your machines for other forms of trust, if he
 can exploit a machine using a CGI vulnerability, he may gain access to a
 hosts /etc/hosts.allow file, for example.

 After analysing the data from the above checks, the cracker will start
 to identify trust between hosts. The next step for the cracker is to
 identify any trusted hosts that are vulnerable to a remote compromise.



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   3.4  Identifying vulnerable network components
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 If a cracker can build lists of your external and internal hosts, he
 will use Linux programs such as ADMhack, mscan, nmap and many smaller
 scanners to scan for specific remote vulnerabilities.

 Usuaully such scans of your external hosts will be launched from machines
 on fast fibre-optic connections, ADMhack requires to be run as root on a
 Linux machine, so a cracker will probably use a Linux machine that he has
 gained unauthorised access to and properly installed a 'rootkit' on. Such a
 'rootkit' is used to backdoor critical system binaries to allow unauthorised
 and undetectable access to the host.

 The systems administrators of the hosts that are used to scan external
 corporate hosts usually have no idea that scans are being launched from
 their machines, as binaries such as 'ps' and 'netstat' are trojaned to
 hide scanning processes.


 Other programs such as mscan and nmap don't require to be run as root, and
 so can be launched from Linux (or other platforms in the case of nmap)
 hosts to effectively identify remote vulnerabilities, although these scans
 are slower, and cannot usually be hidden very well (as the attacker doesn't
 need root access to the host as with ADMhack).


 Both ADMhack and mscan perform the following types of checks on
 remote hosts :

  - A TCP portscan of a host.
  - A dump of the RPC services running via. portmapper.
  - A listing of exports present via. nfsd.
  - A listing of shares present via. samba or netbios.
  - Multiple finger requests to identify default accounts.
  - CGI vulnerability scanning.
  - Identification of vulnerable versions of server daemons, including
    Sendmail, IMAP, POP3, RPC status and RPC mountd.


 Programs such as SATAN are rarely used by crackers nowadays, as they are
 slow.. and scan for outdated vulnerabilities.

 After running ADMhack or mscan on the external hosts, the cracker will have
 a good idea of vulnerable or secure hosts.

 If routers are present that are SNMP capable, the more advanced crackers
 will adopt agressive-SNMP scanning techniques to try and 'brute force'
 the public and private community strings of such devices.



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   3.5  Taking advantage of vulnerable network components
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 So the cracker has identified any trusted external hosts, and also identified
 any vulnerabilities in external hosts. If any vulnerable network components
 were identified, then he will attempt to compromise your hosts.

 A patient cracker won't compromise your hosts during normal hours, he will
 usually launch an attack between 9pm in the evening and 6am the next morning,
 this will reduce the likelyhood of anyone knowing about the attack, and give
 the cracker ample time to install backdoors and sniffers on your hosts
 without having to worry about the presence of Systems Administrators.

 Most crackers have a great deal of spare time over weekends, and attacks
 are usually launched then.

 The cracker will compromise an external trusted host that can be used as
 a point from which to launch an attack on the corporate network. Depending
 on the filtering between the corporate network and the external corporate
 hosts, this technique may or may not work.

 If the cracker compromises an external mailserver, which in turn has total
 access to a segment of the internal corporate network, then he can start
 work on embedding himself deeply into your network.

 To compromise most networked components, crackers will use programs to
 remotely exploit vulnerable versions of server daemons running on external
 hosts, such examples include vulnerable versions of Sendmail, IMAP, POP3
 and RPC services such as statd, mountd and pcnfsd.

 Most remote exploits used by crackers are launched from previously
 compromised hosts, as in some cases they need to be compiled on the same
 platform as the host they are to be used to exploit.

 Upon executing such a program remotely to exploit a vulnerable server daemon
 running on your external host, the cracker will usually gain root access to
 your host, which in turn can be abused to gain access to other hosts and
 the corporate network.



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   3.6  Upon gain access to vulnerable network components
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 After exploiting a server daemon, the cracker will start a 'clean-up'
 operation of doctoring your hosts logs and 'backdooring' service binaries
 so he can access the host undetected later.

 First he will start to implement backdoors, so he can later access the
 host. Most backdoors that crackers use are precompiled, and techniques are
 adopted to change the date and the permissions of the binary that has
 been backdoored, in some cases, even the filesize of the new binary is the
 same as the original binary. Attackers conscious of FTP transfer logs
 may use the 'rcp' program to copy backdoored programs to hosts.

 It is unlikely that such a cracker breaking into a corporate network
 will start to patch your hosts from vulnerabilities, he will usually
 only install backdoors and trojan critical system binaries such as 'ps'
 and 'netstat' to hide any connections he may make to and from the host.

 The following critical binaries are usually backdoored on Solaris 2.x
 machines :

   /usr/bin/login
   /usr/sbin/ping
   /usr/sbin/in.telnetd
   /usr/sbin/in.rshd
   /usr/sbin/in.rlogind


 Some crackers have also been known to place an .rhosts file in the
 /usr/bin directory to allow remote bin access to the host via. rsh
 and csh in interactive mode.

 The next thing that most crackers do is to check the host for any
 presence of logging systems that may have logged his connections to
 the host, he will then proceed to edit such connections out of any
 logs found on the host. It is advisable to log to a lineprinter if
 the machine is very likely to be a prime target of an attack, as
 this makes it extremely difficult for the cracker to edit himself
 from the logs.

 Upon ensuring that his presence has not been logged in any way, the
 cracker will proceed to invade the corporate network. Most crackers
 won't bother exploiting vulnerabilities in other external hosts if
 they have access to the internal network.



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   4.1  Downloading sensitive information
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 If the cracker's goal is to download sensitive information from FTP servers
 or webservers on the internal corporate network, he can do so from the
 external host that is acting as a 'bridge' between the internet and
 corporate network.

 However, if the cracker's goal is to download sensitive information held
 within internally networked hosts, he will proceed to attempt to gain
 access to them by abusing the trust with the external host he already
 has access to.


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   4.2  Cracking other trusted hosts and networks
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 Most crackers will simply repeat the steps taken in sections 3.2, 3.3,
 3.4 and 3.5 to probe and gain access to hosts on the internal corporate
 network, depending on what the cracker is attempting to achieve,
 trojans and backdoors may or may not be installed on your internal
 hosts.

 If the cracker wishes to achieve total network access to the hosts on
 the internal network, he will install trojans and backdoors and remove
 logs as in section 3.6. Crackers will also install sniffers on your
 hosts, these are explained in section 4.3.

 If the cracker merely wishes to download data from key servers,
 he will take different approaches to gaining access to your hosts,
 such as identifying and attacking key hosts that are trusted by the
 target key servers.



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   4.3  Installing sniffers
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 An extremely effective way for crackers to quickly obtain large amounts of
 usernames and passwords for internally networked hosts is to use
 'ethernet sniffer' programs. Because such 'sniffer' programs need to
 operate on the same ethernet as the hosts the cracker wants to gain
 access to, it would be ineffective to run a sniffer on the external host
 he is using as a bridge.

 To 'sniff' data flowing across the internal network, the cracker must
 perform a remote root compromise of an internal host that is on the same
 ethernet as a number of other internal hosts. The techniques mentioned
 in sections 3.2, 3.3, 3.4, 3.5 and 3.6 are adopted here, as the cracker
 must compromise and backdoor the host successfully to ensure that the
 sniffer program can be installed and used effectively.

 Upon compromising, installing a backdoor and installing trojaned 'ps'
 and 'netstat' programs, the cracker must then install the 'ethernet sniffer'
 program on the host. Such sniffer programs are usually installed in the
 /usr/bin or /dev directories under Solaris 2.x, and then modified to seem
 as if they were installed with all the other system binaries.

 Most 'ethernet sniffers' run in the background and output to a log on the
 local machine, it is important to remember that the cracker will usually
 backdoor the 'ps' binary, so the process may not be noticeable.

 Such 'ethernet sniffers' work by turning a network interface into
 'promiscuous mode', the interface then listens and logs to the sniffer
 logfile, any useful usernames, passwords or other data that can be used
 by the cracker to gain access to other networked hosts.

 Because 'ethernet sniffers' are installed on ethernets, literally any
 data travelling across that network can be sniffed, it doesn't have to
 be travelling to or from the host on which the sniffer is installed.

 The cracker will usually return a week later and download the logfile
 created by the 'sniffer' program. In the case of a corporate network
 breach such as this, it is likely that the sniffer will be set up very
 well, and hardly detectable unless a good security policy is implemented.

 A very good utility used by many security-conscious Administrators is
 Tripwire, which is available from COAST (see section 5.2). Tripwire
 makes an MD5 'fingerprint' of your filesystem, and will detect any
 modifications to your files made by malicious users or crackers.

 To detect promiscuous network interfaces (a common sign of a sniffer
 installation), the 'cpm' tool available from CERT is very useful,
 see http://www.cert.org/ftp/tools/cpm/ for more information.



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   4.4  Taking down networks
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 If a cracker can compromise key servers running server applications such
 as databases, network operations systems or any other 'mission critical'
 functions, it is easy for him to take down your network for a period of
 time.

 A crude, but not unusual technique adopted by crackers attempting to
 disable network functions, would be to delete all the files from the
 key servers by issuing an 'rm -rf / &' command on the server. Depending
 on the backup system implemented, the system could be for anything from
 hours, to months.


 If a cracker was to gain access to your internal network, he could abuse
 vulnerabilities present in many routers such as in the Cisco, Bay and
 Ascend brands. In some cases the cracker could restart, or shut down
 routers entirely until an administrator was to reboot them.
 This can cause big problems regarding network functionality,
 as if the cracker was to assemble a list of vulnerable routers that
 performed key networking roles (if they were used on the corporate
 backbone for example), then he could easily disable the corporations
 networking ability for some time.


 For these reasons, it is very important that 'mission critical' routers
 and servers are always patched and secure.