8.4 LAN integration of an IT system via ISDN

Description

ISDN (Integrated Services Digital Network) is a digital telecommunications network which allows the operation of a variety of services such as telephony and facsimile, as well as the transmission of data and images.

This chapter deals with the integration of a remote IT system into a local network via a public ISDN network. The remote IT system is linked by means of an ISDN adapter card possessing an S0 interface to the LAN via a router which is connected to a public ISDN network via an S2M interface.

This type of integration of remote IT systems constitutes a typical possibility for telecommuting workstations as well.

Threat Scenario

The following typical threats are generally assumed as regards baseline protection of an IT system integrated into a LAN via ISDN:

Force Majeure:

• T 1.2 Failure of the IT system
• T 1.10 Failure of a wide area network

Organisational Shortcomings:

• T 2.1 Lack of, or insufficient, rules
• T 2.6 Unauthorised admission to rooms requiring protection
• T 2.7 Unauthorised use of rights
• T 2.9 Poor adjustment to changes in the use of IT
• T 2.19 Inadequate key management for encryption
• T 2.22 Lack of evaluation of auditing data
• T 2.24 Loss of confidentiality of sensitive data of the network to be protected
• T 2.32 Inadequate line bandwidth
• T 2.37 Uncontrolled usage of communications lines

Human Failure:

• T 3.1 Loss of data confidentiality/integrity as a result of IT user error
• T 3.6 Hazards posed by cleaning staff or outside staff
• T 3.8 Improper use of the IT system
• T 3.9 Improper IT system administration
• T 3.13 Transfer of incorrect or undesired data records
• T 3.16 Incorrect administration of site and data access rights

Technical Failure:

• T 4.8 Discovery of software vulnerabilities
• T 4.25 Still active connections

Deliberate Acts:

• T 5.2 Manipulation of data or software
• T 5.7 Interception of lines
• T 5.8 Manipulation of lines
• T 5.9 Unauthorised use of IT systems
• T 5.10 Abuse of remote maintenance ports
• T 5.14 Call charges fraud
• T 5.16 Threat posed by internal staff during maintenance/administration work
• T 5.17 Threat posed by external staff during maintenance work
• T 5.18 Systematic trying-out of passwords
• T 5.25 Masquerade
• T 5.26 Analysis of the message flow
• T 5.39 Infiltrating computer systems via communication cards
• T 5.48 IP spoofing
• T 5.61 Misuse of remote access to management functions on routers
• T 5.62 Misuse of resources via remote IT systems
• T 5.63 Manipulation via the ISDN D-channel

Recommended Countermeasures (S)

For the implementation of IT baseline protection, selection of the required packages of safeguards ("modules") as described in chapters 2.3 and 2.4, is recommended.

The safeguard package for integration of an IT system into a LAN via ISDN is described in the following. The primary objective in this context is to ensure reliable communications. Further safeguards required for the communicating IT systems are specified in the related chapters (refer to Chapter 6 for routers, and Chapter 5 for remote IT systems).

The following measures are additionally recommended:

Infrastructure:

• S 1.43 (2) Secure siting of ISDN routers

Organisation:

• S 2.4 (2) Maintenance/repair regulations
• S 2.9 (2) Ban on using non-approved software
• S 2.35 (2) Obtaining information on security weaknesses of the system
• S 2.42 (1) Determination of potential communications partners
• S 2.46 (2) Appropriate key management
• S 2.64 (2) Checking the log files
• S 2.106 (2) Purchase of suitable ISDN cards
• S 2.107 (2) Documentation of the configuration of ISDN cards
• S 2.108 (2) Relinquishment of remote maintenance of ISDN gateways (optional)
• S 2.109 (1) Assigning rights for remote access

Personnel:

• S 3.4 (1) Training before actual use of a program
• S 3.5 (1) Education on IT security measures

Hardware/Software:

• S 4.7 (1) Change of preset passwords
• S 4.34 (1) Using encryption, checksums or digital signatures (optional)
• S 4.59 (1) Deactivation of ISDN board functions which are not required
• S 4.60 (1) Deactivation of ISDN router functions which are not required
• S 4.61 (1) Use of security mechanisms offered by ISDN components
• S 4.62 (2) Use of a D-channel filter (optional)

Communications:

• S 5.29 (2) Periodic checks of destination addresses and logs
• S 5.32 (1) Secure use of communications software
• S 5.47 (1) Configuration of a Closed User Group (optional)
• S 5.48 (1) Authentication via CLIP/COLP
• S 5.49 (1) Callback based on CLIP/COLP
• S 5.50 (1) Authentication via PAP/CHAP