The twenty most critical internet security vulnerabilities: The experts’ consensus

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The twenty most critical internet security vulnerabilities: The experts’ consensus

Last year we listed the SANS Institute list of the ten most critical Internet security vulnerabilities. This year the list has grown to twenty items, and is a bit too long to print here. Each vulnerability has a description (included here), a list of systems affected, ways to determine if you are vulnerable, and how to protect against it. If you are involved in Internet security, we recommend you go to http://www.sans.org/top20.htm and look at the full report, which is updated regularly.

1 – Default installs of operating systems and applications

Most software, including operating systems and applications, comes with installation scripts or installation programs. The goal of these installation programs is to get the systems installed as quickly as possible, with the most useful functions enabled, with the least amount of work being performed by the administrator. To accomplish this goal, the scripts typically install more components than most users need. The vendor philosophy is that it is better to enable functions that are not needed, than to make the user install additional functions when they are needed. This approach, although convenient for the user, creates many of the most dangerous security vulnerabilities because users do not actively maintain and patch software components they don’t use. Furthermore, many users fail to realize what is actually installed, leaving dangerous samples on a system simply because users do not know they are there.

Those unpatched services provide paths for attackers to take over computers. For operating systems, default installations nearly always include extraneous services and corresponding open ports. Attackers break into systems via these ports. In most cases the fewer ports you have open, the fewer avenues an attacker can use to compromise your network. For applications, default installations usually include unneeded sample programs or scripts. One of the most serious vulnerabilities with web servers is sample scripts; attackers use these scripts to compromise the system or gain information about it. In most cases, the system administrator whose system is compromised did not realize that the sample scripts were installed. Sample scripts are a problem because they usually do not go through the same quality control process as other software. In fact they are shockingly poorly written in many cases. Error checking is often forgotten and the sample scripts offer a fertile ground for buffer overflow attacks.

2 – Accounts with No Passwords or Weak Passwords

Most systems are configured to use passwords as the first, and only, line of defense. User IDs are fairly easy to acquire, and most companies have dialup access that bypasses the firewall. Therefore, if an attacker can determine an account name and password, he or she can log on to the network. Easy to guess passwords and default passwords are a big problem; but an even bigger one is accounts with no passwords at all. In practice all accounts with weak passwords, default passwords, and no passwords should be removed from your system.

In addition, many systems have built-in or default accounts. These accounts usually have the same password across installations of the software. Attackers commonly look for these accounts, because they are well known to the attacker community. Therefore, any default or built-in accounts also need to be identified and removed from the system.

3 – Non-existent or Incomplete Backups

When an incident occurs (and it will occur in nearly every organization), recovery from the incident requires up-to-date backups and proven methods of restoring the data. Some organizations make daily backups, but never verify that the backups are actually working. Others construct backup policies and procedures, but do not create restoration policies and procedures. Such errors are often discovered after a hacker has entered systems and destroyed or otherwise ruined data.

A second problem involving backups is insufficient physical protection of the backup medium. The backups contain the same sensitive information that is residing on the server, and should be protected in the same manner

4 – Large number of open ports

Both legitimate users and attackers connect to systems via open ports. The more ports that are open the more possible ways that someone can connect to your system. Therefore, it is important to keep the least number of ports open on a system necessary for it to function properly. All other ports must be closed.

5 – Not filtering packets for correct incoming and outgoing addresses

Spoofing IP addresses is a common method used by attackers to hide their tracks when they attack a victim. For example, the very popular smurf attack uses a feature of routers to send a stream of packets to thousands of machines. Each packet contains a spoofed source address of a victim. The computers to which the spoofed packets are sent flood the victim’s computer often shutting down the computer or the network. Performing filtering on traffic coming into your network (ingress filtering) and going out (egress filtering) can help provide a high level of protection.

6 – Non-existent or incomplete login

One of the maxims of security is, “Prevention is ideal, but detection is a must.” As long as you allow traffic to flow between your network and the Internet, the opportunity for an attacker to sneak in and penetrate the network, is there. New vulnerabilities are discovered every week, and there are very few ways to defend yourself against an attacker using a new vulnerability. Once you are attacked, without logs, you have little chance of discovering what the attackers did. Without that knowledge, your organization must choose between completely reloading the operating system from original media, and then hoping the data back-ups were OK, or taking the risk that you are running a system that a hacker still controls. You cannot detect an attack if you do not know what is occurring on your network. Logs provide the details of what is occurring, what systems are being attacked, and what systems have been compromised.

Logging must be done on a regular basis on all key systems, and logs should be archived and backed up because you never know when you might need them. Most experts recommend sending all of your logs to a central log server that writes the data to a write once media, so that the attacker cannot overwrite the logs and avoid detection.

7 – Vulnerable CGI Programs

Most web servers, including Microsoft’s IIS and Apache, support Common Gateway Interface (CGI) programs to provide interactivity in web pages enabling functions such as data collection and verification. In fact, most web servers are delivered (and installed) with sample CGI programs.

Unfortunately, too many CGI programmers fail to consider that their programs provide a direct link from any user anywhere on the Internet directly to the operating system of the computer running the web server. Vulnerable CGI programs present a particularly attractive target to intruders because they are relatively easy to locate and operate with the privileges and power of the web server software itself. Intruders are known to have exploited vulnerable CGI programs to vandalize web pages, steal credit card information, and set up back doors to enable future intrusions. When the Department of Justice web site was vandalized, an in-depth assessment concluded that a CGI hole was the most probable avenue of compromise.

Web server applications are similarly vulnerable to threats created by uneducated or careless programmers. As a general rule, sample programs should always be removed from production systems.

8 – Windows Unicode Vulnerability (Web Server Folder Traversal)

Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language. The Unicode Standard has been adopted by most vendors, including Microsoft. By sending an IIS server a carefully constructed URL containing an invalid Unicode UTF-8 sequence an attacker can force the server to literally ‘walk up and out’ of a directory and execute arbitrary scripts. This type of attack is also known as the directory traversal attack.

The Unicode equivalents of / and which are %2f and %5c, respectively. However, you can also represent these characters using so-called “overlong” sequences. Overlong sequences are technically invalid Unicode representations that are longer than what is actually required to represent the character. Both / and can be represented with a single byte. An overlong representation, such as %c0%af for / represents the character using two bytes. IIS was not written to perform a security check on overlong sequences. Thus, passing an overlong Unicode sequence in a URL, will bypass Microsoft’s security checks. If the request is made from a directory marked as “executable” the attacker can cause the executable files to be executed on the server.

9 – Windows ISAPI Extension Buffer Overflows

Microsoft’s Internet Information Server (IIS) is the web server software found on most web sites deployed on Microsoft Windows NT and Windows 2000 servers. When IIS is installed, several ISAPI extensions are automatically installed. ISAPI, which stands for Internet Services Application Programming Interface, allows developers to extend the capabilities of an IIS server using DLLs. Several of the DLLs, like idq.dll, contain programming errors that cause them to do improper error bounds checking. In particular, they do not block unacceptably long input strings. Attackers can send data to these DLLs, in what is known as a buffer overflow attack, and take full control of an IIS web server.

10 – Windows IIS RDS exploit (Microsoft Remote Data Services)

Microsoft’s Internet Information Server (IIS) is the web server software found on most web sites deployed on Microsoft Windows NT 4.0. Malicious users exploit programming flaws in IIS’s Remote Data Services (RDS) to run remote commands with administrator privileges.

11 – Windows NETBIOS – unprotected Windows networking shares

The Server Message Block (SMB) protocol, also known as the Common Internet File System (CIFS), enables file sharing over networks. Improper configuration can expose critical system files or give full file system access to any hostile party connected to the Internet. Many computer owners unknowingly open their systems to hackers when they try to improve convenience for coworkers and outside researchers by making their drives readable and writeable by network users. Administrators of a government computer site used for software development for mission planning made their files world readable, so that people at a different government facility could get easy access. Within two days, attackers had discovered the open file shares and had stolen the mission planning software.

Enabling file sharing on Windows machines makes them vulnerable to both information theft and certain types of quick-moving viruses. Macintosh and Unix computers are also vulnerable to file sharing exploits if users enable file sharing.

The SMB mechanisms that permit Windows File Sharing may also be used by attackers to obtain sensitive system information from Windows systems. User and Group information (usernames, last logon dates, password policy, RAS information), system information, and certain Registry keys may all be accessed via a “null session” connection to the NetBIOS Session Service. This information is useful to hackers because it helps them mount a password guessing or brute force password attack against the Windows target.

12 – Windows Information leakage via null session connections

A Null Session connection, also known as Anonymous Logon, is a mechanism that allows an anonymous user to retrieve information (such as user names and shares) over the network, or to connect without authentication. It is used by applications such as explorer.exe to enumerate shares on remote servers. On Windows NT and Windows 2000 systems, many local services run under the SYSTEM account, known as LocalSystem on Windows 2000. The SYSTEM account is used for various critical system operations. When one machine needs to retrieve system data from another, the SYSTEM account will open a null session to the other machine.

The SYSTEM account has virtually unlimited privileges and it has no password, so you can’t log on as SYSTEM. SYSTEM sometimes needs to access information on other machines such as available shares, user names, etc. — Network Neighborhood type functionality. Because it cannot log into the other systems using a UserID and password, it uses a Null session to get access. Unfortunately attackers can also log in as the Null Session.

13 – Windows Weak hashing in SAM (LM hash)

Though most Windows users have no need for LAN Manager support, Microsoft stores LAN Manager password hashes, by default, on Windows NT and 2000 systems. Since LAN Manager uses a much weaker encryption scheme than do the more current Microsoft approaches, LAN Manager passwords can be broken in a very short period of time. Even strong password hashes can be cracked in under a month. The major weaknesses of LAN Manager hashes is the following:

password truncated to 14 characters

password padded with spaces to become 14 characters

password converted to all upper case characters

password split into two seven character pieces

This means that a password cracking program has to crack only two sevencharacter passwords without even testing lower case letters. In addition, LAN Manager is vulnerable to eavesdropping of the password hashes. Eavesdropping can provide attackers with user passwords.

14 – UNIX Buffer Overflows in RPC Services

Remote procedure calls (RPCs) allow programs on one computer to execute programs on a second computer. They are widely used to access network services such as NFS file sharing andNIS. Multiple vulnerabilities caused by flaws in RPC are being actively exploited. There is compelling evidence that the majority of the distributed denial of service attacks launched during 1999 and early 2000 were executed by systems that had been victimized through the RPC vulnerabilities. The broadly successful attack onU.S.military systems during the Solar Sunrise incident also exploited an RPC flaw found on hundreds of Department of Defense systems.

15 – UNIX Sendmail Vulnerabilities

Sendmail is the program that sends, receives, and forwards most electronic mail processed on UNIX and Linux computers. Sendmail’s widespread use on the Internet makes it a prime target of attackers. Several flaws have been found over the years. In fact, the very first advisory issued by CERT/CC, in 1988, made reference to an exploitable weakness in Sendmail. In one of the most common exploits, the attacker sends a crafted mail message to the machine running Sendmail, and Sendmail reads the message as instructions requiring the victim machine to send its password file to the attacker’s machine (or to another victim) where the passwords can be cracked.

16 – UNIX Bind Weaknesses

The Berkeley Internet Name Domain (BIND) package is the most widely used implementation of Domain Name Service (DNS) — the critical means by which we all locate systems on the Internet by name (e.g., www.sans.org) without having to know specific IP addresses — and this makes it a favorite target for attack. Sadly, according to a mid-1999 survey, as many as 50% of all DNS servers connected to the Internet are running vulnerable versions of BIND. In a typical example of a BIND attack, intruders erased the system logs and installed tools to gain administrative access. They then compiled and installed IRC utilities and network scanning tools, which they used to scan more than a dozen class-B networks in their search for additional systems running vulnerable versions of BIND. In a matter of minutes, they had used the compromised system to attack hundreds of remote systems, resulting in many additional successful compromises. This example illustrates the chaos that can result from a single vulnerability in the software for ubiquitous Internet services such as DNS. Outdated versions of Bind also include buffer overflow exploits that attackers can use to get unauthorized access.

17 – UNIX R Commands

Trust relationships are widely used in the UNIX world, particularly for system administration. Companies frequently assign a single administrator to be responsible for dozens or even hundreds of systems. Administrators often use trust relationships and the related UNIX r commands to switch from system to system conveniently. r commands enable someone to access a remote system without supplying a password. Instead of requiring a username/password combination, the remote machine authenticates anyone coming from a trusted IP addresses. If an attacker gains control of any machine in such a trusted network, he or she can gain access to all other machines that trust the hacked machine. The following r commands are often used:

1. rlogin – remote login

rsh – remote shell

rcp – remote copy

18 – UNIX LPD (remote print protocol daemon)

In Unix, the in.lpd provides services for users to interact with the local printer. LPD listens for requests on TCP port 515. The programmers who developed the code that transfers print jobs from one machine to another made an error that creates a buffer overflow vulnerability. If the daemon is given too many jobs within a short time interval, the daemon will either crash or run arbitrary code with elevated privileges.

19 – UNIX sadmind and mountd

Sadmind allows remote administration access to Solaris systems, providing a graphical user interface for system administration functions. Mountd controls and arbitrates access to NFS mounts on UNIX hosts. Buffer overflows in these applications, enabled by programming errors made by the software developers, can be exploited to allow attackers to gain control with root access.

20 – UNIX Default SNMP Strings

The Simple Network Management Protocol (SNMP) is widely used by network administrators to monitor and administer all types of network connected devices ranging from routers to printers to computers. SNMP uses an unencrypted “community string” as its only authentication mechanism. Lack of encryption is bad enough, but the default community string used by the vast majority of SNMP devices is “public”, with a few “clever” network equipment vendors changing the string to “private” for more sensitive information. Attackers can use this vulnerability in SNMP to reconfigure or shut down devices remotely. Sniffed SNMP traffic can reveal a great deal about the structure of your network, as well as the systems and devices attached to it. Intruders use such information to pick targets and plan attacks.

Note: SNMP is not unique to Unix. However, the reason it is listed under Unix is because the contributors have seen a majority of attacks on Unix systems caused by poor SNMP configurations. The contributors have not seen this as a major problem on Windows Systems.

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