Attacking Linux, Part 2


However, regardless of whether your attacker entered via the front or

back door, his next priority after gaining root access is to cover his

tracks, preventing the administrator from noticing his presence and

locking him out. He'll do that by sabotaging the system logs and

accounting software, disabling any security-monitoring software, and

installing trojan horse (trojaned) software to conceal his activities,

gain additional intelligence, and create back doors in case he needs

another way in.

The trojaned software usually includes replacement binaries for the

genuine login, netstat, ps, ifconfig, du, df, ls, top, syslogd, tcpd,

locate, and various servers run by the inetd superserver. The aim is to

hide the attacker's tools, logs, and processes, so that they are

invisible to the legitimate root user.

And tomorrow the world!

Some of those processes will be spy programs, running to capture login

information entered by local users for remote systems elsewhere. Those

will be logged and conveyed back to the attacker, giving him new

targets. Some may be network sniffers, monitoring the traffic passing

nearby, to or from other nearby machines, and likewise capturing

private information for the bad guys. Those work by putting your

network interface in promiscuous mode, in which the normal disregarding

of other machines' network traffic gets disabled. Some may be

clandestine network services, such as file-swapping, that are useful

for the attacker and his friends. Most distressing of all, some may be

carrying out attacks on other systems. The older variety of those

involved flooding distant machines with either normal or deliberately

malformed network traffic (ping, ping of death, smurf, SYN flooding,

teardrop, land, bonk), as a denial of service (DoS) attack. Then

starting last year, the more-organized DDoS tools (trinoo, Tribal Flood

Network, stacheldraht, Trank, and so on) came to sudden public

attention when they were used to overwhelm popular Internet sites. The

third-party, subverted machines (zombies) used to carry out those

attacks appear to have been university machines, favored for their lax

security and high Internet bandwidth, but your Linux hosts could be the

attackers' next tools.

Even if your machines don't cause you that order of embarrassment, the

other risks are equally grim: you can reveal confidential data with

business and/or personal consequences, lose that data entirely, see it

corrupted or sabotaged, be involved in wrongful or even criminal

activity, lose access to your computing resources, and indirectly cause

harm to your staff and business associates. Your Website can be defaced

or modified, or visitors might be redirected by sabotaged company DNS

servers to entirely different sites.

What would the Master Control Program do?

As Ozancin pointed out, to prevent, detect, and recover from such

attacks, your first step is to spend some time thinking like an

attacker. Spend some time exploring your network with Nessus, nmap2,

and Firewalk, discovering its vulnerabilities as if you were an

outsider peeking in. Set John the Ripper loose on your password files

to discover any trivial-to-break passwords with which your users are

damaging your security posture. Subscribe to the security-alert mailing

list for your Linux distribution. Install one or more security-checking

packages LIDS, LogCheck, Tripwire, or HostSentry, or simply generate

and store (off-system) MD5 checksums for all critical system files (see


Disable all network services you're not sure you need (if you're wrong,

you'll find out), including those in /etc/inetd.conf), and the CGI

scripts on Web servers. (Never place scripting executables such as the

Perl interpreter in your CGI-BIN directory.) If you wish to leave the

user-information service finger running, make sure it's not one that

lists all logged-in users if you run finger @hostname (substituting

your machine's name for hostname). Stay current on security-related

revisions, especially for the network services you leave enabled. The

foregoing measures are probably the second most valuable precautions

you can take.

The most valuable measure would involve password policies. you'll want

to always used shadowed passwords. The utility pwconv will switch you

over to those (populating /etc/shadow, and removing all passwords

from /etc/passwd) if you aren't running shadowed already. That

essentially eliminates the risk of password cracking.

You'll also want to set a minimum password length. Most Linux

distributions require five or six minimum characters, but Ozancin

suggests changing that to require a full eight characters. Since most

Linux systems these days use a Pluggable Authentication Module (PAM)

security architecture, the minimum length can usually be set easily in

the /etc/pam.d/passwd configuration file.

In addition, you should consider avoiding plaintext-password network

services: The POP3, FTP, and Telnet daemons pose a special risk because

their passwords pass unencrypted across the open network, sniffable by

any nearby machine along the way. SSH (the secure-shell suite) and

stunnel can replace or protect the vulnerable protocols, and you can

use SSL encryption for any sensitive Web-based information. For best

protection, Ozancin recommends two-factor authentication3: adding an

additional security mechanism to the password one such as a smart card

or an encryption key pair. Users should be encouraged (and equipped) to

encrypt any sensitive email using PGP or GNU Privacy Guard (see

Resources). Also, given the possibility of network sniffing, use

switched Ethernet hubs wherever possible to isolate traffic (thus

minimizing the amount of sniffable information).

Next Week: Attacking Linux, Part 3


2. Among nmap's interesting features is the ability to estimate

your chances of successfully predicting TCP sequence numbers on

a target system, a method described by Steven Bellovin in 1989

and reportedly used by Kevin Mitnick in 1994 to remotely take

over security consultant Tsutomu Shimomura's Unix host. Ozancin

used nmap to show that such guessing is thousands of times more

difficult against a remote machine running a generic Linux 2.2

kernel than against one running MS Windows NT 4.0 with the

latest service pack. nmap can also operate in decoyed mode, in

which a high percentage of the probe packets purport to come

from elsewhere entirely. One series of probes against Pentagon

systems in December 1998 seemed to originate from addresses all

over Russia, but is now thought to have involved a single

university machine running nmap.

3. The rule of thumb in security is that you can maximize security

through a three-factor approach: something you know, something

you have, and something you are. Passwords typify something you

know, smart cards are an example of something you have, and

biometric scanning techniques exemplify something you are. The

latter may seem science-fictional, but I saw a mouse device that

incorporates a fingerprint reader at the August 2000 Stanford

Cypherpunks meeting: they are being mass produced as you read


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