Hacking Unix/Linux

1. Hacking Unix/Linux

2. Footprinting, Scanning, Enumeration • Footprinting:Similar to Windows but using different tools: • Network enumeration:using whois and finding authoritative name server. • DNS interrogation: using nslookup for zone transfer , host to find e-mail server, and traceroutefor network reconnaissance. Also dig. • Ubuntu: use System, Administration, Network Tools for ping, whois, traceroute, etc. • Scanning: again similar to Windows with different tools : • Nmap and Nmapfe: use sudo apt-get install nmap and sudo apt-get install nmapfe to setup. Use sudo nmapfe for graphical interface. • ping sweeps, port scanning: tcp and udp, OS detection. • Countermeasures: Snort and Psionic Port Sentry: documentation. (seen later in IPS/IDS) • Enumeration: UNIX enumeration. • UNIX Users and group enumeration: finger (see this example) and tftp should be disabled and killed. • Basic daemons: sendmail, rpc, NFS, NIS -- all have known vulnerabilities, setup and latest security patches should be applied carefully.

3. Getting started • Vulnerability mapping:identify specific software vulnerabilities (e.g. sendmail, telnet, ftp, NFS, apache). • Quest for root: root or nothing ? Su - root. • Remote access: gaining access via the network • exploit a listening daemon/service • a UNIX system performing routing with IP forwarding enabled • user-initiated remote execution (hostile Web site, Trojan horses, etc) • Local access: having access to a shell or login to the system • privilege escalation attacks (from login to root) • once a remote access exploits a vulnerability it gains local shell access • Brute force attacks: (we will see John the Ripper but not Hydra) • remote login programs: telnet, ftp, rlogin.rsh,ssh, http plus • a tool to crack the username/password combination. The /etc/passwd file. You should use shadow passwords. • Countermeasures: password education and software (e.g. checkpassd)

4. Remote access • Data driven attacks: sending data to an active service that causes unintended results, generally allowing access to the system • Buffer Overflow:same Windows vulnerabilities (C programs). Countermeasures: basically good programming practices, testing, auditing, safer compilers, etc. • Input Validation: failure in validating input and accepting extraneous input (hack code!!!). Produces similar results to buffer overflow and the countermeasure is the same: safe programming. • Shell access: after gaining access using a data driven attack the first objective is to open a shell window to enter commands. • Traditional shell access: telnet, rlogin,ssh (admins can close most). • Using X-Windows to run xterm in the remote (target) machine and display in the intruder machine, using its client/server features. • Reverse telnet and back channels: admins can remove X, then what? Run nc (netcat) in the intruder and run malicious code in the target creating a telnet connection from target to intruder. • Countermeasures: remove X (servers), chmod 750 telnet (Linux, root only), better yet only use ssh.

5. Common remote access attacks • FTP: do not run anonymous FTP in the same server you have regular accounts and ftp with user authentication. Keep up to date with vulnerabilities (e.g. wu-ftpd, a popular ftp, had a vulnerability in exec). Better yet, only use ssh/scp. • Sendmail:a very complex mail server (over 80,000 lines of code), which few understand. It is needed in almost all UNIX machines and in most cases should not be disabled. Qmail is a potential replacement. • Remote Procedure Call:rpcinfo (as root) let us see the running rpc services. Best defense is to limit the use of RPC services to the minimum. • NFS: do not export the file system to everyone, or enable NFS selectively (not active in the MISLab), files saved locally, not in the network. • DNS: disable BIND in machines NOT used as DNS servers (type ps aux to see what processes are running, also look for /etc/name.boot). Discussing named is beyond the scope of this course (another difficult daemon to setup safely). Test for buffer overflow using dig @ipnumber version.bind chaos txt • X Windows: use xhost to add, delete, names and hosts which can access X-Windows. Not a strong line of defense. Again, remove X-Windows from servers

6. Local access • Password cracking: We cannot decrypt a password, we use a dictionary, or other type of algorithm to generate passwords, encrypt them using the crypt function, DES, etc, and compare with hash in the passwd file. Crack 5 and John the Ripper are two of “the best of the breed.” • Local buffer overflow: same problems and countermeasures. • Symlinks: ln -s /this /that, when you cat this you see that. Signals in UNIX: you re-start, start, stop daemons sending signals: SIGXXX. • Core dump: memory (core) dumps are snapshots of the memory when an error occurs. Delete it or it stays. ulimit set to 0 cancel core dumps. • Shared libraries: replace a standard library file with a rogue one, granting privileges to the intruder, when used. • Kernel problems: upgrades to the kernel may create security flaws, which will be found, patched, etc. Linux particularly vulnerable (growing). • File and directory permissions: SUID files -- a necessary evil. Some applications need to run as root and be used by regular users (solution: SUID). The find command list these files, and there are many of them. World writable files are another problem.