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CIT 380: Securing Computer Systems

CIT 380: Securing Computer Systems. Scanning. FTP Bounce Scan. FTP protocol supports proxy ftp Client requests server send file to another IP, port. If server can open connection, port is open. Advantages: Hide identity of scanning host.

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CIT 380: Securing Computer Systems

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  1. CIT 380: Securing Computer Systems Scanning CIT 380: Securing Computer Systems

  2. FTP Bounce Scan FTP protocol supports proxy ftp • Client requests server send file to another IP, port. • If server can open connection, port is open. Advantages: • Hide identity of scanning host. • Bypass firewalls by using ftp server behind firewall. Disadvantages: • Most ftp servers no longer support proxying. • Printer ftp servers often do still support. CIT 380: Securing Computer Systems

  3. Idle Scan Use intermediate idle host to do scan. • Idle host must increment IP ID for each packet. • Idle host must not receive traffic from anyone other than attacker. Scan Process • Attacker connects to idle host to obtain initial IP ID X. • Send SYN packet to port Y of target with spoofed IP of idle host. • If port is open, target host will send SYN+ACK to idle host. • Idle host with send RST packet with IP ID X+1 to target. • Attacker connects with SYN to idle host to obtain updated IP ID. • Idle host sends back SYN+ACK to attacker. • Note that this action will increment IP ID by 1. • If IP ID is X+2, then port Y on target is open. Advantages: hides attacker IP address from target. CIT 380: Securing Computer Systems

  4. UDP Scans Send 0-byte UDP packet to each UDP port UDP packet returned • Port is open ICMP port unreachable • Port is closed Nothing • Port listed as open|filtered • Could be that packet was lost. • Could be that server only returns UDP on valid input. Disadvantages: • ICMP error rate throttled to a few packets/second (RFC 1812), making UDP scans of all 65535 ports very slow. • MS Windows doesn’t implement rate limiting. CIT 380: Securing Computer Systems

  5. Version Scanning • Port scanning reveals which ports are open • Guess services on well-known ports. • How can we do better? • Find what server: vendor and version • telnet/netcat to port and check for banner • Version scanning CIT 380: Securing Computer Systems

  6. Banner Checking > nc www.nku.edu 80 GET / HTTP/1.1 HTTP/1.1 400 Bad Request Date: Sun, 07 Oct 2007 19:27:08 GMT Server: Apache/1.3.34 (Unix) mod_perl/1.29 PHP/4.4.1 mod_ssl/2.8.25 OpenSSL/0.9.7a Connection: close Transfer-Encoding: chunked Content-Type: text/html; charset=iso-8859-1 127 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <HTML><HEAD> <TITLE>400 Bad Request</TITLE> </HEAD><BODY> <H1>Bad Request</H1> Your browser sent a request that this server could not understand.<P> client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /<P> </BODY></HTML> CIT 380: Securing Computer Systems

  7. Version Scanning • If port is TCP, open connection. • Wait for service to identify self with banner. • If no identification or port is UDP, • Send probe string based on well-known service. • Check response against db of known results. • If no match, test all probe strings in list. CIT 380: Securing Computer Systems

  8. nmap version scan > nmap -sV at204m02 (The 1645 ports scanned but not shown below are in state: closed) PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 3.7.1p2 (protocol 1.99) 80/tcp open http Apache httpd 2.0.48 (mod_python/3.1.3 … DAV/2) 111/tcp open rpcbind 2-4 (rpc #100000) 443/tcp open ssl/http Apache httpd 2.0.48 (mod_python/3.1.3 … DAV/2) 515/tcp open printer? 2049/tcp open nfs 2-3 (rpc #100003) 4045/tcp open nlockmgr 1-4 (rpc #100021) 5432/tcp open postgres? 5901/tcp open vnc VNC (protocol 3.3) 6000/tcp open X11? 32775/tcp open status 1 (rpc #100024) CIT 380: Securing Computer Systems

  9. More nmap Tools Set source port • Bypass firewall by using allowed source port. • Use port 80 for TCP, port 53 for UDP scans. Decoys • Send additional scans from list of decoys. • Spoof IP addresses of decoy hosts. • Defender has to investigate decoys + attacker. CIT 380: Securing Computer Systems

  10. Defenses Prevention • Disable unnecessary services. • Block ports at firewall. • Use a stateful firewall instead of packet filter. Detection • Network Intrusion Detection Systems. • Port scans often have distinct signatures. • IPS can react to scan by blocking IP address. CIT 380: Securing Computer Systems

  11. OS Fingerprinting Identify OS by specific features of its TCP/IP network stack implementation. • Explore TCP/IP differences between OSes. • Build database of OS TCP/IP fingerprints. • Send set of specially tailored packets to host • Match results to identical fingerprint in db to identify operating system type and version. CIT 380: Securing Computer Systems

  12. nmap OS fingerprint examples > nmap –O at204m02 ... Device type: general purpose Running: Sun Solaris 8 OS details: Sun Solaris 8 Uptime 10.035 days (since Sat Mar 27 08:59:38 2004) > nmap –O 10.17.0.1 … Device type: router Running: Bay Networks embedded OS details: Bay Networks BLN-2 Network Router or ASN Processor revision 9 CIT 380: Securing Computer Systems

  13. OS Fingerprinting Techniques FIN probe • RFC 793 requires no response • MS Windows, BSDI, Cisco IOS send RST Bogus flag probe • Bit 7 of TCP flags unused • Linux <2.0.35 keeps flag set in response TCP ISN sampling • Different algorithms for TCP ISNs IP Identification • Different algorithms for incrementing IPID CIT 380: Securing Computer Systems

  14. Passive Fingerprinting • Identify OSes of hosts on network by sniffing packets sent by each host. • Use similar characteristics as active technique: • TTL • MSS • Initial Window Size • Don’t Fragment bit • Tools: p0f CIT 380: Securing Computer Systems

  15. Fingerprinting Defenses • Detection • NIDS • Blocking • Firewalling • Some probes can’t be blocked. • Deception • IPpersonality changes Linux TCP/IP stack signature to that of another OS in nmap db. CIT 380: Securing Computer Systems

  16. OS Mapping • www.netcraft.com CIT 380: Securing Computer Systems

  17. References • William Cheswick, Steven Bellovin, and Avriel Rubin, Firewalls and Internet Security, 2nd edition, 2003. • Fyodor, “The Art of Port Scanning,” http://www.insecure.org/nmap/nmap_doc.html • Fyodor, NMAP man page, http://www.insecure.org/nmap/data/nmap_manpage.html • Fyodor, “Remote OS detection via TCP/IP Stack FingerPrinting,” Phrack 54, http://www.insecure.org/nmap/nmap-fingerprinting-article.html • Simson Garfinkel, Gene Spafford, and Alan Schwartz, Practical UNIX and Internet Security, 3rd edition, O’Reilly & Associates, 2003. • Johnny Long, Google Hacking for Penetration Testers, Snygress, 2004. • Stuart McClure, Joel Scambray, George Kurtz, Hacking Exposed, 5th edition, McGraw-Hill, 2003. • Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006. CIT 380: Securing Computer Systems

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