Using Emulation and More to Analyze Network Security - Using Specific Source Ports to Bypass Filtering
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When using a tool such as Nmap to perform either UDP or TCP port scanning of hosts, it is important to assess responses using specific source ports. Here are four source ports you should use along with UDP, half-open SYN, and inverse FIN scan types:
- TCP or UDP port 53 (DNS)
- TCP port 20 (FTP data)
- TCP port 80 (HTTP)
- TCP or UDP port 88 (Kerberos)
Nmap can be run with the-g <port>flag to provide a source port when performing TCP or UDP port scanning.
Using specific source ports, attackers can take advantage of firewall configuration issues and bypass filtering. UDP port 53 (DNS) is a good candidate when circumventing stateless packet filters because machines inside the network have to communicate with external DNS servers, which in turn respond using UDP port 53. Typically, a rule is put in place allowing traffic from UDP port 53 to destination port 53 or anything above 1024 on the internal client machine. Useful source ports to run scans from are TCP 20 (FTP data), UDP 53 (DNS), and UDP 500 (ISAKMP).
Check Point Firewall-1, Cisco PIX, and other stateful firewalls aren’t vulnerable to these issues (unless grossly misconfigured) because they maintain a state table and allow traffic back into the network only if a relative outbound connection or request has been initiated.
An inverse FIN scan should be attempted when scanning the HTTP service port because a Check Point Firewall-1 option known as fastmode is sometimes enabled for web traffic in high throughput environments (to limit use of firewall processing resources). For specific information regarding circumvention of Firewall-1 in certain configurations, consult the excellent presentation from Black Hat Briefings 2000 by Thomas Lopatic et al. titled “A Stateful Inspection of Firewall-1” (available as a Real Media video stream and PowerPoint presentation from http://www.blackhat.com/html/bh-usa-00/bh-usa-00-speakers.html).
On Windows 2000 and other Microsoft platforms that can run IPsec, a handful of default exemptions to the IPsec filter exist, including one that allows Kerberos (source TCP or UDP port 88) traffic into the host if the filter is enabled. These default exemptions are removed in Windows Server 2003, but they still pose a problem in some environments that rely on filtering at the operating system kernel level.
Next: Low-Level IP Assessment >>
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 This article is excerpted from chapter four of Network Security Assessment, Second Edition, written by Chris McNab (O'Reilly, 2007; ISBN: 0596510306). Check it out today at your favorite bookstore. Buy this book now.
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