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» Attacks on layer two of the OSI modelRelated categories: Networks | Low-level network security | Security Alfredo Andrés, David BarrosoViewed: 18321 | Article date: 2006-02-27 10:41:47 Layer two of OSI model is one of the weakest links when trying to assure network security in an organization. It is also one of the most commonly ignored, because there aren't many public implementations of layer two attacks. However, a successful attack on layer two can be just as dangerous as any other.
Layer two of OSI model is one of the weakest links when trying to assure network security in an organization. It is also one of the most commonly ignored, because there aren't many public implementations of layer two attacks. However, a successful attack on layer two can be just as dangerous as any other.
About the authorsDavid Barroso specialises in incident response and network security. He currently works in a Spanish security company called S21sec. He is also deeply involved in the global security community, writing articles, papers and developing new security tools. Alfredo Andres has been working in the security field for several years and contributing to the Open Source community developing tools and patches. Alfredo also works in S21sec, leading a pen-testing group. Both authors were presenting Yersinia on BlackHat Europe 2005. A Cisco zero-day attack was also presented, related to one of the protocols targeted by Yersinia (Cisco was notified, of course) and discovered when developing the tool.
The Data link layer is one of the least secured and most often forgotten elements of networks. It's quite common that administrators simply connect the switches, configure them to work and then never worry about them. Pen-testing often reveals switches, which use a vulnerable version of IOS and are not hardened in any way. It is also commonly thought, that implementing VLAN in a network keeps malicious attackers away. However, VLAN architecture can just as well be defeated and therefore all higher OS layer attacks such as sniffing passwords, Man-in-the-Middle are possible across VLANs.
What you will learn...
What you should know...
The good thing about layer two is the fact, that Data link layer packets can't go through IP networks, for example the Internet. Therefore all attacks are limited to internal networks. But then again, statistics show that attacks from inside can be just as dangerous as the ones from the outside. It must also be remembered, that if an external intruder traverses our firewall and gets to the DMZ, such attacks can allow him to escape the DMZ and target our whole network. Let's see what common Data link layer vulnerabilities are, how can they be exploited by an attacker and what can we do to protect our equipment. All the examples are related to Cisco equipment, but some of them can just as well affect equipment from other vendors. Most of the observations and data have been obtained by the authors via research and development of the Yersinia tool. Sometimes it has been impossible to find references or publicly available code, therefore certain observations are based on behavioural analysis and not on published standards.
Seven layers of OSIIn 1977 a model called Open Systems Interconnection (OSI) was proposed, with the aim to establish an interoperability standard for different vendor products. This model defines several layers related to data transfer, from the lowest (physical) to the highest (application) layer. They have a strong dependence upon one another and headers are usually added when traversing from a lower layer to a higher one. The seven layers are:
The Yersinia toolIn order to perform described Data Link layer attacks, we'll be using a tool called Yersinia, written by the authors of this article. Yersinia is portable - written in C (using libpcap and libnet), multithreaded (supports multiple users and multiple concurrent attacks). It can be used to analyse, edit and watch network packets and even save the traffic in pcap format. Latest Yersinia version (0.5.5.2) supports the following protocols:
Yersinia operates in one of three main modes:
All the attacks described have been executed in the GUI mode, although they can be run in one of the two other modes just as well. In order to find out what all the features of the tool are, press [h] when running Yersinia in the GUI mode (yersinia -I). Note: the GUI mode needs a large number of rows and columns to run - if running this mode fails, try to maximize your terminal window. Yersinia also incorporates attacks other than the ones on layer two (for example HSRP, DHCP), however we'll be focusing only on its capabilities related to layer two. The name of the tool has been devised from the name of the bacteria which caused Black Death in Europe in the Middle Age - Yersinia pestis.
Decoding packetsAlthough one of Yersinia's use is decoding and watching packets of layer two protocols, we can use other protocol analysers such as tcpdump or Ethereal for that purpose. If we, for example, want to capture STP packets, ethereal can be run with the following options: # ethereal -f stp
STP (Spanning Tree Protocol)The purpose of STP is avoiding network loops when interconnecting network segments. Only one unique path can exist from one device to another. Each STP packet is called BPDU (Bridge Protocol Data Unit), and we can identify it by looking at its format: an IEEE 802.3 packet with a 802.2 header and with destination MAC 01:80:C2:00:00:00 (see Figure 1).
Figure 1. How a BPDU packet is built
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