## 4 Security Threats
### Distribution of Sharing Rules
#### 4.3.3 Unreachable Rules  
A common and important security vulnerability, posing a significant threat to a DSA network's operation, is that of preventing nodes from accessing the rules enforced on them [24]. As already mentioned, there are two discrete types of rules in such networks, those derived by a negotiation process and those predefined by either an administrative authority (e.g., the FCC), or the network administrator itself (which usually remain unchanged for long periods of time). Independent of the category, the managing network must assure that all its clients can access the rules in an uninterrupted manner. Since this access is usually supported by either a common control channel, or the data channels themselves, a number of related security issues can arise and, thus, affect the overall network's stability. Following is a brief description of the most important such vulnerabilities. A careful reader will notice that some of them are closely related to threats already mentioned in previous sections, particularly in Section 4.2.1. To begin with, an attacker may attempt to occupy the common control channel [38], if such exists, or else the backbone of the sharing mechanism of the DSA network. An attacker's alternatives include either jamming [38] or flooding [40] the control channel [63]. In the former case, the attacker has to simply transmit a high-power signal over the required channel to obstruct packet transmissions over it. Consequently, the number of packet collisions will significantly increase and large delays will unavoidably be instigated. In the latter case, an attacker has to inject fake traffic in the control channel with a view to increase the competition for its usage and make it operate under significant congestion. In both cases, packets containing the desired sharing rules can never reach their destination, causing strong disturbance and chaotic conditions. Network nodes are always able to switch to a new control channel to avoid any possible abnormal operating conditions. However, such an alternative does not come at zero cost. All communicating parties have to either communicate with each other to agree on which new channel to move to, or be informed in advance regarding which new channel they should move to each time a problem arises. The high handover costs, additionally, are not easy to be amortized over time. On the other hand, when the rules distribution is carried out over the existing data channels of a DSA network, an attacker can follow a slightly different approach. An adversary may first determine the time periods during which the required control information is transmitted and the respective channel number and then cause significant targeted interference. The unavoidable increase in packet collisions will prevent any new or existing rule from reaching its destination, letting the underlying DSA network operate far from optimally. While such an attack is harder to execute and requires more resources to be spent from the attacker's side, it should not be ignored. A very interesting categorization regarding the enforceable rules in a DSA network is presented in [24]. According to this, there are the positive, as well as the negative ones, a brief description of which is provided below. * _Positive_ rules: This category contains rules defining the conditions under which a node is allowed to use the available resources. * _Negative_ rules: These rules define the requirements to be satisfied to prevent a node from using the aforementioned resources. Surprisingly, this categorization is enough to differentiate the way a DSA network is influenced by such an attack. In the first case, a lost positive rule may result in spectrum under-utilization and users' resource starvation, since no (new) rules to dictate when nodes are allowed to gain access to the spectrum exist. As a consequence, no node will be able to transmit any packet for satisfying its operational needs, leaving the spectrum unexploited.