## 4 Security Threats
### Negotiation
#### 4.2.7 Auction Cheating  
A significant number of methods in the literature borrow ideas from auction theory to provide optimal spectrum allocation results. The intrinsic fairness, as well as the efficiency, characterizing such techniques makes them ideal for use in the CR paradigm. Multi-unit Sealed-Bid [43], Vickrey [44], Second Price [45] and Double Auctions [46] are some representative examples. Not surprisingly, since many such approaches depend on the truthfulness of the requests and information they receive from the negotiating entities, they are also vulnerable to the aforementioned attacks. However, some additional problems could arise, in particular in relation to auction implementation in automated, fast and underpowered environments, a brief description of which is provided below:  
* _Bid Shielding_[47]: An attacking node may announce an extremely high bid to a sharing mechanism with the view to discourage its competitors from making any more offers. Since each node maintains its own affordable upper bid limit, in such a case it would be obliged to retire early from the whole process. To make things worse, if a node has the right to retract its last bid, allowing the second biggest one to be accepted, a number of colluding nodes may exploit such a vulnerability to successfully mislead the underlying mechanism and gain access to the auctioned spectrum at a really low price. * _Shilling_: One or more attacking nodes may announce sequential bids to a sharing mechanism with aiming to increase the winning price of a special spectrum portion. In fact, these nodesmay even have no intention to win the auction, but their sole objective would be to press nodes in real need of the available spectrum to offer more and more money. * _Sniping_: An attacking node may choose to announce a high (or higher than the current) bid, just before an auction closes. As a consequence, none of its opponents would have the time to respond with a higher one, losing automatically the chance to gain access to the spectrum. Since nodes could stop offering higher bids even before reaching the highest price they can afford (for example when they know that there has been no higher bid offered by their competitors until then), a legitimate node could fail to win the auction, even if it had not reached its price limit. * _Bidding ring and loser collusion_[48]: These types of attacks are closely related to those already described in Section 4.2.5. * _Sub-leasing_[48]: One or more cheating nodes may do their best to win an auction at the lowest possible price and, in turn, sublease the spectrum to others (and losers of the original auction may be included). The primary objective of such an attacker would be to earn extra profit at zero cost by gaining benefits which should be credited to the original spectrum auctioneer. All these attacks are problems related to auction design and implementation, studied in the field of auctions and addressed in various ways. What may be different here is that some of these may be neglected to be addressed for various reasons, e.g. from simple oversight, to considerations of the value of the resource to be auctioned _vs._ the cost of addressing some of these problems (and enforcement of the rules). Finally, there are approaches found in the literature that correlate a spectrum opportunity's value with the number of nodes competing for its usage [49]. Unfortunately, such an assumption can allow an important vulnerability to arise. A set of colluding attackers may simultaneously start competing for occupancy of a specific transmission opportunity whose quality is not the highest in the underlying network. Fraudulently increasing an opportunity's \"reputation\" will unavoidably mislead all other network nodes, making them falsely believe that there is a more valuable candidate.