Improving Web Application Security: Threats and - CGISecurity

200 Part III: Building Secure Web ApplicationsCarefully consider before using a link demand because it is easy to introduce securityvulnerabilities if you use them. If you do use link demands, consider the followingissues:● Luring attacks● Performance and link demands● Calling methods with link demands● Mixing class and method level link demands● Interfaces and link demands● Structures and link demands● Virtual methods and link demandsLuring AttacksIf you protect code with a link demand, it is vulnerable to luring attacks, wheremalicious code gains access to the resource or operation exposed by your codethrough a trusted intermediary as shown in Figure 8.5.Luring attackvia intermediaryA B CDCan’t accessXLink demandprotectedFigure 8.5An example of a luring attack with link demandsIn figure 8.5, methods in assembly X, which access a secure resource,are protected with a link demand for a specific public key (using aStrongNameIdentityPermission). Assemblies A, B, and C are signed with the privatekey that corresponds to the public key that assembly X trusts, and so these assembliescan call assembly X. Assemblies A, B, and C are subject to a luring attack if they donot check their callers for specific evidence before making calls to assembly X. Forexample, assembly D that is not signed with the same private key cannot callassembly X directly. It could, however, access assembly X through the trustedassembly A, if A does not check its callers, either with another link demand orthrough a full demand.