Virtual Function Table Attack

Virtual function table (A.K.A vtable) is the key to manage virtual function which is a key mechanism to support polymorphism in C++ language. VTable is used to support dynamic dispatch or runtime method binding in C++. At the same time it is a popular target of malicious programmers to execute their malicious behavior on the system. Attacker exploits the virtual function table mechanism to execute shellcode in the system. In this writing we’ll see how attacker can execute virtual function table attack.

 In C++, for each class with virtual function, the compiler will create one or more virtual function table. The number of created tables depend on the class inheritance hierarchy. Inside each instance of the class, a pointer is created. This pointer is called virtual function pointer or vfptr and it points to the VTable of the corresponding class. For example, take a look on the following class hierarchy, its object layout and the corresponding VTable.

class B1{
	int B1_i;
	int B1_j;
	virtual void B1_f1();
	virtual void B1_f2();
};

class B2{
	int B2_i;
	int B2_j;
	virtual void B2_f1();
	virtual void B2_f2();
};

class A : public B1, public B2{
	int A_i;
	int A_j;
	virtual void A_f1();
	virtual void A_f2();
};
Object Layout with VTable Pointer
Object Layout with VTable Pointer

As the figure dictates, the virtual function of the derived class and the first base class are normally placed together on the same VTable. During runtime, if there is a need to make a virtual function call, system will first read out vfptr from the object. Using the vfptr, it will read out the target function from the VTable and finally call the target function.

Now from an attacker’s perspective, there are 3 possible ways to execute a virtual function table attack.

  1. VTable corruption attack: Modify the VTable content so it can point to some malicious function
  2. VTable Inject Attack: Inject a fake VTable into the application and modify vfptr to point to this injected malicious VTable.
  3. VTable Reuse Attack: Modify the vfptr to point to some already  existing VTable or existing code or data in memory. It differs from VTable injection attack by its use of existing code, not attacker crafted code.

The attacker normally make use of some existing vulnerability in the software to execute any of these 3 attacks. Among these, VTable injection attack is most popular because it provides attacker more control of the attack.