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As we mentioned earlier, the current code segment often spans the entire linear memory range (such as zero to 4 GB). The near jump is a jump within the current code segment. Intel processors have several different jump modes that have evolved over time, but a number of modes are no longer used. The operand can be an immediate value, a register, or a memory location. The destination operand specifies the address of the instruction we wish to execute next. Jump operations transfer control to a different point in the program stream without recording any return information. By downloading within a spawned process, the malware can be hidden from the user (and the browser). #Vxworks Program Exception Current Instruction Address downloadThe shellcode then spawns a process to download and execute malware. It is usually possible to predict an appropriate EIP value that will land execution within the NOPs which will “execute” until the payload (usually shellcode) is encountered. A standard buffer overflow is used to overwrite the EIP. Next, a browser’s vulnerability in a component (such as a plug-in) is exploited to alter the execution flow to jump into the heap. Spraying the heap by filling chunks of memory with payload results in payload at predictable addresses. The nop_sled is appended to the payload and written to the heap in the form of JavaScript strings mapping to a new block of memory. The “\0×90” represents the NOP instruction and the Unicode encoding of NOP instruction is “%u90”. #Vxworks Program Exception Current Instruction Address softwareįirst, create what is known as a nop_sled (NOP sled), a block of NOP instructions with a Unicode encoding which is an industry standard of representing the strings that is understood by the software application (browser, etc.).Basically, the heap exploitation takes the following steps: When the return address pointer is overwritten with an address controlled by the attacker, the pointer lands on the NOP sled leading to the execution of the attacker supplied payload. The instruction pointer moves forward instruction-by-instruction until it reaches the target code. NOP sled is a collection of NOP instructions placed in the memory to delay the execution in the scenarios where the target address is unknown. ![]() It is an assembly instruction (x86 programming) which does not perform any operation when placed in the code. A specific size of memory is allocated to JavaScript strings containing NOP sled (also known as NOP ramp) and shellcode (payload) and in most cases the specific address range points to a NOP sled. The heap is allocated in the form of blocks and the JavaScript engine stores the allocated strings to new blocks. The idea is to fill chunks of heap memory with payload before taking control of the Extended Instruction Pointer (EIP). This technique exploits the fact that it is possible to predict heap locations (addresses). Heap spraying is a stage of browser exploitation where a payload is placed in a browser’s heap. ![]() In this case, we will use dumpbin for simplicity, since it can quickly tell us what we need. To do this, we could use the depends.exe program that ships with visual studio, or the dumpbin.exe utility that will allow you to examine a program's imports. To do this effectively, we need to see what DLLs are imported into our attacked program and examine those loaded DLLs for potentially vulnerable pieces of code. Since we have already written findjmp, we'll use that to find an effective place to get our ESP into the EIP. So now we examine the memory space of the attacked program for useful pieces of code that would allow us to get the EIP register to point to the ESP. Also notice how in the function epilogue, the saved EBP was restored, but this time with our 0x90 string instead of its original contents. This means that if we can get the contents of the ESP register into the EIP, we can execute code at that point. Also, control should continue from there. After this ret, the ESP will move up 4 bytes and what is there should be moved to the EIP. We notice that the ESP points right into the stack, right after where the saved EIP should be. Typically, I put a debugging break point at the end of the function that returns, so I can see what the state of the registers are when we are right before the vulnerable functions ret instruction. As mentioned before, there are numerous ways to get the EIP to point to our code. Next, we need to write out where we want the EIP to go. In Hack Proofing Your Network (Second Edition), 2002 Finding a Jump Point ![]()
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