The 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 software#
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