What is indicated by the value 0x90 in a network IDS entry?

• A. The NOOP instruction commonly used by Intel processors, often indicating a buffer overflow attempt.
• B. A signature for legitimate processor maintenance traffic.
• C. An error code indicating a failed packet checksum.
• D. Encrypted payload data indicating secure transmission.

Answer: (A)

0x90 is the NOP (No Operation) opcode in x86 machine code. In attack payloads, especially buffer overflow exploits, defenders often insert a string of NOPs—called a NOP sled—to pad the shellcode and increase the chances that execution lands in the actual payload even if the exact jump address is off. Seeing 0x90 in a network IDS context is a red flag for this kind of exploitation technique, because it signals the presence of a NOP sled rather than legitimate, meaningful data. It’s not an indicator of legitimate processor maintenance traffic, nor an error code, nor encrypted payload data. The key idea is that 0x90 is a simple instruction used for alignment and padding in exploits, which is why IDS entries may flag it as suspicious.