Impossible Disassembly

Created the Monday 18 March 2019. Updated 5 months, 3 weeks ago.

Impossible disassembly is an anti-disassembling technique that involves inserting data bytes after a conditional jump instruction in order to prevent the real instruction that follows from being disassembled. This technique takes advantage of a basic assumption in disassembly, which states that one byte is only interpreted in the context of one instruction. By inserting a byte that is the opcode for a multibyte instruction, the disassembler will be unable to correctly interpret the next instruction, and will therefore generate incorrect disassembly output.

Technique Identifier

U0211

Technique Tags

Impossible disassembly Anti-disassembling Disassembler Conditional jump Data byte Disassembly output Disassembly accuracy

Code Snippets

#![no_main]

macro_rules! rogue_byte {
    ($byte:expr) => {
        unsafe {
            core::arch::asm!(
                "lea r8, [rip]",           // Get next position
                "add r8, 8",               // Offset after rogue
                "push r8",                 // Jump after rogue
                "ret",
                concat!(".byte ", $byte),  // inject rogue byte
                options(nostack, nomem)
            )
        }
    };
}

// $ pwn shellcraft amd64.linux.sh
#[no_mangle]
#[link_section=".text"]
static SHELLCODE: [u8; 48] = [
    0x6a,0x68,0x48,0xb8,0x2f,0x62,0x69,0x6e,0x2f,0x2f,0x2f,0x73,
    0x50,0x48,0x89,0xe7,0x68,0x72,0x69,0x01,0x01,0x81,0x34,0x24,
    0x01,0x01,0x01,0x01,0x31,0xf6,0x56,0x6a,0x08,0x5e,0x48,0x01,
    0xe6,0x56,0x48,0x89,0xe6,0x31,0xd2,0x6a,0x3b,0x58,0x0f,0x05
];

#[no_mangle]
fn main() -> usize {
    // Get shellcode function pointer
    let malicious: extern "C" fn() -> usize = unsafe { 
        std::mem::transmute(&SHELLCODE as *const _ as *const ())
    };

    rogue_byte!(0xe8);
    malicious();

    return 0;
}

About Author Open Snippet

Author: Gelven / Target Platform: Linux

Detection Rules

rule RogueByte
{
    meta:
        description = "Detect disassembly obfuscation with a rogue byte"
        author = "Joakim (Gelven) Pettersen"
        date = "2024-11-08"
    strings:
        /* rax..rdi */
        $s1 = { 48 8D ?? 00 00 00 00 [0-10] 48 83 ?? 07 [0-10] ( FF E? | 5? C3 ) }
        /* r8..r15 */
        $s2 = { 4C 8D ?? 00 00 00 00 [0-10] 49 83 ?? 08 [0-10] ( 41 FF E? | 41 5? C3 ) }
    condition:
        any of them
}

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.