Overview of Traditional Security Techniques

This section will introduce some traditional, fundamental yet effective assembly language security techniques, rather than cutting-edge rootkits or virus/antivirus technologies. This foundational knowledge is crucial for understanding modern security mechanisms.

Basic Concepts of Viruses and Trojans

In the realm of traditional security, the distinctions between viruses and trojans are as follows:

1. Virus: Spreads by infecting other executable files
2. Trojan: Acts as a backdoor to the system, potentially leaking information or downloading more malicious programs

Modern malware often combines these two techniques, making it difficult to strictly differentiate, hence they are collectively referred to as “malware”.

Behavioral Characteristics of Malware

Malware may possess the following capabilities:

1. Download and execute more malicious code
2. Receive remote control commands
3. Use rootkit techniques to hide itself
4. Cause system instability (blue screens, performance degradation, etc.)

Challenges of Kernel-Level Malware

; Simplified kernel hook example (x86)
hook_syscall:
    cli                 ; Disable interrupts
    mov eax, [syscall_table+4*0x80]  ; Get original system call address
    mov [original_syscall], eax      ; Save original address
    mov dword [syscall_table+4*0x80], our_handler ; Replace with our handler
    sti                 ; Restore interrupts
    ret

our_handler:
    ; Malicious code logic
    cmp eax, 0x01      ; Check system call number
    je monitor_file_access
    jmp [original_syscall] ; Jump to original handler

The reasons why kernel-level malware is extremely difficult to remove are:

1. It has the same privilege level as the operating system
2. It can intercept and modify system calls
3. It can hide its existence

Prevention is Better than Cure

The basic principle of security isprevention is better than cure. Preventing malicious code execution is more effective than cleaning it up afterward. Modern defense mechanisms include:

1. User Account Control (UAC)
2. Code signing verification
3. Principle of least privilege

Analysis of UAC Mechanism

Although UAC introduced in Vista is not perfect, it provides important protection:

; Simplified logic simulating UAC privilege check
check_privilege:
    call get_current_token
    test eax, ADMIN_FLAG
    jnz allow_execution
    ; Show UAC prompt
    call show_uac_prompt
    cmp eax, USER_CONFIRMED
    jne deny_execution
allow_execution:
    ; Elevate privileges to execute code
    ret
deny_execution:
    ; Deny execution
    mov eax, ACCESS_DENIED
    ret

Advantages and disadvantages of UAC:

• Advantages: Prevents automatically running malicious programs
• Disadvantages: Frequent prompts affect user experience

Examples of Traditional Defense Techniques

1. Code Checksums and Verification:

verify_checksum:
    mov esi, code_start
    mov ecx, code_length
    xor eax, eax
calc_loop:
    add al, [esi]
    inc esi
    loop calc_loop
    cmp eax, [stored_checksum]
    jne corruption_detected
    ret

1. System Call Monitoring:

monitor_syscalls:
    mov eax, [esp+4]    ; Get system call number
    cmp eax, DANGEROUS_CALL
    jne allow_call
    ; Log or block dangerous calls
    call log_suspicious_activity
allow_call:
    jmp [original_syscall]

These traditional techniques, although simple, form the foundation of modern security defenses. Understanding these principles is essential for further learning of advanced security technologies.