analyzing-linux-system-artifacts

分析 Linux 系统制品

适用场景

• 调查遭到入侵的 Linux 服务器或工作站时
• 识别持久化机制（cron、systemd、SSH 密钥）时
• 通过 Shell 历史记录和身份验证日志追踪用户活动时
• 在事件响应过程中确定 Linux 入侵事件的影响范围时
• 检测 Rootkit、后门和未经授权的修改时

前提条件

• Linux 系统的取证镜像或实时访问权限（只读）
• 了解 Linux 文件系统层次结构（FHS）
• 熟悉常见的 Linux 日志位置（/var/log/）
• 工具：chkrootkit、rkhunter、AIDE、auditd 日志
• 熟悉 systemd、cron 和 PAM 配置
• 拥有 root 权限，以便完整收集制品

工作流

第 1 步：挂载并收集系统制品

# Mount forensic image read-only
mount -o ro,loop,offset=$((2048*512)) /cases/case-2024-001/images/linux_evidence.dd /mnt/evidence

# Create collection directories
mkdir -p /cases/case-2024-001/linux/{logs,config,users,persistence,network}

# Collect authentication logs
cp /mnt/evidence/var/log/auth.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/secure* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/syslog* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/kern.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/audit/audit.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/wtmp /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/btmp /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/lastlog /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/faillog /cases/case-2024-001/linux/logs/

# Collect user artifacts
for user_dir in /mnt/evidence/home/*/; do
    username=$(basename "$user_dir")
    mkdir -p /cases/case-2024-001/linux/users/$username
    cp "$user_dir"/.bash_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.zsh_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp -r "$user_dir"/.ssh/ /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.bashrc /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.profile /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.viminfo /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.wget-hsts /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.python_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
done

# Collect root user artifacts
cp /mnt/evidence/root/.bash_history /cases/case-2024-001/linux/users/root/ 2>/dev/null
cp -r /mnt/evidence/root/.ssh/ /cases/case-2024-001/linux/users/root/ 2>/dev/null

# Collect system configuration
cp /mnt/evidence/etc/passwd /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/shadow /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/group /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/sudoers /cases/case-2024-001/linux/config/
cp -r /mnt/evidence/etc/sudoers.d/ /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/hosts /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/resolv.conf /cases/case-2024-001/linux/config/
cp -r /mnt/evidence/etc/ssh/ /cases/case-2024-001/linux/config/

第 2 步：分析用户帐户和身份验证

# Analyze user accounts for anomalies
python3 << 'PYEOF'
print("=== USER ACCOUNT ANALYSIS ===\n")

# Parse /etc/passwd
with open('/cases/case-2024-001/linux/config/passwd') as f:
    for line in f:
        parts = line.strip().split(':')
        if len(parts) >= 7:
            username, _, uid, gid, comment, home, shell = parts[0], parts[1], int(parts[2]), int(parts[3]), parts[4], parts[5], parts[6]

            # Flag accounts with UID 0 (root equivalent)
            if uid == 0 and username != 'root':
                print(f"  ALERT: UID 0 account: {username} (shell: {shell})")

            # Flag accounts with login shells that shouldn't have them
            if shell not in ('/bin/false', '/usr/sbin/nologin', '/bin/sync') and uid >= 1000:
                print(f"  User: {username} (UID:{uid}, Shell:{shell}, Home:{home})")

            # Flag system accounts with login shells
            if uid < 1000 and uid > 0 and shell in ('/bin/bash', '/bin/sh', '/bin/zsh'):
                print(f"  WARNING: System account with shell: {username} (UID:{uid}, Shell:{shell})")

# Parse /etc/shadow for account status
print("\n=== PASSWORD STATUS ===")
with open('/cases/case-2024-001/linux/config/shadow') as f:
    for line in f:
        parts = line.strip().split(':')
        if len(parts) >= 3:
            username = parts[0]
            pwd_hash = parts[1]
            last_change = parts[2]

            if pwd_hash and pwd_hash not in ('*', '!', '!!', ''):
                hash_type = 'Unknown'
                if pwd_hash.startswith('$6$'): hash_type = 'SHA-512'
                elif pwd_hash.startswith('$5$'): hash_type = 'SHA-256'
                elif pwd_hash.startswith('$y$'): hash_type = 'yescrypt'
                elif pwd_hash.startswith('$1$'): hash_type = 'MD5 (WEAK)'
                print(f"  {username}: {hash_type} hash, last changed: day {last_change}")
PYEOF

# Analyze login history
last -f /cases/case-2024-001/linux/logs/wtmp > /cases/case-2024-001/linux/analysis/login_history.txt
lastb -f /cases/case-2024-001/linux/logs/btmp > /cases/case-2024-001/linux/analysis/failed_logins.txt 2>/dev/null

第 3 步：检查持久化机制

# Check cron jobs for all users
echo "=== CRON JOBS ===" > /cases/case-2024-001/linux/persistence/cron_analysis.txt

# System cron
for cronfile in /mnt/evidence/etc/crontab /mnt/evidence/etc/cron.d/*; do
    echo "--- $cronfile ---" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    cat "$cronfile" 2>/dev/null >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
done

# User cron tabs
for cronfile in /mnt/evidence/var/spool/cron/crontabs/*; do
    echo "--- User crontab: $(basename $cronfile) ---" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    cat "$cronfile" 2>/dev/null >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
done

# Check systemd services for persistence
echo "=== SYSTEMD SERVICES ===" > /cases/case-2024-001/linux/persistence/systemd_analysis.txt
find /mnt/evidence/etc/systemd/system/ -name "*.service" -newer /mnt/evidence/etc/os-release \
   >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt

for svc in /mnt/evidence/etc/systemd/system/*.service; do
    echo "--- $(basename $svc) ---" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
    cat "$svc" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
done

# Check authorized SSH keys (backdoor detection)
echo "=== SSH AUTHORIZED KEYS ===" > /cases/case-2024-001/linux/persistence/ssh_keys.txt
find /mnt/evidence/home/ /mnt/evidence/root/ -name "authorized_keys" -exec sh -c \
   'echo "--- {} ---"; cat {}; echo ""' \; >> /cases/case-2024-001/linux/persistence/ssh_keys.txt

# Check rc.local and init scripts
cat /mnt/evidence/etc/rc.local 2>/dev/null > /cases/case-2024-001/linux/persistence/rc_local.txt

# Check /etc/profile.d/ for login-triggered scripts
ls -la /mnt/evidence/etc/profile.d/ > /cases/case-2024-001/linux/persistence/profile_scripts.txt

# Check for LD_PRELOAD hijacking
grep -r "LD_PRELOAD" /mnt/evidence/etc/ 2>/dev/null > /cases/case-2024-001/linux/persistence/ld_preload.txt
cat /mnt/evidence/etc/ld.so.preload 2>/dev/null >> /cases/case-2024-001/linux/persistence/ld_preload.txt

第 4 步：分析 Shell 历史记录和命令执行情况

# Analyze bash history for each user
python3 << 'PYEOF'
import os, glob

print("=== SHELL HISTORY ANALYSIS ===\n")

suspicious_commands = [
    'wget', 'curl', 'nc ', 'ncat', 'netcat', 'python -c', 'python3 -c',
    'perl -e', 'base64', 'chmod 777', 'chmod +s', '/dev/tcp', '/dev/udp',
    'nmap', 'masscan', 'hydra', 'john', 'hashcat', 'passwd', 'useradd',
    'iptables -F', 'ufw disable', 'history -c', 'rm -rf /', 'dd if=',
    'crontab', 'at ', 'systemctl enable', 'ssh-keygen', 'scp ', 'rsync',
    'tar czf', 'zip -r', 'openssl enc', 'gpg --encrypt', 'shred',
    'chattr', 'setfacl', 'awk', '/tmp/', '/dev/shm/'
]

for hist_file in glob.glob('/cases/case-2024-001/linux/users/*/.bash_history'):
    username = hist_file.split('/')[-2]
    print(f"User: {username}")

    with open(hist_file, 'r', errors='ignore') as f:
        lines = f.readlines()

    print(f"  Total commands: {len(lines)}")
    flagged = []
    for i, line in enumerate(lines):
        line = line.strip()
        for cmd in suspicious_commands:
            if cmd in line.lower():
                flagged.append((i+1, line))
                break

    if flagged:
        print(f"  Suspicious commands: {len(flagged)}")
        for lineno, cmd in flagged:
            print(f"    Line {lineno}: {cmd[:120]}")
    print()
PYEOF

第 5 步：检查 Rootkit 和被修改的二进制文件

# Check for known rootkit indicators
# Compare system binary hashes against known-good
find /mnt/evidence/usr/bin/ /mnt/evidence/usr/sbin/ /mnt/evidence/bin/ /mnt/evidence/sbin/ \
   -type f -executable -exec sha256sum {} \; > /cases/case-2024-001/linux/analysis/binary_hashes.txt

# Check for SUID/SGID binaries (potential privilege escalation)
find /mnt/evidence/ -perm -4000 -type f 2>/dev/null > /cases/case-2024-001/linux/analysis/suid_files.txt
find /mnt/evidence/ -perm -2000 -type f 2>/dev/null > /cases/case-2024-001/linux/analysis/sgid_files.txt

# Check for suspicious files in /tmp and /dev/shm
find /mnt/evidence/tmp/ /mnt/evidence/dev/shm/ -type f 2>/dev/null \
   -exec file {} \; > /cases/case-2024-001/linux/analysis/tmp_files.txt

# Check for hidden files and directories
find /mnt/evidence/ -name ".*" -not -path "*/\." -type f 2>/dev/null | \
   head -100 > /cases/case-2024-001/linux/analysis/hidden_files.txt

# Check kernel modules
ls -la /mnt/evidence/lib/modules/$(ls /mnt/evidence/lib/modules/ | head -1)/extra/ 2>/dev/null \
   > /cases/case-2024-001/linux/analysis/extra_modules.txt

# Check for modified PAM configuration (authentication backdoors)
diff /mnt/evidence/etc/pam.d/ /cases/baseline/pam.d/ 2>/dev/null \
   > /cases/case-2024-001/linux/analysis/pam_changes.txt

核心概念

概念	说明
/var/log/auth.log	Debian/Ubuntu 系统上的主要身份验证日志
/var/log/secure	RHEL/CentOS 系统上的主要身份验证日志
wtmp/btmp	记录成功和失败登录会话的二进制日志
.bash_history	用户命令历史记录文件（攻击者可能会清除该文件）
crontab	常用于实现持久化的计划任务系统
authorized_keys	授予帐户免密码访问权限的 SSH 公钥
SUID 位	允许以文件所有者身份执行文件的权限（权限提升途径）
LD_PRELOAD	在加载其他库之前加载共享库的环境变量（挂钩技术）

工具与系统

工具	用途
chkrootkit	用于 Linux 系统的 Rootkit 检测扫描器
rkhunter	Rootkit Hunter——检查 Rootkit、后门和本地漏洞利用
AIDE	Advanced Intrusion Detection Environment——文件完整性监控工具
auditd	用于监控系统调用和文件访问的 Linux 审计框架
last/lastb	解析 wtmp/btmp，查看登录和登录失败历史记录
Plaso/log2timeline	创建包含 Linux 制品的超级时间线
osquery	基于 SQL 的系统查询工具，用于实时取证调查
Velociraptor	具备 Linux 制品收集功能的端点 Agent

常见场景

场景 1：SSH 暴力破解后遭到入侵 分析 auth.log，查找先出现 SSH 登录失败、后出现登录成功的记录，识别攻击者 IP，检查 .bash_history 中入侵后的命令，查看 authorized_keys 中是否添加了后门密钥，检查 crontab 中是否存在持久化任务，并检查网络连接。

场景 2：通过应用程序漏洞入侵 Web 服务器 检查 Web 服务器访问日志和错误日志中的利用尝试，检查 /tmp 和 /dev/shm 中是否存在 Webshell，分析 Web 服务器用户（www-data）的活动，检查是否通过 SUID 二进制文件或内核漏洞提升权限，并查看出站连接。

场景 3：数据库服务器上的内部威胁 分析嫌疑用户的 bash_history，查找数据库转储命令，检查主目录或 /tmp 中是否存在大型 tar/zip 文件，查看 scp/rsync 命令以查找数据传输记录，检查 cron 任务中是否存在自动外传数据的行为，并查看 USB 设备日志。

场景 4：云实例上的加密货币挖矿程序 检查 /proc 中的高 CPU 占用进程（实时）或 systemd 服务文件，查看 crontab 条目中是否存在重启挖矿程序的脚本，检查 /tmp 中是否存在挖矿二进制文件，分析网络连接以查找与矿池的通信，并检查 authorized_keys 中是否存在攻击者的访问密钥。

输出格式

Linux Forensics Summary:
  System: webserver01 (Ubuntu 22.04 LTS)
  Hostname: webserver01.corp.local
  Kernel: 5.15.0-91-generic

  User Accounts:
    Total: 25 (3 with UID 0 - 1 ANOMALOUS)
    Interactive shells: 8 users
    Recently created: admin2 (created 2024-01-15)

  Authentication Events:
    Successful SSH logins: 456
    Failed SSH attempts: 12,345 (from 23 unique IPs)
    Sudo executions: 89

  Persistence Mechanisms Found:
    Cron jobs: 3 suspicious (reverse shell, miner restart)
    Systemd services: 1 unknown (update-checker.service)
    SSH keys: 2 unauthorized keys in root authorized_keys
    rc.local: Modified with download cradle

  Suspicious Activity:
    - bash_history contains wget to pastebin URL
    - SUID binary /tmp/.hidden/escalate found
    - /dev/shm/ contains compiled ELF binary
    - LD_PRELOAD in /etc/ld.so.preload pointing to /lib/.hidden.so

  Report: /cases/case-2024-001/linux/analysis/