The vmlinuz file is the compressed Linux kernel image that gets loaded during system boot. Unlike its uncompressed counterpart vmlinux, this file is specifically designed to be space-efficient while containing all essential kernel components. In Ubuntu systems, it's typically located in /boot or at the root directory.

# Typical location of vmlinuz in Ubuntu
ls -lh /boot/vmlinuz-$(uname -r)
-rw------- 1 root root 7.8M Sep 15 14:23 /boot/vmlinuz-5.4.0-84-generic

A changing checksum for vmlinuz is significant because:

• Kernel files shouldn't change unless through official updates
• Unauthorized modifications could indicate rootkit installation
• The kernel runs with highest privileges (ring 0)

To verify if the change was legitimate:

# Compare with package manager's records
dpkg --verify linux-image-$(uname -r)

The combination of a modified kernel and empty Apache logs strongly suggests malicious activity. Common attack patterns include:

# Check for hidden kernel modules (potential rootkits)
lsmod | grep -Ev '^(Module|ip_tables|nf_conntrack)'

# Verify kernel module signatures
for module in /lib/modules/$(uname -r)/kernel/*; do
    modinfo "$module" | grep -E 'signer|sig_key'
done

1. Isolate the affected system from the network
2. Capture forensic artifacts before reboot:

   # Memory capture
   sudo dd if=/proc/kcore of=/tmp/memdump.img bs=1M count=1024
   
   # Kernel symbol table
   sudo cat /proc/kallsyms > /tmp/kallsyms.txt
   

3. Restore from known-good backup
4. Analyze the backup's vmlinuz checksum:

   sha256sum /path/to/backup/vmlinuz
   

For WordPress deployments, consider these hardening steps:

# Kernel hardening via sysctl
echo "kernel.kptr_restrict = 2" >> /etc/sysctl.conf
echo "kernel.dmesg_restrict = 1" >> /etc/sysctl.conf
sysctl -p

# Filesystem monitoring configuration (example for auditd)
sudo auditctl -w /vmlinuz -p wa -k kernel_binary
sudo auditctl -w /boot -p wa -k boot_files

Implement real-time monitoring with checks like:

# Zabbix item prototype for kernel file monitoring
UserParameter=kernel.checksum[*],sha256sum $1 | cut -d' ' -f1

# Cron job for periodic verification
0 3 * * * root /usr/bin/rkhunter --check --sk --quiet

The vmlinuz file is the compressed Linux kernel image that gets loaded during system boot. It's typically located in the root directory (/vmlinuz) or in /boot. Unlike its uncompressed counterpart vmlinux, vmlinuz is smaller and contains the essential components for system startup.

Under normal circumstances, the vmlinuz checksum only changes when:

• The kernel is updated via package manager (apt/yum)
• Manual kernel compilation and installation occurs

If neither of these operations were performed, a checksum change is a red flag. Here's how to verify the current kernel package:

# For Debian/Ubuntu
dpkg -l | grep linux-image

# For RHEL/CentOS
rpm -qa kernel

When you encounter a changed vmlinuz checksum coupled with suspicious signs (like empty logs), follow these steps:

# 1. Check file metadata
ls -lh /vmlinuz
stat /vmlinuz

# 2. Verify package integrity
debsums -s linux-image-$(uname -r)

# 3. Compare against known good version
md5sum /vmlinuz
# Compare with output from:
cat /var/lib/dpkg/info/linux-image-$(uname -r).md5sums | grep vmlinuz

For WordPress servers, kernel-level compromises often follow these patterns:

• Modified kernel modules for rootkit functionality
• Kernel-based process hiding
• Network traffic interception hooks

Check for suspicious kernel modules with:

lsmod | grep -Ev '^Module|autofs4|bnep|bluetooth|bridge|cpufreq|drm|ecb|ip6|ipv6|joydev|llc|mac80211|nf_conntrack|nls|parport|ppdev|rfcomm|sco|uvcvideo|video|x_tables|xt_|8021q|acpi_cpufreq|aes|ahci|atkbd|ehci|i8042|intel|libahci|memstick|pata|processor|psmouse|rng|serio|sg|soundcore|sparse_keymap|snd|thermal|thinkpad|tpm|usb|virtio|wmi|xhci'

When compromise is suspected:

1. Isolate the server from the network
2. Preserve forensic evidence (create disk snapshots)
3. Restore from known-good backups
4. Perform post-mortem analysis

Implement these security practices:

# Monitor critical files with auditd
apt install auditd
auditctl -w /vmlinuz -p wa -k kernel_binary
auditctl -w /boot -p wa -k boot_dir

Configure regular integrity checking with AIDE or similar tools:

apt install aide
aideinit
mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
aide.wrapper --check