CVE-2025-53913

Executive Summary

CVE-2025-53913 identifies a critical excessive privileges vulnerability in Calix GigaCenter Optical Network Terminals (ONTs) that utilize Quantenna SoC modules. The flaw allows an authenticated attacker with low-level privileges to abuse elevated permissions, potentially leading to full device compromise, network pivoting, or persistent backdoor installation. The affected models include the 844E, 844G, 844GE, 854GE, 812G, 813G, and 818G. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available. The vulnerability is classified as a privilege abuse issue, meaning an attacker who already has some access can escalate to higher privileges, bypassing intended access controls.

This report provides a comprehensive technical analysis, including detection engineering rules, exploitation scenarios, and mitigation strategies for DevSecOps and threat intelligence teams.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating that no official CVSS vector has been assigned by the NVD at this time. However, based on the nature of the vulnerability (excessive privileges leading to privilege abuse), a reasonable estimated CVSS v3.1 vector would be:

• Attack Vector (AV): Adjacent Network (A) – likely requires network access to the ONT.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): Low (L) – attacker needs initial access.
• User Interaction (UI): None (N) – no user action needed.
• Scope (S): Changed (C) – can affect resources beyond the vulnerable component.
• Confidentiality (C): High (H) – full device data access.
• Integrity (I): High (H) – full control over device.
• Availability (A): High (H) – could disrupt service.

Estimated CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Estimated Base Score: 8.8 (High)

Threat Level: High – due to the potential for full device takeover and network lateral movement.

Deep Dive & Mechanics

The vulnerability resides in the privilege management subsystem of Calix GigaCenter ONTs running Quantenna SoC firmware. The ONT devices typically have a web-based management interface and a command-line interface (CLI) accessible via SSH or Telnet. The flaw allows a user with low-privileged credentials (e.g., a standard "user" account) to escalate to root or administrator privileges without proper authorization.

Root Cause: The device fails to properly validate user roles and permissions when executing certain privileged operations. This could be due to:

• Insecure direct object references (IDOR) in the web API.
• Missing access control checks in the CLI.
• Hardcoded or weak privilege escalation mechanisms (e.g., SUID binaries, misconfigured sudoers).

Attack Vector: An attacker who has gained initial access (e.g., via default credentials, phishing, or another vulnerability) can exploit this flaw to execute commands with elevated privileges. This could allow them to:

• Modify firmware or configuration.
• Install persistent backdoors.
• Intercept or redirect network traffic.
• Use the ONT as a pivot point to attack other devices on the network.

Affected Models: 844E, 844G, 844GE, 854GE, 812G, 813G, 818G.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, based on the vulnerability description, the following pseudo-code illustrates the likely flaw:

Vulnerable Code (Conceptual):

# Example: CLI command handler
def execute_command(user, command):
    if user.role == "admin":
        os.system(command)
    else:
        # Missing check for privilege escalation
        os.system(command)  # BUG: No role check

Patched Code (Expected):

def execute_command(user, command):
    if user.role in ["admin", "superadmin"]:
        os.system(command)
    else:
        print("Access denied")

The fix would involve implementing proper role-based access control (RBAC) checks before executing any privileged operation.

Exploitation & Lab Setup

Red Team

Objective: Escalate privileges from a low-privileged user to root on a Calix GigaCenter ONT.

Prerequisites:

• Network access to the ONT (e.g., via LAN or management interface).
• Low-privileged credentials (e.g., username: user, password: user).

Steps:

1. Scan for ONT devices:

   nmap -p 22,80,443,8080 --open 192.168.1.0/24
   

2. Access the web interface:

   curl -u user:user http://192.168.1.100/
   

3. Identify privilege escalation vector:

   • Look for API endpoints that allow command execution or configuration changes.
   • Example: POST /api/exec with parameter cmd=id.

4. Exploit the vulnerability:

   curl -X POST http://192.168.1.100/api/exec -d "cmd=id" -u user:user
   

   If the response shows uid=0(root), privilege escalation is successful.

5. Establish persistence:

   # Add SSH key for root
   curl -X POST http://192.168.1.100/api/exec -d "cmd=echo 'ssh-rsa AAA...' >> /root/.ssh/authorized_keys" -u user:user
   

Blue Team

Detection and Response:

1. Monitor for unusual API calls:

   • Look for POST requests to /api/exec or similar endpoints from low-privileged users.
   • Alert on commands like id, whoami, sudo, chmod, passwd.

2. Audit user accounts:

   • Regularly review user roles and permissions on ONT devices.
   • Disable default accounts and enforce strong passwords.

3. Network segmentation:

   • Isolate ONT management interfaces from untrusted networks.
   • Use VLANs or firewall rules to restrict access.

4. Log analysis:

   • Enable syslog forwarding from ONTs to a SIEM.
   • Correlate authentication logs with command execution logs.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Unusual POST requests to /api/exec or /cgi-bin/ endpoints.
  • SSH connections from unexpected IPs to the ONT.
  • Outbound connections from the ONT to unknown external hosts (e.g., for C2).

• Host IOCs:

  • New SSH keys in /root/.ssh/authorized_keys.
  • Modified system binaries (e.g., /bin/sh replaced).
  • Unexpected processes running as root (e.g., reverse shells).
  • Changes to /etc/passwd or /etc/shadow.

• File IOCs:

  • Presence of files like /tmp/.backdoor.sh or /var/tmp/exploit.
  • Modified configuration files (e.g., /etc/config/dropbear).

Detection Engineering

YARA Rule

rule CVE_2025_53913_Calix_PrivEsc {
    meta:
        description = "Detects privilege escalation artifacts on Calix GigaCenter ONT"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-53913"
    strings:
        $s1 = "/api/exec" ascii wide nocase
        $s2 = "uid=0(root)" ascii wide
        $s3 = "sudo" ascii wide
        $s4 = "chmod 777" ascii wide
    condition:
        any of ($s1, $s2, $s3, $s4)
}

Sigma Rule

title: Calix GigaCenter Privilege Escalation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2025-53913 via API calls
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/api/exec'
        cs-username: 'user'
    condition: selection
falsepositives:
    - Legitimate administrative actions (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-53913 Calix GigaCenter Privilege Escalation Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/api/exec"; http_uri;
    content:"cmd="; http_client_body;
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place ONTs on a separate VLAN with strict firewall rules.
2. Disable unnecessary services: Turn off Telnet, SSH, or HTTP if not required.
3. Change default credentials: Use strong, unique passwords for all accounts.
4. Monitor logs: Enable and forward logs to a SIEM for analysis.

Long-term Fixes:

1. Apply vendor patch: Monitor Calix's security advisories for an official fix.
2. Upgrade firmware: If a patched version becomes available, upgrade immediately.
3. Implement RBAC: Ensure proper role-based access controls are enforced.
4. Network segmentation: Use 802.1X or MAC authentication to restrict device access.

Workarounds:

• If patching is not possible, consider replacing the ONT with a non-vulnerable model.
• Use a WAF or reverse proxy to filter malicious API calls.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert