CVE-2026-6058

Executive Summary

CVE-2026-6058 is a denial-of-service (DoS) vulnerability in the Zyxel WRE6505 v2 wireless range extender, specifically in firmware version V1.00(ABDV.3)C0. The vulnerability resides in the CGI program responsible for rendering the “AP Select” page. An adjacent attacker on the WLAN can exploit this by injecting a malformed SSID into the environment. When an authenticated administrator visits the “AP Select” page, the improper encoding or escaping of the SSID causes the web management interface to crash, resulting in a DoS condition. The product is marked as “UNSUPPORTED WHEN ASSIGNED,” meaning it is End-of-Life (EOL) and no patches will be released. The CVSS score is 4.5 (Medium), with a low attack complexity but requiring adjacent network access and user interaction.

Supply Chain & Package Security

This vulnerability does not involve NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in an embedded device.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:H/UI:R/S:U/C:N/I:N/A:H
• Base Score: 4.5 (Medium)
• Attack Vector (AV): Adjacent Network (AV:A) – The attacker must be on the same WLAN.
• Attack Complexity (AC): Low (AC:L) – No special conditions required beyond a malformed SSID.
• Privileges Required (PR): High (PR:H) – Requires an authenticated administrator session.
• User Interaction (UI): Required (UI:R) – The admin must visit the “AP Select” page.
• Scope (S): Unchanged (S:U) – The vulnerability affects only the web management interface.
• Confidentiality (C): None (C:N)
• Integrity (I): None (I:N)
• Availability (A): High (A:H) – The web interface becomes unavailable.

Threat Level: Medium. Exploitation requires an adjacent attacker and admin interaction, but the impact is a complete DoS of the management interface, which could disrupt network configuration and monitoring.

Deep Dive & Mechanics

The vulnerability is a classic improper encoding/escaping issue (CWE-116). The CGI program that renders the “AP Select” page does not properly sanitize or encode the SSID (Service Set Identifier) broadcast by nearby access points. When an administrator navigates to this page, the CGI script processes the list of detected SSIDs. If one of these SSIDs contains special characters (e.g., quotes, angle brackets, or control characters) that are not properly escaped, it can cause the CGI process to crash or enter an infinite loop, leading to a DoS.

Attack Flow:

1. The attacker sets up a rogue access point or modifies an existing one to broadcast a malformed SSID (e.g., containing null bytes, long strings, or HTML/JavaScript code).
2. The attacker waits for an authenticated administrator to log into the WRE6505 web interface.
3. The admin navigates to the “AP Select” page (e.g., for configuring WDS or repeater mode).
4. The CGI script attempts to parse and display the malformed SSID, but due to improper encoding, it crashes or hangs.
5. The web management interface becomes unresponsive, requiring a physical reboot or power cycle to recover.

Root Cause: The CGI program likely uses a function like sprintf or strcpy without proper bounds checking, or it fails to escape HTML entities when rendering the SSID in the web page. This is a common issue in embedded firmware where input validation is minimal.

Vulnerable vs. Patched Code

Since the product is EOL and no patch is available, there is no patched code to compare. However, a hypothetical fix would involve:

• Vulnerable Code (Conceptual):

  // CGI handler for AP Select page
  void handle_ap_select() {
      char ssid[32];
      get_ssid_from_scan(ssid);
      // Directly output without encoding
      printf("<td>%s</td>", ssid);
  }
  

• Patched Code (Conceptual):

  void handle_ap_select() {
      char ssid[32];
      char escaped_ssid[128];
      get_ssid_from_scan(ssid);
      // Properly escape HTML entities
      html_escape(ssid, escaped_ssid, sizeof(escaped_ssid));
      printf("<td>%s</td>", escaped_ssid);
  }
  

The fix would involve using a safe string formatting function and HTML entity encoding to prevent injection.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• A wireless adapter capable of monitor mode and packet injection (e.g., Alfa AWUS036ACH).
• A tool to create a rogue AP with a custom SSID (e.g., hostapd or airbase-ng).
• Knowledge of the target WRE6505’s IP address and admin credentials (or social engineering to get an admin to log in).

Lab Setup:

1. Create a malformed SSID: Use a script to generate an SSID with special characters. For example, a long string of %s or null bytes.

   # Generate a malformed SSID with 100 'A' characters
   python3 -c "print('A'*100)" > malformed_ssid.txt
   

2. Set up a rogue AP: Use hostapd with the malformed SSID.

   # hostapd.conf
   interface=wlan0
   driver=nl80211
   ssid=$(cat malformed_ssid.txt)
   channel=6
   

3. Run hostapd:

   sudo hostapd hostapd.conf
   

4. Wait for admin interaction: The attacker must convince an authenticated admin to visit the “AP Select” page. This could be done via social engineering (e.g., “Please check the available networks to improve performance”).
5. Trigger DoS: Once the admin navigates to the page, the CGI crashes, and the web interface becomes unresponsive.

Expected Outcome: The web management interface is unavailable until the device is rebooted.

Blue Team (Defender)

Detection:

• Monitor for unusual SSIDs in the environment using wireless intrusion detection systems (WIDS).
• Look for repeated admin logins followed by navigation to the “AP Select” page.
• Check device logs for CGI crashes or memory exhaustion.

Mitigation:

• Since no patch is available, the only mitigation is to replace the EOL device with a supported model.
• If replacement is not immediate, restrict administrative access to the web interface (e.g., only allow access from a wired management VLAN).
• Use network segmentation to isolate the WRE6505 from untrusted wireless clients.

Indicators of Compromise (IOCs)

• Network:
  • Presence of an SSID containing unusual characters (e.g., long strings, null bytes, or HTML tags).
  • Repeated HTTP requests to /cgi-bin/ap_select or similar endpoints.
• System:
  • Web interface unresponsive (HTTP 500 or connection refused).
  • Device logs showing CGI process crashes (e.g., “CGI process terminated unexpectedly”).
• File:
  • No specific file IOCs, as the vulnerability is memory-based.

Detection Engineering

YARA Rule

rule CVE_2026_6058_Zyxel_WRE6505_DoS {
    meta:
        description = "Detects malformed SSID patterns that could trigger DoS on Zyxel WRE6505"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-6058"
    strings:
        $ssid_long = /[A-Za-z0-9]{50,}/  // SSID longer than 50 characters
        $ssid_null = /\x00/               // Null byte in SSID
        $ssid_html = /<[^>]+>/            // HTML tags in SSID
    condition:
        any of them
}

Sigma Rule

title: Zyxel WRE6505 DoS via Malformed SSID
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects attempts to exploit CVE-2026-6058 by broadcasting a malformed SSID
author: Valters IT
logsource:
    category: wireless
    product: wids
detection:
    selection:
        ssid|contains|all:
            - '\x00'  # Null byte
            - '<'     # HTML tag start
            - '>'     # HTML tag end
        ssid|length: > 50
    condition: selection
falsepositives:
    - Legitimate long SSIDs (rare)
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 80 (msg:"CVE-2026-6058 Zyxel WRE6505 DoS Attempt"; flow:to_server,established; content:"GET"; http_method; content:"/cgi-bin/ap_select"; http_uri; pcre:"/SSID=[^&]{50,}/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Replace the device: The Zyxel WRE6505 v2 is End-of-Life. Purchase a supported wireless range extender.
2. Restrict admin access: If replacement is not possible, limit access to the web interface to trusted IPs only (e.g., via firewall rules).
3. Disable remote management: Ensure the web interface is not exposed to the internet.

Long-Term:

• Implement a policy to replace EOL network devices.
• Use wireless intrusion prevention systems (WIPS) to detect and block rogue APs with malformed SSIDs.
• Train administrators to avoid visiting the “AP Select” page unless necessary.

Workaround: There is no official patch. The only workaround is to avoid using the “AP Select” page and configure the device via other means (e.g., CLI if available).

Valters IT
DevSecOps & Cyber Threat Intelligence Expert

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