CVE-2025-26007

Executive Summary

CVE-2025-26007 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 login interface when processing requests to the systemtil.cgi endpoint. An attacker can exploit this vulnerability by sending a specially crafted, oversized request, leading to a stack overflow. This can result in remote code execution (RCE) with the privileges of the web server, typically root, allowing for full device compromise. The vulnerability is currently unpatched, and no public exploit code is available, but the risk is high due to the device's exposure on the internet.

Supply Chain & Package Security

This vulnerability is not related to a software package (NPM, PyPI, Go, Ruby) but is a firmware-level issue in an embedded IoT device. The supply chain risk involves the manufacturer (Telesquare) failing to provide a patch, leaving all deployed devices vulnerable.

CVSS Breakdown & Threat Level

• CVSS Score: 9.8 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• 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 access leads to data exposure.
• Integrity (I): High (H) - Attacker can modify device configuration and firmware.
• Availability (A): High (H) - Overflow can cause denial of service or full control.

Threat Level: Critical. The combination of remote exploitability, 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 systemtil.cgi endpoint. The systemtil.cgi script is part of the router's web management interface, likely used for system tools or diagnostics. When a request is made to this endpoint, the CGI program copies user-supplied input (e.g., from POST data or query strings) into a fixed-size stack buffer without proper bounds checking.

Root Cause: The use of unsafe C functions like strcpy() or sprintf() instead of their bounded counterparts (strncpy(), snprintf()). An attacker can send a request with a payload that exceeds the buffer size, overwriting the saved return address on the stack.

Exploitation Flow:

1. The attacker sends an HTTP POST request to http://<target>/cgi-bin/systemtil.cgi with an oversized parameter.
2. The CGI program copies the input into a stack buffer, overflowing it.
3. The overflow overwrites the saved EIP (return address) and potentially other stack data.
4. When the function returns, execution jumps to an address controlled by the attacker.
5. The attacker can redirect execution to a shellcode payload (e.g., a reverse shell) placed in the overflow data or elsewhere in memory.

Why it's dangerous: The router runs with root privileges, so successful exploitation grants full control over the device. This can be used for:

• Adding the device to a botnet (e.g., Mirai).
• Exfiltrating network traffic.
• Launching further attacks on the internal network.

Vulnerable vs. Patched Code

Since no patch is available, we can only analyze the vulnerable code pattern. The vulnerable code likely resembles:

// Vulnerable code (pseudo)
void handle_systemtil() {
    char buffer[256];
    char *input = get_cgi_input("command");
    strcpy(buffer, input);  // No bounds check
    // ... process command
}

A patched version would use safe functions:

// Patched code (pseudo)
void handle_systemtil() {
    char buffer[256];
    char *input = get_cgi_input("command");
    strncpy(buffer, input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... process command
}

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Environment: Use a virtualized MIPS or ARM environment (QEMU) to emulate the router's firmware, or use a physical device in an isolated lab.
2. Tools: Python3, pwntools, gdb-multiarch, Metasploit (for payload generation).
3. Firmware: Obtain the firmware from Telesquare's support site (if available) or extract it from a device.

Exploitation Steps:

1. Identify the vulnerability: Fuzz the systemtil.cgi endpoint with large inputs to confirm the crash.
2. Control EIP: Use a cyclic pattern (e.g., pattern_create.rb) to find the offset to the return address.
3. Find gadgets: Use ROPgadget to find ROP gadgets in the binary or libc for MIPS/ARM.
4. Craft payload: Create a payload that overflows the buffer, overwrites the return address with a ROP chain or direct jump to shellcode.
5. Execute: Send the payload via HTTP POST.

Example Python exploit skeleton:

#!/usr/bin/env python3
import socket

target = "192.168.1.1"
port = 80

# Offset to return address (example)
offset = 268
# Shellcode (MIPS little-endian reverse shell example)
shellcode = b"\x00\x00\x00\x00"  # Placeholder

payload = b"A" * offset
payload += b"BBBB"  # Overwrite return address
payload += shellcode

http_request = (
    f"POST /cgi-bin/systemtil.cgi HTTP/1.1\r\n"
    f"Host: {target}\r\n"
    f"Content-Length: {len(payload)}\r\n"
    f"Content-Type: application/x-www-form-urlencoded\r\n"
    f"\r\n"
    f"command={payload.decode('latin-1')}"
)

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target, port))
s.send(http_request.encode())
s.close()

Blue Team (Defensive)

Detection & Response:

1. Network Monitoring: Look for HTTP POST requests to /cgi-bin/systemtil.cgi with abnormally large payloads (e.g., > 1024 bytes).
2. Log Analysis: Check web server logs for repeated 500 errors or crashes from the CGI process.
3. Honeypot: Deploy a honeypot mimicking the router to capture exploit attempts.
4. Mitigation: If patching is not possible, restrict access to the web interface to trusted IPs only using firewall rules.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /cgi-bin/systemtil.cgi with payloads > 1024 bytes.
  • Outbound connections from the router to unknown IPs on high ports (e.g., 4444, 31337).
  • Unusual DNS queries from the router.
• Host IOCs:
  • Unexpected processes running on the router (e.g., /tmp/bot).
  • Modified firmware or configuration files.
  • Presence of shellcode in memory (detectable via memory forensics).
• File IOCs:
  • New files in /tmp/ or /var/ with suspicious names.
  • Changes to /etc/passwd or /etc/shadow.

Detection Engineering

YARA Rule

rule CVE_2025_26007_Exploit_Payload {
    meta:
        description = "Detects exploit payloads targeting CVE-2025-26007 in Telesquare TLR-2005KSH"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26007"
    strings:
        $cgi_path = "/cgi-bin/systemtil.cgi" ascii wide nocase
        $overflow_pattern = { 41 41 41 41 41 41 41 41 } // 8 A's as a pattern
        $shellcode_mips = { 24 0f ff ff 27 e4 ff ff 24 0c 01 01 } // MIPS execve
    condition:
        $cgi_path and ( #overflow_pattern > 10 or $shellcode_mips )
}

Sigma Rule

title: CVE-2025-26007 Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests to /cgi-bin/systemtil.cgi with large payloads
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/systemtil.cgi'
        sc-status: 500
    condition: selection
falsepositives:
    - Legitimate large configuration uploads (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-26007 Telesquare TLR-2005KSH Stack Overflow Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/systemtil.cgi"; http_uri;
    content:!"Referer"; http_header;
    pcre:"/command=[A-Za-z0-9+/]{100,}/Ri";
    sid:1000001;
    rev:1;
    reference:cve,2025-26007;
    classtype:attempted-admin;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place the TLR-2005KSH routers on a separate VLAN with strict firewall rules.
2. Disable remote management: If not required, disable the web interface from the WAN side.
3. Monitor logs: Enable detailed logging on the router and SIEM for suspicious activity.

Long-term Solutions:

1. Vendor patch: Contact Telesquare for a firmware update. If unavailable, consider replacing the device.
2. WAF rules: Deploy a Web Application Firewall (WAF) to block oversized POST requests to /cgi-bin/systemtil.cgi.
3. Network segmentation: Ensure IoT devices are isolated from critical systems.

Workaround:

• Use iptables to drop packets with large payloads to the router's web interface:

  iptables -A INPUT -p tcp --dport 80 -m length --length 1500:65535 -j DROP
  

Valters IT
Securing Digital Frontiers