CVE-2023-40250

Executive Summary

CVE-2023-40250 is a critical buffer overflow vulnerability in Hancom HCell, a spreadsheet application for Windows. The flaw, classified as a Classic Buffer Overflow (CWE-120), allows an attacker to overflow buffers by providing a specially crafted input that exceeds the allocated memory buffer. This can lead to arbitrary code execution, data corruption, or denial of service. The vulnerability affects HCell version 12.0.0.893, and as of the latest intelligence, no official patch has been released. The CVSS score of 8.8 indicates high severity, with local access required but no privileges needed, and user interaction is required. The EPSS score is 0.0%, suggesting low exploitation activity in the wild, but the risk remains significant for targeted attacks.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a standalone Windows application vulnerability.

CVSS Breakdown & Threat Level

• CVSS Score: 8.8 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Attack Vector: Local (AV:L) – The attacker must have local access to the system.
• Attack Complexity: Low (AC:L) – No special conditions are required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: Required (UI:R) – The victim must open a malicious file.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – Full data disclosure possible.
• Integrity Impact: High (I:H) – Data can be modified.
• Exploitability: Low due to local access requirement, but high impact once exploited.

Deep Dive & Mechanics

The vulnerability resides in the HCell component of Hancom Office, specifically in the handling of buffer operations. The function responsible for copying data into a buffer does not check the size of the input, leading to a classic buffer overflow. When a user opens a maliciously crafted spreadsheet file (e.g., .hcell or .xlsx), the application reads data into a fixed-size buffer without bounds checking. If the input exceeds the buffer size, it overwrites adjacent memory, potentially corrupting the stack or heap.

Technical Details:

• Vulnerable Function: Likely a memcpy or strcpy call in the file parsing routine.
• Trigger: Opening a specially crafted file with oversized data in a specific field (e.g., cell value, formula, or metadata).
• Impact: Overwriting the return address or function pointers can lead to arbitrary code execution. The attacker can inject shellcode to execute commands, install malware, or exfiltrate data.

Attack Vector:

1. Attacker crafts a malicious .hcell file with a buffer overflow payload.
2. Victim opens the file using Hancom HCell 12.0.0.893.
3. The overflow triggers, executing the attacker's code in the context of the application.

Vulnerable vs. Patched Code

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

void parse_cell_data(char *input) {
    char buffer[256];
    strcpy(buffer, input); // No bounds checking
    // Process buffer...
}

A patched version would include bounds checking:

void parse_cell_data(char *input) {
    char buffer[256];
    strncpy(buffer, input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // Process buffer...
}

Exploitation & Lab Setup

Red Team

Objective: Achieve arbitrary code execution on a target system running Hancom HCell 12.0.0.893.

Lab Setup:

• Windows 10/11 VM with Hancom HCell 12.0.0.893 installed.
• Attacker machine with Python and Metasploit.

Exploit Development:

1. Fuzzing: Use a fuzzer to identify the exact offset where the buffer overflows.
2. Craft Payload: Create a malicious .hcell file with a payload that overwrites the return address with a controlled value.
3. Shellcode: Use a Windows shellcode (e.g., reverse shell) to gain remote access.

Example Python script to generate malicious file:

import struct

# Offset to return address (example)
offset = 260
# Shellcode (placeholder)
shellcode = b"\x90" * 100  # NOP sled + actual shellcode

# Craft payload
payload = b"A" * offset + struct.pack("<I", 0x12345678) + shellcode

# Write to file
with open("malicious.hcell", "wb") as f:
    f.write(payload)

Execution:

• Send the malicious file to the victim via email or other means.
• When the victim opens the file, the exploit triggers.

Blue Team

Detection:

• Monitor for abnormal behavior in Hancom HCell processes (e.g., crashes, high memory usage).
• Use endpoint detection and response (EDR) tools to detect shellcode execution.

Mitigation:

• Restrict execution of Hancom HCell to trusted users only.
• Use application whitelisting to prevent unknown executables.
• Enable DEP (Data Execution Prevention) and ASLR (Address Space Layout Randomization) to mitigate exploitation.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – actual hash depends on exploit)
  • e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
• Process Names: HCell.exe
• Network Connections: Outbound connections to attacker IP on port 4444 (if reverse shell used).
• Registry Changes: Unusual entries in HKCU\Software\Microsoft\Windows\CurrentVersion\Run.
• File Artifacts: Malicious .hcell files with unusual size or structure.

Detection Engineering

YARA Rule

rule CVE_2023_40250_Hancom_HCell_Buffer_Overflow {
    meta:
        description = "Detects malicious HCell files exploiting CVE-2023-40250"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2023-40250"
    strings:
        $buffer_overflow = { 41 41 41 41 41 41 41 41 } // Pattern of 'A's
        $shellcode = { 90 90 90 90 90 90 90 90 } // NOP sled
    condition:
        $buffer_overflow or $shellcode
}

Sigma Rule

title: Hancom HCell Buffer Overflow Exploit
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2023-40250 in Hancom HCell
author: Valters IT
date: 2025/04/10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\HCell.exe'
        CommandLine|contains: '.hcell'
    condition: selection
falsepositives:
    - Legitimate use of HCell
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2023-40250 Hancom HCell Buffer Overflow"; flow:to_server,established; content:"|41 41 41 41|"; depth:260; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Discontinue use of Hancom HCell 12.0.0.893 until a patch is available.
• Workaround: Use alternative spreadsheet software (e.g., Microsoft Excel, LibreOffice Calc).
• Mitigation: Enable DEP and ASLR on all Windows systems. Use application whitelisting to block untrusted executables.
• Monitoring: Deploy YARA and Sigma rules to detect exploitation attempts.
• Patch: Await official patch from Hancom. Monitor vendor advisories.

Valters IT Signature:
This report was generated by Valters IT Cyber Threat Intelligence. For questions, contact threatintel@valtersit.com.