CVE-2023-50234

Executive Summary

CVE-2023-50234 is a stack-based buffer overflow vulnerability in Hancom Office Cell, a component of the Hancom Office suite used for spreadsheet processing. The flaw resides in the parsing of XLS files, where insufficient validation of user-supplied data length leads to a buffer overflow on the stack. An attacker can exploit this by convincing a user to open a malicious XLS file or visit a crafted web page, resulting in remote code execution in the context of the current process. With a CVSS score of 7.8, this vulnerability poses a significant risk to end-user systems, especially in environments where Hancom Office is deployed. No public proof-of-concept or patch is currently available, but the vulnerability has been disclosed via the Zero Day Initiative (ZDI-CAN-20386).

CVSS Breakdown & Threat Level

• CVSS Score: 7.8 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Attack Vector: Local (requires user interaction to open a malicious file)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

Threat Level: High. Exploitation requires user interaction, but the impact is complete compromise of the affected system. The lack of a patch increases the risk for unpatched installations.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow (CWE-121) in the XLS file parser of Hancom Office Cell. When parsing a specially crafted XLS file, the parser copies user-supplied data into a fixed-size stack buffer without proper length validation. By providing an overly long value in a specific field (likely a cell value, formula, or record), an attacker can overwrite the return address and other stack data.

Technical Details:

• Root Cause: The memcpy or similar function is used to copy data from the XLS file into a stack buffer of size N, but the length of the source data is not checked against N.
• Exploitation: The attacker crafts an XLS file with a malicious record that contains a long string. When the parser processes this record, it overflows the stack buffer, overwriting the saved return address. The attacker can then redirect execution to a ROP chain or shellcode.
• Process Context: The exploit runs in the context of the current user process (Hancom Office Cell), so it inherits the user's privileges.

Attack Vector:

• The victim must open the malicious XLS file (e.g., via email attachment, download, or web page).
• No additional privileges are required beyond user-level access.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, based on the vulnerability description, the vulnerable code pattern is:

// Vulnerable code (hypothetical)
void parse_xls_record(char *data, int len) {
    char buffer[256];
    // No length check
    memcpy(buffer, data, len);  // Stack overflow if len > 256
}

A patched version would include bounds checking:

// Patched code (hypothetical)
void parse_xls_record(char *data, int len) {
    char buffer[256];
    if (len > sizeof(buffer)) {
        // Handle error
        return;
    }
    memcpy(buffer, data, len);
}

Exploitation & Lab Setup

Red Team

Objective: Achieve remote code execution on a target running Hancom Office Cell.

Prerequisites:

• Hancom Office Cell installed (any version).
• Victim must open the malicious XLS file.

Steps:

1. Craft Malicious XLS File: Use a tool like msfvenom or a custom Python script to create an XLS file with an oversized field in a record (e.g., a cell value or formula). The overflow should overwrite the return address with a pointer to shellcode.
2. Deliver Payload: Send the file via email, social engineering, or host it on a malicious website.
3. Trigger Exploit: When the victim opens the file, the overflow triggers, executing the shellcode.

Example Python Script (Conceptual):

import struct

# Shellcode (e.g., calc.exe for testing)
shellcode = b"\x90" * 100 + b"\xcc" * 4  # Replace with actual shellcode

# Craft XLS record with overflow
buffer_size = 256
overflow_data = b"A" * (buffer_size + 8)  # Fill buffer and overwrite return address
overflow_data += struct.pack("<I", 0x41414141)  # Overwritten return address (placeholder)
overflow_data += shellcode

# Write to file (simplified)
with open("malicious.xls", "wb") as f:
    f.write(overflow_data)

Note: This is a conceptual example. Actual exploitation requires precise knowledge of the stack layout and ASLR/DEP bypass techniques.

Blue Team

Detection & Response:

• Monitor for Hancom Office crashes: Unexpected crashes of cell.exe or related processes may indicate exploitation attempts.
• Analyze XLS files: Use static analysis tools to detect oversized records or anomalous structures.
• Endpoint Detection: Deploy EDR rules to detect stack-based buffer overflow patterns (e.g., memcpy with large size).
• User Awareness: Train users not to open untrusted XLS files.

Indicators of Compromise (IOCs)

• File Hashes: (None publicly available; generate upon discovery)
• Network Indicators: Outbound connections from cell.exe to unknown IPs (if shellcode connects back).
• Process Anomalies: cell.exe spawning child processes (e.g., cmd.exe, powershell.exe).
• Registry Changes: Unauthorized modifications to startup keys.
• File System: Creation of new executable files in temporary directories.

Detection Engineering

YARA Rule

rule CVE_2023_50234_Hancom_XLS_Overflow {
    meta:
        description = "Detects malicious XLS files exploiting CVE-2023-50234"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-50234"
    strings:
        $xls_magic = { D0 CF 11 E0 A1 B1 1A E1 }  // OLE2 magic
        $long_record = /[\x00-\xff]{300,}/  // Record longer than 300 bytes
    condition:
        $xls_magic at 0 and #long_record > 0
}

Sigma Rule

title: Hancom Office Cell Stack Buffer Overflow Exploitation
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2023-50234 via process crash or anomalous behavior
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\cell.exe'
        CommandLine|contains: '*.xls'
    condition: selection
falsepositives:
    - Legitimate large XLS files
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2023-50234 Hancom XLS Exploit Attempt"; flow:to_server; content:"|D0 CF 11 E0 A1 B1 1A E1|"; depth:8; content:"|00 00 00 00|"; distance:0; within:4; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

• No Patch Available: As of this report, Hancom has not released a patch. Monitor vendor updates.
• Mitigation:
  • Disable automatic opening of XLS files from untrusted sources.
  • Use application sandboxing or virtualization for Hancom Office.
  • Implement endpoint protection with exploit prevention capabilities (e.g., ASLR, DEP, CFG).
  • Restrict execution of cell.exe via AppLocker or similar.
• Workaround: Convert XLS files to a safer format (e.g., CSV) before opening.

Valters IT
Securing Digital Frontiers