CVE-2019-25589

Executive Summary

CVE-2019-25589 is a buffer overflow vulnerability in Emtec ZOC Terminal version 7.23.4. The flaw resides in the Shell field of Program Settings, where an excessively long string can be supplied by a local attacker. When the crafted payload is pasted into the Shell configuration field and the Command Shell feature is accessed, the application crashes due to a buffer overflow. The vulnerability has a CVSS score of 6.2 (Medium), with a local attack vector and low complexity. No patch has been confirmed, and the EPSS score is 0.0%, indicating low exploitation probability in the wild. This report provides a comprehensive technical analysis, detection rules, and exploitation guidance.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
• Base Score: 6.2 (Medium)
• Attack Vector: Local (AV:L) – requires local access to the system.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: Low (PR:L) – attacker needs user-level access.
• User Interaction: None (UI:N) – no user action required beyond pasting the payload.
• Scope: Unchanged (S:U) – impact is confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – no data exposure.
• Integrity Impact: None (I:N) – no data modification.
• Availability Impact: High (A:H) – application crash leads to denial of service.

Threat Level: Medium. The vulnerability is local and requires user interaction to paste the payload, but it can cause a denial of service. No remote exploitation is possible.

Deep Dive & Mechanics

The vulnerability is a classic buffer overflow in the Shell field of Program Settings in ZOC Terminal 7.23.4. The application does not properly validate the length of the input string before copying it into a fixed-size buffer. When a user pastes an excessively long string into the Shell configuration field and then accesses the Command Shell feature, the overflow corrupts adjacent memory, leading to a crash.

Root Cause: Lack of bounds checking on the Shell field input. The application likely uses a function like strcpy or sprintf without length limits, allowing an attacker to overwrite the stack or heap.

Attack Vector: Local. The attacker must have access to the system and be able to modify the ZOC settings (e.g., via the GUI or configuration file). The payload is pasted into the Shell field, and the crash occurs when the Command Shell is launched.

Impact: Denial of service. The application crashes, potentially causing loss of unsaved work. No code execution has been confirmed, but buffer overflows can sometimes be leveraged for arbitrary code execution if the attacker controls the overflow data.

Vulnerable vs. Patched Code

Vulnerable Code (ZOC Terminal 7.23.4): The Shell field input is read without length validation. Example pseudo-code:

char shell_buffer[256];
strcpy(shell_buffer, user_input); // No bounds check

Patched Code (Hypothetical): A patched version would include bounds checking:

char shell_buffer[256];
strncpy(shell_buffer, user_input, sizeof(shell_buffer) - 1);
shell_buffer[sizeof(shell_buffer) - 1] = '\0';

Since no official patch is available, users must rely on workarounds.

Exploitation & Lab Setup

Red Team

Exploitation Steps:

1. Gain local access to a system running ZOC Terminal 7.23.4.
2. Open ZOC Terminal and navigate to Program Settings.
3. In the Shell field, paste a crafted payload of excessive length (e.g., 1000 'A' characters).
4. Save the settings and access the Command Shell feature.
5. The application will crash, causing a denial of service.

Proof-of-Concept (PoC):

# PoC for CVE-2019-25589
payload = "A" * 1000
print("Paste the following into the Shell field:")
print(payload)

Note: No public exploit code exists beyond this simple crash PoC. No code execution has been demonstrated.

Blue Team

Detection and Mitigation:

• Monitor for unusual configuration changes in ZOC Terminal.
• Use application whitelisting to prevent unauthorized modifications.
• Restrict local user permissions to limit access to application settings.
• Consider using a sandbox or virtualized environment for ZOC Terminal.

Indicators of Compromise (IOCs)

• File: ZOC Terminal configuration file (e.g., zoc.ini or ZOCSettings.dat) containing an abnormally long Shell field value.
• Process: ZOC Terminal process crash (Event ID 1000 in Windows Application Event Log).
• Memory: Buffer overflow patterns in crash dumps (e.g., repeated 'A' characters).
• Network: No network indicators; this is a local vulnerability.

Detection Engineering

YARA Rule

rule CVE_2019_25589_ZOC_BufferOverflow {
    meta:
        description = "Detects ZOC Terminal configuration files with excessively long Shell field"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25589"
    strings:
        $shell_field = "Shell="
        $long_string = /Shell=[A-Za-z0-9]{256,}/
    condition:
        $shell_field and $long_string
}

Sigma Rule

title: ZOC Terminal Buffer Overflow Configuration
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects modification of ZOC Terminal Shell field with excessively long string
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2019-25589
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\zoc.exe'
        CommandLine|contains: 'Shell='
        CommandLine|re: 'Shell=.{256,}'
    condition: selection
falsepositives:
    - Legitimate long paths (unlikely)
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2019-25589 ZOC Terminal Buffer Overflow Attempt"; content:"Shell="; depth:100; pcre:"/Shell=.{256,}/"; classtype:attempted-dos; sid:1000001; rev:1;)

Note: Suricata rule is for network-based detection if the configuration is transferred over the network (unlikely for local vulnerability). Adjust as needed.

Remediation & Mitigation

• No Official Patch: As of this report, no patch is available. Users should monitor Emtec for updates.
• Workaround: Avoid pasting untrusted strings into the Shell field. Use only short, validated paths.
• Restrict Access: Limit local user permissions to prevent modification of ZOC settings.
• Application Control: Use software restriction policies to block unauthorized changes.
• Alternative Software: Consider using a different terminal emulator if ZOC is not essential.

Valters IT Signature This report is provided for educational and defensive purposes only. Unauthorized exploitation is illegal.