CVE-2020-37128: ZOC Terminal 7.25.5 DoS Vulnerability

CVE-2020-37128 Overview

CVE-2020-37128 is a stack-based buffer overflow vulnerability affecting ZOC Terminal version 7.25.5. The vulnerability exists in the application's REXX script processing functionality, which fails to properly validate the size of input data when loading script files. Local attackers can exploit this flaw by crafting a malicious REXX script file containing approximately 20,000 repeated characters, causing the application to crash and resulting in a denial of service condition.

Critical Impact

Successful exploitation allows local attackers to crash ZOC Terminal through maliciously crafted REXX script files, disrupting terminal emulation services and potentially affecting critical administrative workflows that depend on the application.

Affected Products

• ZOC Terminal version 7.25.5
• Emtec ZOC Terminal (Windows and macOS versions)

Discovery Timeline

• 2026-02-05 - CVE-2020-37128 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2020-37128

Vulnerability Analysis

The vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), which occurs when a program writes data beyond the boundaries of a pre-allocated stack buffer. In the case of ZOC Terminal 7.25.5, the script processing engine does not adequately check the length of input data when parsing REXX script files.

When a user opens or executes a REXX script through ZOC Terminal, the application reads the script content into a stack-allocated buffer. The absence of proper bounds checking allows an attacker to provide input that exceeds the buffer's capacity, overwriting adjacent memory on the stack. While this specific vulnerability results in application termination (denial of service), stack-based buffer overflows can potentially be leveraged for code execution in certain circumstances.

The attack requires local access and user interaction—specifically, the victim must load the malicious script file into the application.

Root Cause

The root cause of CVE-2020-37128 is insufficient input validation in ZOC Terminal's REXX script parser. The application allocates a fixed-size buffer on the stack to process script content but does not verify that incoming data fits within these boundaries. When processing a script containing approximately 20,000 repeated characters, the buffer overflows, corrupting stack memory and causing the application to crash.

Attack Vector

The attack vector is local, requiring the attacker to either:

1. Have direct access to the target system to place a malicious REXX script file
2. Convince a user to download and open a malicious script file through social engineering

The exploitation is straightforward—an attacker generates an oversized REXX script file with repeated characters (approximately 20,000 bytes of padding) and delivers it to the victim. When the victim opens this script in ZOC Terminal, the application crashes due to the stack buffer overflow.

Technical details about the exploitation mechanism can be found in the Exploit-DB #48302 entry and the VulnCheck Security Advisory.

Detection Methods for CVE-2020-37128

Indicators of Compromise

• Presence of unusually large REXX script files (.rexx, .rex) in user directories or download locations
• ZOC Terminal crash logs or Windows Error Reporting events indicating application termination
• Unexpected ZOC Terminal process terminations without user-initiated closure
• Script files containing repetitive character patterns or suspicious padding

Detection Strategies

• Monitor for ZOC Terminal crash events in Windows Event Logs (Application log, event ID 1000 for application crashes)
• Implement file integrity monitoring for directories containing REXX scripts used by ZOC Terminal
• Deploy endpoint detection rules to identify unusually large script files being opened by zoc.exe
• Use SentinelOne's behavioral AI to detect anomalous process terminations and potential exploitation attempts

Monitoring Recommendations

• Enable application crash logging and forward crash reports to centralized logging infrastructure
• Monitor for repeated ZOC Terminal crashes that may indicate exploitation attempts
• Track file system activity for creation of REXX script files exceeding normal size thresholds
• Implement user awareness training regarding suspicious script files received via email or download

How to Mitigate CVE-2020-37128

Immediate Actions Required

• Upgrade ZOC Terminal to the latest available version from Emtec
• Restrict execution of REXX scripts to trusted, validated sources only
• Implement application whitelisting to control which scripts can be loaded into ZOC Terminal
• Educate users about the risks of opening script files from untrusted sources

Patch Information

Users should check the Emtec website for the latest version of ZOC Terminal that addresses this vulnerability. The vendor advisory and patch information should be reviewed for specific remediation guidance. Additional technical details are available in the VulnCheck Security Advisory.

Workarounds

• Disable or restrict REXX script functionality if not required for operational needs
• Implement file size validation for script files before opening them in ZOC Terminal
• Use file filtering solutions to block oversized script files from reaching end users
• Deploy endpoint protection solutions like SentinelOne to detect and prevent exploitation attempts

# Example: Block large REXX files using Windows PowerShell
# Monitor and alert on REXX files larger than 10KB in user directories
Get-ChildItem -Path "$env:USERPROFILE" -Recurse -Include *.rexx,*.rex | 
Where-Object { $_.Length -gt 10KB } | 
ForEach-Object { Write-Warning "Suspicious script file detected: $($_.FullName)" }