CVE-2019-25569: Crun Realterm DOS Vulnerability

CVE-2019-25569 Overview

CVE-2019-25569 is a stack-based buffer overflow vulnerability affecting RealTerm Serial Terminal version 2.0.0.70. The vulnerability exists in the Echo Port field input handling, where insufficient boundary checking allows local attackers to overflow the stack buffer by providing a specially crafted input string. This overflow corrupts the Structured Exception Handler (SEH) chain, resulting in application crash and denial of service conditions.

Critical Impact

Local attackers can cause denial of service by crashing the RealTerm Serial Terminal application through SEH chain corruption, potentially disrupting serial communication workflows and industrial monitoring systems.

Affected Products

• RealTerm Serial Terminal 2.0.0.70
• Crun RealTerm (cpe:2.3:a:crun:realterm:2.0.0.70)

Discovery Timeline

• 2026-03-21 - CVE CVE-2019-25569 published to NVD
• 2026-03-24 - Last updated in NVD database

Technical Details for CVE-2019-25569

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), specifically manifesting as a stack-based buffer overflow. The flaw resides in the Echo Port input field of RealTerm Serial Terminal, where user-supplied data is copied to a fixed-size stack buffer without adequate length validation.

When an attacker provides an input string exceeding the expected buffer size, the excess data overwrites adjacent stack memory, including critical control structures. The exploitation technique involves crafting a payload with 268 bytes of padding followed by carefully positioned values designed to corrupt the SEH chain. This corruption causes the application's exception handling mechanism to fail catastrophically when triggered.

The local attack vector requires an attacker to have access to the target system where RealTerm is installed. While this limits the attack surface compared to network-exploitable vulnerabilities, it remains a concern in shared workstation environments or scenarios where malicious input could be supplied through automated scripts or configuration files.

Root Cause

The root cause of CVE-2019-25569 is improper input validation in the Echo Port field handler. The application allocates a fixed-size buffer on the stack to store port configuration data but fails to verify that user-supplied input does not exceed this buffer's capacity before copying the data. This classic buffer overflow scenario allows attackers to write beyond the intended memory boundaries, corrupting the SEH chain and other stack-based structures.

Attack Vector

The attack requires local access to a system running RealTerm Serial Terminal 2.0.0.70. An attacker can exploit this vulnerability by:

1. Opening the RealTerm application
2. Navigating to the Echo Port configuration field
3. Pasting a malicious input string consisting of 268 bytes of padding characters followed by SEH overwrite values
4. The application crashes when processing the malformed input, causing denial of service

The vulnerability leverages the application's lack of input sanitization to achieve SEH chain corruption. Technical details regarding the specific exploitation methodology can be found in the Exploit-DB #46391 entry and the VulnCheck RealTerm Advisory.

Detection Methods for CVE-2019-25569

Indicators of Compromise

• Unexpected crashes of realterm.exe process with access violation exceptions
• Windows Event Log entries showing application faults in RealTerm with exception code 0xC0000005
• Abnormally long strings (268+ bytes) present in configuration files or clipboard history associated with RealTerm usage
• SEH chain corruption artifacts in crash dump analysis

Detection Strategies

• Monitor for repeated RealTerm application crashes using Windows Application Event logs (Event ID 1000)
• Implement endpoint detection rules to identify processes with SEH-related exception handling failures
• Deploy file integrity monitoring on RealTerm configuration directories to detect suspicious modifications
• Use application crash monitoring tools to alert on patterns consistent with exploitation attempts

Monitoring Recommendations

• Enable enhanced crash reporting for desktop applications in enterprise environments
• Configure SentinelOne Singularity Platform to monitor for buffer overflow exploitation patterns targeting RealTerm
• Implement user activity monitoring on systems where RealTerm is deployed for industrial or development purposes
• Review clipboard contents and input field data for anomalously long strings in security-sensitive environments

How to Mitigate CVE-2019-25569

Immediate Actions Required

• Inventory all systems running RealTerm Serial Terminal version 2.0.0.70
• Restrict local access to workstations running vulnerable RealTerm installations
• Consider replacing RealTerm with alternative serial terminal applications until a patch is available
• Implement application whitelisting to control who can execute RealTerm on enterprise systems
• Enable Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR) at the OS level to increase exploitation difficulty

Patch Information

As of the last NVD update on 2026-03-24, no official vendor patch has been identified for this vulnerability. Users should monitor the RealTerm SourceForge project page for any security updates or newer versions that may address this issue. Consider reaching out to the RealTerm development community for remediation guidance.

Workarounds

• Limit RealTerm usage to trusted, isolated systems not connected to sensitive networks
• Run RealTerm in a sandboxed environment or virtual machine to contain potential crashes
• Implement input validation at the operating system level using Enhanced Mitigation Experience Toolkit (EMET) or Windows Defender Exploit Guard
• Restrict clipboard paste functionality when possible to prevent easy exploitation via copy-paste attacks
• Deploy endpoint protection solutions like SentinelOne that can detect and prevent exploitation of buffer overflow vulnerabilities

# Enable DEP for RealTerm executable (Windows)
# Run as Administrator in PowerShell
Set-ProcessMitigation -Name realterm.exe -Enable DEP
Set-ProcessMitigation -Name realterm.exe -Enable SEHOP