CVE-2019-25679: RealTerm Serial Terminal Buffer Overflow

CVE-2019-25679 Overview

RealTerm Serial Terminal 2.0.0.70 contains a structured exception handling (SEH) buffer overflow vulnerability in the Echo Port tab that allows local attackers to execute arbitrary code. This vulnerability is classified as CWE-787 (Out-of-bounds Write), enabling attackers to craft a buffer overflow payload with a POP POP RET gadget chain and shellcode that triggers code execution when a malicious payload is pasted into the Port field and the Change button is clicked.

Critical Impact

Local attackers can achieve arbitrary code execution on systems running the vulnerable RealTerm Serial Terminal application by exploiting the SEH buffer overflow through user interaction with malicious input.

Affected Products

• RealTerm Serial Terminal version 2.0.0.70

Discovery Timeline

• 2026-04-05 - CVE-2019-25679 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2019-25679

Vulnerability Analysis

This vulnerability exploits improper input validation in the Echo Port tab functionality of RealTerm Serial Terminal. When a user pastes an oversized or specially crafted payload into the Port field and clicks the Change button, the application fails to properly validate the input length, leading to a buffer overflow condition that overwrites the structured exception handler (SEH) chain.

The SEH overflow technique is a classic Windows exploitation method where an attacker overwrites exception handler pointers on the stack. When an exception occurs, Windows traverses the SEH chain to find an appropriate handler. By corrupting this chain with attacker-controlled values, execution flow can be redirected to shellcode placed in the overflow buffer.

Root Cause

The root cause of this vulnerability is insufficient boundary checking on user-supplied input in the Echo Port tab's Port field. The application allocates a fixed-size buffer for port input but does not enforce length limits before copying user data, allowing an attacker to write beyond the buffer boundaries and corrupt adjacent memory structures including the SEH chain.

Attack Vector

This is a local attack vector requiring user interaction. An attacker must craft a malicious payload containing:

1. A buffer of sufficient size to reach the SEH handler pointers
2. A POP POP RET gadget address to bypass SafeSEH protections
3. A short jump instruction (nSEH) to redirect execution to shellcode
4. Shellcode to perform the desired malicious actions

The attack is triggered when a user pastes the malicious payload into the Port field within the Echo Port tab and clicks the Change button. The resulting buffer overflow corrupts the SEH chain, and when an exception is raised, the attacker's shellcode executes with the privileges of the current user.

For detailed exploitation information, refer to the Exploit-DB #46441 entry and the VulnCheck RealTerm Advisory.

Detection Methods for CVE-2019-25679

Indicators of Compromise

• Presence of RealTerm Serial Terminal version 2.0.0.70 installed on systems
• Unusual process behavior or crashes originating from realterm.exe
• Memory access violations or exception handling anomalies in RealTerm processes
• Suspicious child processes spawned by realterm.exe

Detection Strategies

• Monitor for abnormal memory allocations or stack corruption in RealTerm Serial Terminal processes
• Deploy endpoint detection rules to identify SEH exploitation patterns characteristic of buffer overflow attacks
• Implement application whitelisting to control which versions of RealTerm are permitted to run
• Use behavior-based detection to identify unusual code execution following user interaction with RealTerm

Monitoring Recommendations

• Enable Windows Event Logging for application crashes and access violations related to realterm.exe
• Deploy EDR solutions capable of detecting SEH manipulation and ROP gadget chain execution
• Monitor clipboard activity for unusually large or encoded payloads being pasted into applications
• Implement process monitoring to detect anomalous child process creation from RealTerm

How to Mitigate CVE-2019-25679

Immediate Actions Required

• Remove or disable RealTerm Serial Terminal version 2.0.0.70 from production systems until a patched version is available
• Restrict access to systems where RealTerm is required to authorized personnel only
• Consider using alternative serial terminal applications that are actively maintained and patched
• Implement application sandboxing to limit the impact of potential exploitation

Patch Information

Users should check the RealTerm Project Homepage and RealTerm Download Files for updated versions that may address this vulnerability. If no patched version is available, consider migrating to an alternative serial terminal application with active security support.

Workarounds

• Avoid pasting untrusted or unknown content into the Port field of the Echo Port tab
• Run RealTerm Serial Terminal with minimal user privileges to limit the impact of successful exploitation
• Use application virtualization or sandboxing technologies to isolate RealTerm from critical system components
• Consider network segmentation to limit access to systems running vulnerable RealTerm installations

# Identify vulnerable RealTerm installations
# Check installed version on Windows systems
wmic product where "name like '%RealTerm%'" get name,version

# Alternative: Search for realterm.exe files
dir /s /b C:\realterm.exe 2>nul