CVE-2019-25627: FlexHEX Buffer Overflow Vulnerability

CVE-2019-25627 Overview

FlexHEX 2.71 contains a local buffer overflow vulnerability in the Stream Name field that allows local attackers to execute arbitrary code by triggering a structured exception handler (SEH) overflow. Attackers can craft a malicious text file with carefully aligned shellcode and SEH chain pointers, paste the contents into the Stream Name dialog, and execute arbitrary commands when the exception handler is triggered.

Critical Impact

Local attackers can achieve arbitrary code execution by exploiting the SEH overflow in FlexHEX 2.71's Stream Name field, potentially leading to complete system compromise.

Affected Products

• FlexHEX version 2.71

Discovery Timeline

• 2026-03-24 - CVE CVE-2019-25627 published to NVD
• 2026-03-24 - Last updated in NVD database

Technical Details for CVE-2019-25627

Vulnerability Analysis

This vulnerability is a classic stack-based buffer overflow that specifically targets Windows Structured Exception Handler (SEH) mechanisms. When a user provides an oversized input in the Stream Name field, the application fails to properly validate the input length before copying it to a fixed-size buffer on the stack. This overflow corrupts the SEH chain stored on the stack, allowing an attacker to redirect execution flow when an exception is triggered.

The SEH overwrite technique is particularly effective against older Windows applications that lack modern exploit mitigations such as SafeSEH or SEHOP (Structured Exception Handler Overwrite Protection). By carefully crafting the payload with proper alignment and Unicode-compatible shellcode, an attacker can bypass basic protections and achieve reliable code execution.

Root Cause

The root cause of this vulnerability is improper input validation in FlexHEX 2.71's Stream Name handling functionality. The application allocates a fixed-size buffer for storing stream names but does not enforce length restrictions on user input. When excessively long input is provided through the Stream Name dialog, it overflows the stack buffer and corrupts adjacent memory structures, including the SEH chain pointers. This is categorized under CWE-434, though the actual weakness is more accurately described as a buffer overflow leading to SEH corruption.

Attack Vector

The attack requires local access to the target system where FlexHEX 2.71 is installed. An attacker crafts a malicious text file containing a specially constructed payload with the following components:

1. A buffer of controlled characters to fill the stack buffer
2. Carefully aligned bytes to reach the SEH chain on the stack
3. A pointer to a POP-POP-RET instruction sequence that bypasses SafeSEH (if present)
4. Unicode-compatible shellcode or an egghunter to locate and execute the final payload

The attacker then pastes this crafted content into the Stream Name dialog field, triggering the buffer overflow. When the application subsequently encounters an exception (which can be forced by the overflow itself), the corrupted SEH handler redirects execution to the attacker's code.

The attack mechanism involves overflowing the Stream Name input buffer to overwrite the SEH chain on the stack. When an exception occurs, Windows walks the SEH chain and executes the attacker-controlled handler address, which typically points to a POP-POP-RET gadget followed by shellcode execution. For detailed technical analysis, see the Exploit-DB #46665 entry.

Detection Methods for CVE-2019-25627

Indicators of Compromise

• Presence of FlexHEX version 2.71 installed on endpoints
• Unusual crash dumps or application exceptions from flexhex.exe
• Unexpected child processes spawned by FlexHEX (e.g., calc.exe, cmd.exe, or PowerShell)
• Text files containing long strings of repeated characters or NOP-sled patterns in user directories

Detection Strategies

• Deploy endpoint detection rules to monitor for buffer overflow exploitation attempts targeting FlexHEX
• Configure application whitelisting to detect unexpected process spawning from FlexHEX
• Monitor for crash events associated with flexhex.exe that may indicate exploitation attempts
• Use memory protection tools to detect SEH chain corruption attempts

Monitoring Recommendations

• Enable Windows Event Log monitoring for application crashes and exception events
• Implement file integrity monitoring on systems where FlexHEX is installed
• Configure SentinelOne behavioral AI to detect anomalous process execution chains originating from FlexHEX
• Review installed software inventories to identify vulnerable FlexHEX installations across the enterprise

How to Mitigate CVE-2019-25627

Immediate Actions Required

• Uninstall or disable FlexHEX 2.71 until a patched version is available
• Restrict access to systems where FlexHEX must remain installed to trusted users only
• Deploy endpoint protection capable of detecting buffer overflow and SEH exploitation techniques
• Enable Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR) at the system level

Patch Information

No official vendor patch is currently available for this vulnerability. Users should check the FlexHex Official Website for potential updates. Consider migrating to alternative hex editor software that is actively maintained and implements modern security mitigations.

For additional technical details on this vulnerability, refer to the VulnCheck Buffer Overflow Advisory.

Workarounds

• Run FlexHEX in a sandboxed environment or virtual machine to contain potential exploitation
• Enable Windows Exploit Guard or Enhanced Mitigation Experience Toolkit (EMET) protections if available
• Implement strict access controls limiting who can execute FlexHEX on enterprise systems
• Use SentinelOne's exploit prevention capabilities to block SEH overwrite attempts at runtime

System administrators can enable additional Windows exploit mitigations by configuring Windows Defender Exploit Guard policies or deploying mandatory ASLR and DEP enforcement through Group Policy. These mitigations can make exploitation significantly more difficult, though they do not address the underlying vulnerability.