CVE-2019-25627: FlexHEX Buffer Overflow Vulnerability

CVE-2019-25627 Overview

CVE-2019-25627 is a local buffer overflow vulnerability in FlexHEX 2.71, a Windows hex editor distributed by FlexHEX. The flaw resides in the Stream Name field, where pasted input is copied without proper bounds checking. An attacker can craft a malicious text file containing aligned shellcode and a Structured Exception Handler (SEH) chain pointer. When the user pastes this content into the Stream Name dialog and the application raises an exception, the corrupted SEH chain redirects execution to attacker-controlled shellcode. The original public exploit demonstrates spawning calc.exe on the victim host. The issue is tracked under [CWE-434] and impacts local users only.

Critical Impact
Successful exploitation grants arbitrary code execution in the context of the FlexHEX user, enabling local code execution and full compromise of confidentiality, integrity, and availability on the host.

Affected Products

FlexHEX 2.71 on Windows
FlexHEX installer distributed via flexhex_setup.exe
Any environment where users interact with untrusted text files inside FlexHEX dialogs

Discovery Timeline

2026-03-24 - CVE-2019-25627 published to NVD
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2019-25627

Vulnerability Analysis

The vulnerability is a classic Windows SEH-based stack buffer overflow triggered by user input in the Stream Name dialog. FlexHEX 2.71 fails to validate the length of the Unicode string pasted into this field. When the input exceeds the destination buffer, adjacent stack data — including the SEH chain — is overwritten. The application subsequently raises an exception while processing the malformed input, transferring control to the corrupted SEH record. Because the overflow accepts Unicode-encoded data, exploitation relies on Unicode-safe shellcode and venetian-style alignment for return addresses. The published proof of concept on Exploit-DB demonstrates reliable execution of calc.exe on vulnerable installs.

Root Cause

The root cause is improper handling of user-controlled input length in the Stream Name field. FlexHEX copies pasted Unicode data into a fixed-size stack buffer without verifying its size against the buffer boundary. This produces a stack-based overflow that clobbers the SEH chain and the next-SEH pointer, satisfying the conditions for an SEH overwrite exploitation primitive.

Attack Vector

Exploitation requires local interaction. An attacker delivers a specially crafted text file to the victim — for example, through phishing, shared drives, or removable media. The victim opens FlexHEX, navigates to the Stream Name dialog, and pastes the file contents. The application processes the input, triggers an exception, and follows the overwritten SEH pointer into attacker-supplied shellcode. No network access or elevated privileges are required, and authentication is not needed beyond standard local user access. Public exploit details are available in Exploit-DB #46665 and the VulnCheck Advisory on FlexHex Buffer Overflow.

// No verified code sample is reproduced here. Refer to Exploit-DB #46665
// for the published proof-of-concept payload structure consisting of:
//   [ Unicode NOP-equivalent padding ]
//   [ next-SEH overwrite (short jump) ]
//   [ SEH handler overwrite (POP POP RET gadget address) ]
//   [ Unicode-encoded shellcode invoking calc.exe ]

Detection Methods for CVE-2019-25627

Indicators of Compromise

Unexpected child processes spawned by flexhex.exe, particularly calc.exe, cmd.exe, or powershell.exe.
Crash dumps or Windows Error Reporting events referencing access violations inside FlexHEX 2.71.
Text or hex files containing long Unicode strings with embedded POP/POP/RET-style return addresses delivered to user workstations.

Detection Strategies

Monitor process lineage for flexhex.exe and alert on any spawned interactive or scripting binaries.
Inspect Application event logs for repeated exception events tied to the FlexHEX process image.
Hunt for files with extreme line lengths or repeating Unicode patterns staged near FlexHEX installations.

Monitoring Recommendations

Enable command-line and process-creation auditing (Windows Event ID 4688) on hosts where FlexHEX is installed.
Forward endpoint telemetry to a centralized analytics platform to correlate FlexHEX crashes with subsequent process activity.
Track installations of FlexHEX 2.71 across managed endpoints and flag systems still running the vulnerable build.

How to Mitigate CVE-2019-25627

Immediate Actions Required

Inventory all endpoints running FlexHEX 2.71 and restrict use until a fixed version is deployed.
Block delivery of untrusted text files to users who operate FlexHEX, particularly via email and removable media.
Instruct users not to paste content from untrusted sources into FlexHEX dialogs, especially the Stream Name field.

Patch Information

No vendor advisory or fixed version is referenced in the available CVE data. Consult the FlexHEX Official Website for the latest release and replace FlexHEX 2.71 with a newer version if one is published. Where no patched build exists, remove FlexHEX 2.71 from production endpoints.

Workarounds

Uninstall FlexHEX 2.71 from systems where hex editing is not required.
Run FlexHEX inside a low-privilege account or an isolated virtual machine to limit the impact of local code execution.
Apply application allowlisting to prevent flexhex.exe from launching child processes such as shells or scripting engines.

bash

# Example: enumerate FlexHEX installs across Windows endpoints via PowerShell
Get-CimInstance Win32_Product | Where-Object { $_.Name -like 'FlexHEX*' } | Select-Object Name, Version, InstallLocation