CVE-2023-47174: Thorntech Sftp Gateway Firmware RCE Flaw

CVE-2023-47174 Overview

CVE-2023-47174 is a critical insecure deserialization vulnerability affecting Thorn SFTP gateway versions 3.4.x before 3.4.4. The vulnerability stems from the use of Pivotal Spring Framework for Java deserialization of untrusted data, which is not a supported use case by Pivotal. This flaw allows remote attackers to execute arbitrary code on affected systems without authentication, potentially leading to complete system compromise.

Critical Impact

Remote code execution via insecure Java deserialization enables unauthenticated attackers to execute arbitrary commands on vulnerable Thorn SFTP gateway instances, potentially compromising sensitive file transfer operations and gaining persistent access to enterprise networks.

Affected Products

• Thorntech SFTP Gateway Firmware versions 3.4.x before 3.4.4
• Thorntech SFTP Gateway hardware appliance
• GCP deployments of Thorntech SFTP Gateway

Discovery Timeline

• 2023-10-31 - CVE-2023-47174 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-47174

Vulnerability Analysis

This vulnerability is classified as CWE-502 (Deserialization of Untrusted Data) and represents a significant security flaw in how the Thorn SFTP gateway processes serialized Java objects. The vulnerable component leverages the Pivotal Spring Framework's deserialization capabilities in a manner that was never intended or supported by the framework developers.

The attack can be executed remotely over the network without requiring any authentication or user interaction. When successfully exploited, an attacker can achieve complete compromise of the target system, gaining the ability to read, modify, or delete sensitive data, as well as execute arbitrary commands with the privileges of the SFTP gateway service.

This vulnerability is related to CVE-2016-1000027, which documented the inherent risks of using Spring Framework's HTTP invoker for deserialization of untrusted data. The Thorn SFTP gateway's implementation exacerbates this known issue by exposing the vulnerable deserialization endpoint to network-accessible attack vectors.

Root Cause

The root cause lies in the improper use of Java deserialization mechanisms through the Pivotal Spring Framework. The application accepts serialized Java objects from untrusted sources and deserializes them without proper validation or sanitization. This architectural decision violates secure coding principles, as arbitrary object instantiation during deserialization can trigger malicious code execution through carefully crafted gadget chains.

The Spring Framework's HTTP invoker functionality, while useful for internal trusted communications, was never designed to safely handle untrusted input. When exposed to external network traffic, as in the case of the SFTP gateway, this becomes a critical attack surface.

Attack Vector

The attack vector is network-based, allowing remote exploitation. An attacker can craft a malicious serialized Java object containing a gadget chain that, when deserialized by the vulnerable application, triggers arbitrary code execution. The attack requires no authentication, no user interaction, and can be automated at scale.

Typical exploitation involves identifying the vulnerable endpoint, constructing a payload using known Java deserialization gadget chains (such as those found in common libraries like Commons Collections or Spring itself), and sending the malicious serialized object to the target server. Upon deserialization, the embedded commands execute with the privileges of the SFTP gateway service.

For technical exploitation details, refer to the ThornTech RCE Analysis documentation.

Detection Methods for CVE-2023-47174

Indicators of Compromise

• Unexpected outbound network connections from the SFTP gateway server to unknown external hosts
• Unusual process spawning from the Java process running the SFTP gateway service
• Suspicious serialized Java object payloads in network traffic to the gateway
• Anomalous file system activity or new files created in unexpected directories on the SFTP gateway host

Detection Strategies

• Monitor network traffic for serialized Java objects being sent to the SFTP gateway, particularly looking for known gadget chain signatures
• Implement Java deserialization monitoring through JVM agents or application-level logging
• Deploy network intrusion detection rules to identify common deserialization exploit payloads
• Review application logs for deserialization exceptions or unexpected class loading activity

Monitoring Recommendations

• Enable verbose logging on the SFTP gateway to capture deserialization attempts
• Implement endpoint detection and response (EDR) solutions to monitor for post-exploitation activity
• Configure alerting for any new processes spawned by the SFTP gateway Java process
• Monitor for changes to system files and configurations on gateway hosts

How to Mitigate CVE-2023-47174

Immediate Actions Required

• Upgrade Thorn SFTP gateway to version 3.4.4 or later immediately
• Restrict network access to the SFTP gateway management interfaces to trusted IP addresses only
• Place the SFTP gateway behind a web application firewall (WAF) with deserialization attack detection capabilities
• Audit current deployments for signs of compromise before and after patching

Patch Information

ThornTech has addressed this vulnerability in SFTP gateway version 3.4.4. Organizations should immediately upgrade all affected installations to this version or later. The vendor advisory provides detailed upgrade instructions and additional security hardening recommendations. Review the ThornTech security documentation for complete patch guidance.

Workarounds

• Implement network segmentation to isolate SFTP gateway instances from untrusted networks
• Deploy a reverse proxy with request filtering to block suspicious serialized object payloads
• Disable or restrict access to Spring HTTP invoker endpoints if not required for business operations
• Use firewall rules to limit inbound connections to the SFTP gateway from known, trusted sources only

# Example: Restrict access to SFTP gateway using iptables
# Allow only trusted management IP addresses
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Enable logging for dropped connections
iptables -A INPUT -p tcp --dport 443 -j LOG --log-prefix "SFTP-GW-BLOCKED: "