Network Time Security (NTS) specified in RFC8915 is a mechanism to provide cryptographic security for clock synchronization using the Network Time Protocol (NTP) as foundation. By using Transport Layer Security (TLS) and Authenticated Encryption with Associated Data (AEAD) NTS is able to ensure integrity and authenticity between server and clients synchronizing time. However, in the past it was shown that time synchronisation protocols such as the Network Time Protocol (NTP) and the Precision Time Protocol (PTP) might be leveraged as carrier for covert channels, potentially infiltrating or exfiltrating information or to be used as Command-and-Control channels in case of malware infections. By systematically analyzing the NTS specification, we identified 12 potential covert channels, which we describe and discuss in this paper. From the 12 channels, we exemplary selected an client-side approach for a proof-of-concept implementation using NTS random UIDs. Further, we analyze and investigate potential countermeasures and propose a design for an active warden capable of mitigating the covert channels described in this paper.

Covert channels in network protocols are a technique aiming to hide the very existence of secret communication in computer networks. In this work we present a systematic in-depth analysis of covert channels by modification for the Network Time Protocol (NTP). Our analysis results in the identification of 49 covert channels, by applying a covert channel pattern-based taxonomy. The summary and comparison based on nine selected key attributes show that NTP is a plausible carrier for covert channels. The analysis results are evaluated in regards to common behavior of NTP implementations in six major operating systems. Two channels are selected and implemented to be evaluated in network test-beds. By hiding encrypted high entropy data in a high entropy field of NTP we show in our first assessment that practically undetectable channels can be implemented in NTP, motivating the required further research. In our evaluation, we analyze 40,000 NTP server responses from public NTP server providers. We discuss the general approach of the research community that detection of covert channels is the more promising countermeasure, compared to active suppression of covert channels. Therefore, normalization approaches and a secure network environment are introduced.

Steganography embraces several hiding techniques which spawn across multiple domains. However, the related terminology is not unified among the different domains, such as digital media steganography, text steganography, cyber-physical systems steganography, network steganography (network covert channels), local covert channels, and out-of-band covert channels. To cope with this, a prime attempt has been done in 2015, with the introduction of the so-called hiding patterns, which allow to describe hiding techniques in a more abstract manner. Despite significant enhancements, the main limitation of such a taxonomy is that it only considers the case of network steganography.

In this paper, we present an Information Hiding approach that would be suitable for exfiltrating sensible information of Industrial Control Systems (ICS) by leveraging the long-term storage of process data in historian databases. We show how hidden messages can be embedded in sensor measurements as well as retrieved asynchronously by accessing the historian. We evaluate this approach at the example of water-flow and water-level sensors of the Secure Water Treatment (SWAT) dataset from iTrust. To generalize from specific cover channels (sensors and their transmitted data), we reflect upon general challenges that arise in such Information Hiding scenarios creating network covert channels and discuss aspects of cover channel selection and and sender receiver synchronisation as well as temporal aspects such as the potential persistence of hidden messages in Cyber Physical Systems (CPS). For an empirical evaluation we design and implement a covert channel that makes use of different embedding strategies to perform an adaptive approach in regards to the noise in sensor measurements, resulting in dynamic capacity and bandwidth selection to reduce detection probability. The results of this evaluation show that, using such methods, the exfiltration of sensible information in long-term scaled attacks would indeed be possible. Additionally, we present two detection approaches for the introduced hidden channel and carry out an extensive evaluation of our detectors with multiple test data sets and different parameters. We determine a detection accuracy of up to 87.8% on test data at a false positive rate (FPR) of 0%.

Industrial Control Systems (ICS) help to automate various cyber-physical systems in our world. The controlled processes range from rather simple traffic lights and elevators to complex networks of ICS in car manufacturing or controlling nuclear power plants. With the advent of industrial Ethernet ICS are increasingly connected to networks of Information Technology (IT). Thus, novel attack vectors on ICS are possible. In IT networks information hiding and steganography is increasingly used in advanced persistent threats to conceal the infection of the systems allowing the attacker to retain control over the compromised networks. In parallel ICS are more and more a target for attacks as well. Here, simple automated attacks as well as targeted attacks of nation state actors with the intention of damaging components or infrastructures as a part of cyber crime have already been observed. Information hiding could bring such attacks to a new level by integrating backdoors and hidden/covert communication channels that allow for attacking specific processes whenever it is deemed necessary. This paper sheds light on potential attack vectors on Programmable Logic Controllers (PLCs) using OPC Unified Architecture (OPC UA) network protocol based communication. We implement an exemplary supply chain attack consisting of an OPC UA server (Bob, B) and a Siemens S7-1500 PLC as OPC UA client (Alice, A). The hidden storage channel is using source timestamps to embed encrypted control sequences allowing for setting digital outputs to arbitrary values. The attack is solely relying on the programming of the PLC and does not require firmware level access. Due to the potential harm to life caused by attacks on cyber-physical systems any presentation of novel attack vectors need to present suitable mitigation strategies. Thus, we investigate potential approaches for the detection of the hidden storage channel for a warden W as well as potential countermeasures in order to increase the warden-compliance. Our machine learning based detection approach using a One-Class-Classifier yields a detection performance of 89.5% with zero false positives within an experiment with 46,159 OPC UA read responses without a steganographic message and 7,588 OPC UA read responses with an embedded steganographic message.

Abstract A feature of search engines is prediction and suggestion to complete or extend input query phrases, i.e. search suggestion functions (SSF). Given the immediate temporal nature of this functionality, alongside the character submitted to trigger each suggestion, adequate data is provided to derive keystroke features. The potential of such biometric features to be used in identification and tracking poses risks to user privacy.For our initial experiment, we evaluate SSF traffic with different browsers and search engines on a Linux PC and an Android mobile phone. The keystroke network traffic is captured and decrypted using mitmproxy to verify if expected keystroke information is contained, which we call quality assurance (QA). In our second experiment, we present first results for identification of five subjects searching for up to three different phrases on both PC and phone using naive Bayesian and nearest neighbour classifiers. The third experiment investigates potential for identification and verification by an external observer based purely on the encrypted traffic, thus without QA, using the Euclidean distance. Here, ten subjects search for two phrases across several sessions on a Linux virtual machine, and statistical features are derived for classification. All three test cases show positive tendencies towards the feasibility of distinguishing users within a small group. The results yield lowest equal error rates of 5.11% for the single PC and 11.37% for the mobile device with QA and 23.61% for various PCs without QA. These first tendencies motivate further research in feature analysis of encrypted network traffic and prevention approaches to ensure protection and privacy.