一、摘要

本文提出了一种检测工业过程控制攻击的新方案，称为SHADOWPLCS。该方案首先对PLC控制代码进行自动分析，然后提取PLC的关键参数，包括有效寄存器地址、有效值范围、控制逻辑规则等，作为评估攻击的依据。通过主动与PLC通信和被动监控网络流量，从不同角度实时检测攻击行为。

二、介绍

目前仍存在现有工业网络入侵检测系统(IDS)无法检测或阻止的关键攻击,IDS很难从根源上检测隐蔽的攻击。主要有三方面的原因：

从目标系统中并不总是观察到一致的特征，限制了检测规则的提取。
每个ICS都是唯一的，因此，领域专家必须密切参与检测规则的创建。但是，这个过程是非常昂贵和耗时的，可能源于专家的错误，从而错过了检测规则中的关键细节。
攻击者通常使用各种策略来逃避检测。

为了克服现有解决方案的局限性，我们提出了一种新的攻击检测方案SHADOWPLCS。通过分析五种具有代表性的工业过程控制攻击:非法访问地址攻击、恶意数据注入攻击、配置篡改攻击、控制逻辑感染攻击和控制程序替换攻击，激发了ICS对SHADOWPLCS的需求。
SHADOWPLCS自动分析PLC控制代码，提取PLC关键参数，包括有效寄存器地址、有效值范围、控制逻辑规则等，作为检测攻击的基本。基于这些规则SHADOWPLCS主动与PLC通信，映射其存储空间，被动监控网络流量，并从不同角度实时检测过程控制攻击。

三、攻击建模

（一）对手模型

对手可以对PLC进行远程网络攻击，如获取控制中心的控制权限或作为中间人攻击PLC。我们认为对手的目标是操纵或扰乱实际的工业控制过程。在本文中，我们将近年来针对PLC的网络攻击方法分为两类：1)虚假数据注入；2)控制逻辑注入。
虚假数据注入是指将不符合工业控制过程的恶意数据注入PLC，最终导致过程控制的破坏。例如，写反转或最小/最大，修改关键设定点变量和进程值。
控制逻辑注入是指篡改或替换在目标PLC上运行的原始控制逻辑，攻击者通过干扰正常的下载/上传控制逻辑工程操作，向目标PLC注入恶意控制逻辑。

（二）攻击场景

基于上面讨论的对手模型，我们考虑以下可能严重扰乱工业控制过程的攻击场景: 非法访问地址攻击、恶意数据注入攻击、配置篡改攻击、控制逻辑感染攻击、控制程序替换攻击。
非法地址访问攻击。 在这个攻击场景中，假设PLC是对手的黑盒，但对手有能力远程访问PLC的寄存器空间。为了有效地攻击PLC，攻击者首先会探测PLC的寄存器空间，找到攻击的关键执行点。这种攻击很容易发动，而且对手不需要知道实际的工业流程。
恶意数据注入攻击。 假设对手已经知道PLC中的一些关键控制点，例如控制阀门打开或关闭的寄存器Q0.0。对手聚焦在这些关键控制点上，导致控制混乱。一种常见的攻击方法是强制对一个寄存器地址进行写操作，覆盖寄存器的当前值。
配置篡改攻击。 假设攻击者获得了工业流程的一些知识，他们通过恶意篡改关键程序配置信息间接地破坏了工业流程。例如，篡改计时器和计数器设置等，发动延迟攻击。
控制逻辑感染攻击。 假设对手有能力窃取PLC当前的控制程序。它们通过感染控制程序干扰实际的工业生产过程。例如，将I/O寄存器中的值替换为内存中某个地方的可控值，以控制执行器的状态。
控制程序替换攻击。 在这种攻击场景中，攻击者并不感染原PLC控制程序，而是植入恶意控制代码，试图让PLC执行。因为这种攻击完全由对手控制，摧毁任何他们想要完成的工业过程，它具有很强的隐蔽性和破坏性，随时可以发动。

四、SHADOWPLCS方案原理

（一）规则生成

利用PLC代码进行基于签名的检测规则生成。 通过分析PLC代码，可以识别的IDS签名集主要有三种白名单规则:有效地址规则、有效取值范围规则和控制逻辑规则。 如下图所示，演示如何使用PLC代码生成基于签名的检测规则。

（二）攻击检测

本文提出了一种主动和被动相结合的入侵检测方案。 如下图所示，该IDS包括两个部分：被动检测引擎和主动检测引擎。被动检测引擎通过被动监控网络流量，检测非法地址操作、非法值违规等异常行为。主动检测引擎主动与PLC通信，映射低中断和轮询的PLC内存空间，实时监控工业控制过程中的异常情况。对于攻击者来说，要避免主动和被动相结合的入侵检测方案，实现隐身是非常困难和昂贵的。因为他们需要不断模拟正常的工业场状态，符合控制逻辑，甚至需要篡改设备的固件或网络通信模块，而不是通过简单的ARP欺骗和重放报文等传统手段欺骗IDS和控制中心。

①被动检测

被动检测引擎首先将流经工业交换机的网络流量镜像，通过对工业控制协议的深入解析，提取出五元组信息(源IP、目的IP、源端口、目的端口、传输层协议)、操作地址、操作指令和具体值。然后检查上述信息是否符合有效地址白名单和有效值范围白名单，检测非法链接、非法地址操作、非法值等异常行为。
算法1给出了被动检测的过程。第1行表示一个深度解析协议，并提取五元组、地址、操作和值。第2行为非法链路检测。当网络流量中出现未知的IP、未知的端口、未知的通信时，会上报非法链路。第4行检测非法地址。当网络流量中有PLC实际未使用的地址时，会报警非法地址；第6行表示检测非法值范围。当值超出范围等，它将报警非法的值范围。

②主动检测

主动检测引擎主要检测控制逻辑异常，防止重要内存地址被恶意数据注入，防止关键配置被篡改，防止控制程序被篡改或替换。主动检测模块首先主动连接PLC并映射相应的内存空间，然后验证其是否满足有效值范围白名单和控制逻辑白名单。
主动检测引擎以主动、低干扰、轮询的方式检测PLC是否受到攻击，其核心部分如算法2所示。首先，与PLC(第1行)建立通信连接; 然后攻击检测(第2- 16行);最后，当有任务更改需求时，连接断开并销毁任务(第17行，第18行)。在攻击检测阶段，第一步是加载控制逻辑规则、有效取值范围规则和有效地址规则(第4行)，获取寄存器地址和数据类型。其次，根据协议格式构建请求报文并发送给PLC(第5、6行)。第7行和第8行表示接收响应数据并解析协议以提取寄存器变量值; 最后，检查有效值范围和控制逻辑是否存在异常(第9行第15行)。第16行表示对请求频率的设置，以实现对PLC的低干扰。

五、实验评估

为了评估SHADOWPLCS对工业过程控制攻击的检测效果，我们使用两台西门子S7-300 PLC部署在一个燃气管网平台上进行实验。首先，我们验证了将S7-300 PLC的MC7代码反编译为STL源代码的准确性，我们的反编译模块可以准确的将MC7机器码反编译成STL代码，准确率达到100%。其次，分析GitHub、Shodan以及我们的燃气管网平台的的多个PLC程序并生成检测规则。然后，我们比较了SHADOWPLCS与其他四种检测模型（自回归(AR)模型、SRID、LSTM和S-IDS）的检测性能，下表是与其他异常检测模型的性能比较。
最后，为了证明我们的方案对PLC几乎没有影响，我们在检测阶段监控PLC的运行周期变化，并通过压力测试测试PLC处理网络流量的能力。为了避免路由器的当前限制，我们直接用网线连接PLC，每秒钟向PLC发送10、100、1000、5000、10000、20000个请求包。如下图所示，在无外界干扰的情况下，燃气管网平台PLC的周期在0-3ms之间，大部分维持在2ms左右。当S7-300 PLC受到不同程度的外部干扰时，我们发现其周期时间几乎没有受到影响。

六、总结

SHADOWPLCS 通过PLC代码生成的检测规则比基于网络流量的建模和规则提取更加全面，能够充分反映真实的受控过程；通过对PLC控制代码的自动分析，自动综合生成权威的检测规则，不需要人类依靠经验和手动配置建模；该方案采用主动和被动相结合的方法，从不同的角度检测攻击行为。由于我们对受控过程有了深刻的理解，并主动映射相应的PLC寄存器变量，这使得攻击者更难进行隐蔽的攻击。为了逃避检测，攻击者需要了解真实的工业流程，并能够对IDS遵守受控流程的假数据进行实时响应。这通常需要深入PLC，攻击PLC固件并修改其网络处理模块，而不是简单地通过中间人来避免被发现。此外，根据我们提出的检测方案，我们可以定制开发不同的PLC设备，并可以实际应用，对ICS做出实质性的防御。

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