All Your PLCs Belong to Me: ICS Ransomware Is Realistic

你所有的PLC都属于我：ICS勒索软件是现实的

一、摘要

我们介绍了ICS-BROCK，一个成熟的ICS勒索软件，它可以破坏现实世界。为了演示ICS-BROCK的功能，我们使用西门子S7-300 PLC（ICSs中使用最广泛的设备之一）构建了一个真实的水处理环境。实验结果证明了在实际环境中发起ICS勒索软件攻击的可行性。最后，我们对ICS勒索软件提出了一些建议，以帮助今后的研究和防御。

二、介绍

现有的ICS勒索软件研究大多是概念性的，尚未克服上述所有挑战。例如，公共文献没有报道任何ICS勒索软件能够突破物理隔离并保持ICS稳定。如何构建能够危害现实世界ICS的ICS勒索软件尚不清楚。我们的研究填补了这一空白。

为了使研究具体化，我们在实际的水处理环境中演示了具体的攻击方法，该环境使用西门子S7-300 PLC，这是ICS中使用最广泛的设备类型之一。我们执行详细的协议逆向工程分析，并演示如何在不影响PLC扫描周期的情况下编写恶意控制逻辑程序，如逻辑炸弹。

我们的实现构建了一个ICS勒索软件ICSBROCK，它可以在监控层和PLC上完成勒索软件攻击。在监控层，ICSBROCK扫描本地网络，查找PLC和PC等设备。在PC上，ICS-BROCK像传统勒索软件一样加密文件和数据。在PLC上，ICS-BROCK锁定PLC并注入逻辑炸弹。除设备类型外，所有操作都可以在没有任何预先信息的情况下完成。我们之所以需要设备类型，是因为不同的PLC与ICS协议具有不同的通信细节。

我们作出以下贡献。
• 我们构建了第一个成熟的ICS勒索软件攻击ICS-BROCK，它克服了ICS勒索软件面临的所有独特挑战。
• 与现有的理论ICS勒索软件相比，ICSBROCK能够以较少的信息完成勒索攻击，更接近真实的攻击场景。
• 我们演示了对集成最广泛使用的PLC设备类型之一的真实水处理系统的攻击。我们对PLC的通信协议和加密算法进行了详细的逆向工程分析。
• 对于监控计算机上的勒索软件攻击，我们优化了传统勒索软件的攻击方法，以提高效率和抵抗逆向工程分析。
• 我们说明了如何在不影响ICS稳定性的情况下注入逻辑炸弹，这是现实ICS勒索软件的关键要求。

三、系统模型和威胁模型

（一）系统模型

为了证明ICS-BROCK的能力，我们构建了一个水处理系统。本系统主要包括以下模块：上位机下载和编辑PLC程序；HMI主要用于水处理过程中的具体操作。电源用于向整个ICS供电，提供能量；PLC和其他工业控制设备是ICS的主要功能部件，用于模拟整个自来水处理过程。系统模型如图1所示。

系统模型

图1 系统模型

（二）威胁模型

在真实ICS环境中成功的ICS勒索软件应满足以下要求。

  1. 隐身。隐身的目的是尽快完成勒索软件攻击，并且不被发现。在现实世界中，生产环境非常复杂，操作员几乎总是用HMI监控ICS状态。因此，如果攻击者想要快速有效地完成攻击，必须具备隐蔽性。

  2. 多终端攻击。ICS不仅包括工业设备，如PLC、RTU等，还包括PC，如监控计算机。现有的工业勒索软件主要针对PC机，而忽略了工业设备，这也是其他工业勒索软件对ICS影响有限的原因。

  3. 低成本。对于经验丰富的攻击者，较低的攻击成本通常意味着较高的攻击利润。目标ICS的信息收集非常重要，但由于物理隔离意味着更高的成本，因此一直很困难。

在勒索软件的传播中，我们更关注如何突破ICS的物理隔离，这是ICS的主要保护方法。在这项工作中，我们提出了一种可能的攻击方法，改进了传统的ferry攻击方法，并使用BadUSB作为攻击载体。BadUSB的攻击过程如图2所示。
BadUSB攻击过程

图2 BadUSB的攻击过程

四、实验评估

我们选择西门子S7-300PLC作为研究对象，S7-300采用的通信协议为S7comm协议，这是西门子开发的专用ICS协议。ICS-BROCK的大多数操作将基于S7comm协议，例如编写恶意代码和锁定PLC。ICS-BROCK的主要模块如下。

  1. 漏洞检测
  2. PLC检测
  3. 锁定PLC
  4. 逻辑炸弹

ICS-BROCK将使用随机生成的对称密钥加密文件。加密完成后，对称密钥将由公钥和清除内存数据加密。由于对称密钥是在加密过程中随机生成的，因此该攻击方法可以有效抵抗逆向分析。毕竟，时间越短越好。收到付款后，攻击者将使用私钥解密密钥，并将其与解密程序一起发送给受害者。

（一）评估指标

1. PLC评估

在计划时间内，ICSBROCK不会对ICS环境产生任何影响，因此PLC的评估主要是检测ICSBROCK是否会对生产环境产生影响。检测生产环境的影响，主要从两个方面：执行时间和PLC内存消耗。

2.监控计算机评估

与其他传统勒索软件不同，ICS-BROCK主要关注ICS设备。但监控层也是攻击目标。我们将ICS-BROCK与一些知名勒索软件进行了比较，由于ICS环境的特点，监控计算机的性能总是很差，硬件如下表所示，结果如图3所示。
监控计算机的硬件
实验结果比较

图3 实验结果比较

（二）攻击实验

在建立水处理环境并评估ICS-BROCK后，我们在ICS环境中进行了勒索软件攻击，以测试ICS-BROCK的实战能力。具体过程如图4所示。
实验过程

图4 攻击过程

我们通过BadUSB在工作站感染ICS。ICS-BROCK将自动执行进一步的攻击，感染其他监控计算机，最后将逻辑炸弹写入PLC并锁定。

五、结论

我们介绍了第一个成熟的ICS勒索软件ICSBROCK，它可以在不影响ICS设备稳定性的情况下将逻辑炸弹悄悄地注入ICS设备。唯一需要的信息是PLC设备类型，它解决了物理隔离问题。并且，我们构建了一个真正的水处理环境，它采用了应用最广泛的PLC类型之一，我们举例说明了协议和入侵过程的逆向工程等全部细节。评估表明，ICS-BROCK能够成功地对ICS进行妥协。

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