This is the most urgent and effective action to prevent further damage or compromise of the organization's network and data. The other options are less important or irrelevant in this situation.
According to How to identify suspicious insider activity using Active Directory, one of the steps to detect and respond to suspicious activity is to isolate the affected device from the network. This can be done by disabling the network adapter, unplugging the network cable, or blocking the device's IP address on the firewall1. This will prevent the device from communicating with any malicious actors or spreading malware to other devices on the network.
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he greatest concern for an information security manager of a large multinational organization when outsourcing data processing to a cloud service provider is the local laws and regulations that may apply to the data and the cloud service provider. Local laws and regulations may vary significantly across different jurisdictions and may impose different requirements or restrictions on the data protection, privacy, security, sovereignty, retention, disclosure, transfer, or access. These laws and regulations may also create potential conflicts or inconsistencies with the organization's own policies, standards, or contractual obligations.
Therefore, an information security manager should conduct a thorough legal and regulatory analysis before outsourcing data processing to a cloud service provider and ensure that the cloud service provider complies with all the applicable laws and regulations in the relevant jurisdictions.
References = CISM Manual1, Chapter 3: Information Security Program Development (ISPD), Section 3.1:
Outsourcing2
1: https://store.isaca.org/s/store#/store/browse/cat/a2D4w00000Ac6NNEAZ/tiles 2: 1 Outsourcing data processing to a cloud service provider may expose the organization to different legal and regulatory requirements depending on the location of the data and the vendor. This could affect the organization's compliance and liability in case of a breach or dispute. Therefore, the information security manager should be most concerned about the local laws and regulations that apply to the outsourcing arrangement.

The best way to test for the existence of backdoors in a mission-critical business application that was outsourced to a third-party developer is to perform security code reviews on the entire application. A backdoor is a hidden or undocumented feature or function in a software application that allows unauthorized or remote access, control, or manipulation of the application or the system it runs on. Backdoors can be intentionally or unintentionally introduced by the developers, or maliciously inserted by the attackers, and they can pose serious security risks and threats to the organization and its data. Security code reviews are the process of examining and analyzing the source code of a software application to identify and eliminate any security vulnerabilities, flaws, or weaknesses, such as backdoors, that may compromise the functionality, performance, or integrity of the application or the system. Security code reviews can be performed manually by the security experts, or automatically by the security tools, or both, and they can be done at different stages of the software development life cycle, such as design, coding, testing, or deployment. Security code reviews can help to detect and remove any backdoors in the application before they can be exploited by the attackers, and they can also help to improve the quality, reliability, and security of the application.
References = CISM Review Manual, 16th Edition, Chapter 3: Information Security Program Development and Management, Section: Information Security Program Development, page 1581; CISM Review Questions, Answers & Explanations Manual, 10th Edition, Question 87, page 812; CISM ITEM DEVELOPMENT GUIDE, page 63.