This is a case study. Read the following scenario and answer Questions $1$ to $5.$

"Souq.com" is a notable multinational e$-$commerce company that was established in $2005.$ Souq.com is often referred to as the "Amazon of the Middle East" and has gained popularity as a prominent online marketplace serving customers across various countries in the Middle East, including the United Arab Emirates, Saudi Arabia, Egypt, and others. It became a subsidiary of Amazon in $2017,$ further solidifying its presence in the e$-$commerce landscape.

Souq.com is preparing to launch a new AI$-$based mobile app that allows users to purchase products and manage their accounts on the go$.$ You have been hired by Souq.com as a Cybersecurity Professional who will head the Security Team, which includes the Risk Assessment Committee. As the software development team finalizes the app, the security team identifies a potential vulnerability during a thorough code review. The vulnerability involves insufficient input validation in the payment processing module, which could potentially allow attackers to manipulate payment transactions and steal customer payment information.

The risk assessment team is alerted, and they assess the situation. They determine that if exploited, this vulnerability could lead to severe financial losses due to fraudulent transactions and compromised customer data, not to mention the negative impact on the company's reputation. To mitigate this risk, you take a series of actions:

$1.$ Patch Development: You inform the software development team about the vulnerability. They immediately start working on a patch to address the input validation issue.

$2.$ Security Audit: You conduct a comprehensive security audit to identify any other vulnerabilities that might be present in the application. This audit involves code analysis, penetration testing, and vulnerability scanning.

$3.$ Communication Strategy: You develop a communication strategy to inform customers about the potential risk and advise them to monitor their accounts for any suspicious activity. You also recommend to the board of directors of Souq.com to delay the launch of the mobile app until the patch is thoroughly tested and implemented.

$4.$ Employee Training: You recommend to the board of directors of Souq.com to invest in employee training programs to educate developers and other relevant personnel about secure coding practices, emphasizing the importance of input validation and other security measures.

$5.$ Third$-$Party Vendors: You review the security practices of third$-$party vendors used in the app's development, ensuring they adhere to rigorous security standards.

$6.$ Ongoing Monitoring: After the patch is implemented and the app is launched, you recommend to the board of directors of Souq.com to implement continuous monitoring and incident response procedures to detect and address any potential breaches or security incidents.

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What vulnerability was identified in the mobile application?