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.