Many modern hardware and software products are built by combining multiple smaller components together into one larger entity, often during the design or architecture phase. For example, a hardware component might be built by a separate supplier, or the product might use an open-source software library from a third party. Regardless of the source, each component should be sufficiently trusted to ensure correct, secure operation of the product. If a component is not trustworthy, it can produce significant risks for the overall product, such as vulnerabilities that cannot be patched fast enough (if at all)

hidden functionality such as malware.

inability to update or replace the component if needed for security purposes.

hardware components built from parts that do not meet specifications in ways that can lead to weaknesses.

etc. Note that a component might not be trustworthy even if it is owned by the product vendor, such as a software component whose source code is lost and was built by developers who left the company, or a component that was developed by a separate company that was acquired and brought into the product's own company. Note that there can be disagreement as to whether a component is sufficiently trustworthy, since trust is ultimately subjective. Different stakeholders (e.g., customers, vendors, governments) have various threat models and ways to assess trust, and design/architecture choices might make tradeoffs between security, reliability, safety, privacy, cost, and other characteristics.