Greybox

Greybox, or grey-box testing, is a software testing approach that uses a middle-ground level of information about the internal structure of the component under test. Testers have access to some internal details, such as module boundaries, data flows, or design documents, but not full visibility of the source code or internal logic as in white-box testing. This contrasts with black-box testing, which offers no internal information, and white-box testing, which provides complete access to the internal implementation.

The term is commonly used to describe testing strategies that blend external interface validation with selective internal knowledge. Grey-box testing can be applied to functional, integration, and security testing, and it is often employed when some internal information is available through documentation, architecture diagrams, or partial code access. Test design may use this information to target critical data paths, interfaces, and state changes while still simulating real-world usage from a user or API perspective.

Advantages of grey-box testing include more efficient test-case design by focusing on integration points and potential failure modes, improved fault detection at interfaces, and faster feedback in development environments. Limitations include the reliance on partial internal information, which may introduce bias or overlook deeper internal defects, and the need for instrumentation or access that may not always be permitted in proprietary or regulated settings.

Grey-box testing sits between black-box and white-box approaches and is often used to enhance coverage at interfaces and integration points. Related concepts include black-box testing, white-box testing, and grey-box security testing.