Policy regression is an unintended breakage that happens when a software update changes how a security or access rule is enforced. The policy itself may still look correct on paper, but the code path that evaluates permissions, tokens, or elevation behaves differently after patching. In practice, that can turn a valid login, remote session, or admin workflow into a denied request or a failed connection.
This matters because many defenses depend on policy enforcement, not just the policy text. A regression can create denial of service, block support tools, or force teams to disable protections to keep work flowing. In real environments, it shows up after updates to identity, authorization, or hardening features, especially when least-privilege controls interact with remote access. Security teams defend against it by testing patches in pilot rings, checking policy combinations, and validating that authentication and authorization still produce the expected result after changes.