pam_permit

Used in PAM configuration files:
module-type control-flag pam_permit.so

No module-specific options
    The pam_permit module does not accept any specific parameters that alter its behavior. It always succeeds. General PAM control flags (e.g., debug, no_warn) can be applied to its entry in a PAM configuration file, but these are handled by the PAM framework itself, not by the pam_permit module.

The pam_permit module is a fundamental component of the Pluggable Authentication Modules (PAM) framework on Linux. Unlike typical commands invoked directly from the shell, pam_permit is a shared library (pam_permit.so) used within PAM configuration files (e.g., in /etc/pam.d/). Its sole and simple purpose is to unconditionally succeed, meaning it always grants access or authorizes the action for which it is called within a PAM stack, without performing any checks. This makes it a useful building block, often placed at the end of a PAM stack or used in scenarios where no specific authentication or authorization is required but a PAM module must still return a success code. It's crucial for configuring services that might not require a traditional login but still interact with the PAM system.

Using pam_permit carelessly can introduce significant security vulnerabilities, as it bypasses all authentication and authorization checks. It should only be used when absolutely certain that no checks are needed, or as a fail-safe at the end of a carefully constructed PAM stack where other modules have already performed necessary validations. Placing it too early in a PAM stack with an inappropriate control flag (e.g., required or sufficient) could effectively nullify all subsequent security measures.

pam_permit is never invoked directly from the command line by a user. Instead, it is configured within PAM service files located typically in /etc/pam.d/. Each entry specifies a module type (e.g., auth, account, password, session), a control flag (e.g., required, sufficient, optional, requisite), and the module path (pam_permit.so). The PAM system loads and executes these modules in sequence based on the configuration.

While seemingly counterintuitive, pam_permit plays an important role in complex PAM stacks. For instance, it can be used with the sufficient control flag to provide a default 'allow' if no other module explicitly denies access, or with optional to indicate that its success is not strictly necessary but contributes to the overall result. It's often seen at the very end of a stack, or in conjunction with other modules that might fail gracefully without a specific 'allow' module to finalize the decision.

PAM (Pluggable Authentication Modules) was originally developed by Sun Microsystems in the early 1990s to provide a flexible authentication framework. The Linux-PAM project later adopted and expanded upon this concept. The pam_permit module, being a fundamental primitive that simply returns success, has been an integral part of the PAM system since its early iterations. Its existence is crucial for enabling the modularity of PAM, allowing administrators to construct authentication policies by combining various modules, including those that offer unconditional success or failure.

pam(8), pam_deny(8), pam_unix(8), pam.d(5), login(1)