FreeBSD -- Incorrect error handling in PAM policy parser
The OpenPAM library searches for policy definitions in
several locations. While doing so, the absence of a policy
file is a soft failure (handled by searching in the next
location) while the presence of an invalid file is a hard
failure (handled by returning an error to the caller).
The policy parser returns the same error code (ENOENT)
when a syntactically valid policy references a non-existent
module as when the requested policy file does not exist.
The search loop regards this as a soft failure and looks
for the next similarly-named policy, without discarding the
A similar issue can arise if a policy contains an include
directive that refers to a non-existent policy.
If a module is removed, or the name of a module is
misspelled in the policy file, the PAM library will proceed
with a partially loaded configuration. Depending on the
exact circumstances, this may result in a fail-open scenario
where users are allowed to log in without a password, or
with an incorrect password.
In particular, if a policy references a module installed
by a package or port, and that package or port is being
reinstalled or upgraded, there is a brief window of time
during which the module is absent and policies that use it
may fail open. This can be especially damaging to Internet-facing
SSH servers, which are regularly subjected to brute-force
Copyright © 2003-2005 Jacques Vidrine and contributors.
Please see the source of this document for full copyright