RE: Windows NT does not check permissions after HANDLEs are open

From: Michael Wojcik (
Date: 08/30/01

Message-ID: <>
From: Michael Wojcik <>
Subject: RE: Windows NT does not check permissions after HANDLEs are open
Date: Thu, 30 Aug 2001 08:49:33 -0700

> From: Syzop []
> Sent: Thursday, August 30, 2001 5:59 AM
> To:
> Cc:;
> Subject: Re: Windows NT does not check permissions after HANDLEs are
> open
> wrote:
> > The check against the ACL only occurs when the HANDLE
> > is first opened, however. If a HANDLE is opened and
> > permissions on the objecect subsiquently change, the original
> > requestor of the object retains the original access-permissions.

> Isn't this normal?

Yes, it's pretty typical for discretionary access control (DAC) mechanisms
to perform access checking when a subject first requests access to an object
(eg. opens a file) and grant that access (if allowed by the DAC) for the
subject's lifetime or until it's specifically relinquished (eg. by closing
the handle).

In principle, a reference monitor should check every access, but "every
access" is pretty vague. Should the monitor check every operation a subject
performs on an object - eg. every read and write on a file - against the
current ACL? That would be expensive, so instead most systems establish a
boundary at which a subject (eg. a program, running with the rights of a
particular user, who may belong to one or more groups) is granted access to
an object for any number of subsequent operations (generally of certain
kinds - eg. you open a file for reading or writing or read-write). Once the
subject is past that boundary the monitor remembers its decision for that
{subject, object, operation(s)} tuple and doesn't need to revisit the ACL.

This is indeed an issue for timely revocation of access, but it's a
well-known one, and arguably not much of a threat in most cases. See for
example the aside about revocation in capability-based DAC implementations
in section 7.1 of the Rainbow book on DAC mechanisms.[1]


Michael Wojcik
Principal Software Systems Developer, Micro Focus
Department of English, Miami University

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