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RE: address@hidden: Emacs security bug]

From: Marshall, Simon
Subject: RE: address@hidden: Emacs security bug]
Date: Mon, 12 May 2008 11:10:50 +0100

> > - --> Observe that code from foo.c.flc is run.  Not good.
> > (This is with Emacs 21.3.1; XEmacs is also affected, although step 1
needs to
> > be adjusted.)
> >
> > Suggestions:
> >
> > a. Remove "." from fast-lock-cache-directories.  Littering little
> > files everywhere is not a good idea anyway.
> >
> > b. Don't use load to handle the .flc file.  Instead read it into a
> > buffer and read one s-expression at a time and verify that it is
> > before evaluating it.
> Simon, could you take a look at this (you're listed as the author of
> fast-lock.el)?

OK, it seems fast-lock writes a single (fast-lock-cache-data ...) form
into the .flc file, and fast-lock loads the .flc file.  Is there a way
to restrict the forms evaluated during a load, rather than parsing the
.flc file?  Or a better way?

Also note that fast-lock-cache-data calls font-lock-compile-keywords
(when setting font-lock-keywords amongst others) which can ultimately
eval a keyword.  That would also be a security hole, right?  (I assume
that isn't a security issue in general because font-lock-keywords is a
risky local variable, but I think that is a security issue here.)
Someone could put their nasty form in the top-level call of
fast-lock-cache-data, for font-lock-compile-keyword to eval, rather than
at the top-level of the .flc file itself.  I don't see how that could be
fixed with the current design of font-lock.el.

I know this is a bit cheeky of me, but fast-lock.el is so old that even
its successor, lazy-lock.el, is in lisp/obsolete.  It was last updated
for functional purposes in 1996.  

Probably the most reasonable fix, in the circumstances, is to make
fast-lock-cache-directories a risky local variable and remove "." from
its default value?


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