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Message-ID: <20180720154723.5rgdhg2n5nj5bgce@jwilk.net>
Date: Fri, 20 Jul 2018 17:47:24 +0200
From: Jakub Wilk <jwilk@...lk.net>
To: oss-security@...ts.openwall.com
Subject: Re: accountsservice: insufficient path check in
user_change_icon_file_authorized_cb()
* Matthias Gerstner <mgerstner@...e.de>, 2018-07-02, 16:38:
>>>I think the easiest way to fix this is to normalize the user supplied
>>>filename e.g. using realpath()
>>
>>Using realpath(3) for access control is almost always a mistake: this
>>function expands symlinks, including attacker-controlled symlinks.
>
>can you elaborate what your main worry of using realpath is in this
>context?
AIUI, in your original patch, canonicalized path was used for prefix
check, but then the orignal was stored. If you used realpath() for
canonicalization, the attacker could make a symlink that points to
/usr/share/icons/moo.png, so that the check passes, and then switch
the symlink to something else.
But in the patch that went upstream[0], it's the canonicalized path that
is stored, which is probably a good idea anyway.
Another problem with realpath(), unrelated to symlinks, is that if it's
run as root, it could reveal to the attacker whether an
otherwise-inaccessible directory exists. For example,
realpath("/home/bob/foobar/../../../usr/share/icons/moo.png", ...) would
succeed iff /home/bob/foobar/ existed.
[0] https://cgit.freedesktop.org/accountsservice/commit/?id=f9abd359f71a5bce421b9ae23432f539a067847a
--
Jakub Wilk
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