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Date: Fri, 20 Jul 2018 17:47:24 +0200
From: Jakub Wilk <>
Subject: Re: accountsservice: insufficient path check in

* Matthias Gerstner <>, 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 

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.


Jakub Wilk

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