Re: DoS in debian (potato) proftpd: 1.2.0pre10-2.0potato1

From: Alun Jones (alunat_private)
Date: Wed Apr 03 2002 - 18:45:00 PST

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    At 03:40 PM 3/29/2002, martin f krafft wrote:
    >   ls */../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../*
    
    ...
    
    >   DenyFilter \*.*/
    
    Just as a quick question, why not deny the string "/../" (you may have to 
    deny the regex "/\.\./", depending how the filter in question works)?
    
    As far as I can tell, it's the ability to embed "/../" into a path that is 
    at the root of this, far more than the ability to embed wildcards.  I can't 
    think of a situation in which "/../" should appear in a user-supplied path, 
    except after a string of repeated "../"s.
    
    The workaround suggested by Mr Krafft would disable some useful 
    functionality - one large user of mine, for instance, was keen to have my 
    own software evaluate wildcards in the body of the path, which Mr Krafft's 
    workaround disables completely.  They even paid for the privilege (not 
    enough, but they paid ;-))
    
    So, let's see, a regex that would deny "/../", except as part of a string 
    of such...
    
    One bash would be "[^/.].*/\.\./" - matching "/../" if it's after any 
    character other than '/' or '.'.  Doubtless someone can come up with 
    something better.
    
    Alun.
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