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From "Graham Dumpleton (JIRA)" <j...@apache.org>
Subject [jira] Updated: (MODPYTHON-191) Tampering with signed cookies.
Date Thu, 26 Oct 2006 06:29:18 GMT
     [ http://issues.apache.org/jira/browse/MODPYTHON-191?page=all ]

Graham Dumpleton updated MODPYTHON-191:

    Fix Version/s:     (was: 3.3)

After looking at this, I have come to the conclusion that in the context used it is not an
issue. FWIW, the whole idea of allowing a Session to have a secret associated with it and
thus have a signed cookie used seems to be a pointless exercise which adds little value except
making the length of the cookie string longer and perhaps therefore harder to guess. To understand
one needs to look at what happens when one uses a signed cookie.

When one uses a signed cookie, the string stored in the header will be something like:


That is, the first 32 characters represent the hash of the value of the cookie. As one can
see, the actual value is stored in whatever follows the first 32 characters. The actual value
is not encoded in any way and just appears as is.

When cookies are used for sessions, the session ID is itself a value containing 32 characters.
This 32 characters are an md5 hash of  a string made up of the Apache child process ID, the
remote client IP address and two random numbers.

Thus, the session ID would be something like:


When one signs this, one gets:


As you can see, since the value is as is, the second set of 32 characters is still the same
session ID.

The original concern expressed was the code even though told to use a signed cookie, will
revert to accepting a non signed cookie. That is, if the first 32 characters are the correct
hash for the second 32 characters it will reject it. It will instead take the whole value
and try and use that as the SID instead.

At this point, two things still need to happen before the session can work. The first is that
the whole value must be 32 characters in length and must only contain lower case alpha numeric
characters. If it isn't, it will fail the initial precondition check of what constitues a
valid session ID.

Even if it gets past that, the value presented as the session ID still has to actually match
an active session in the session database. If it doesn't it is flagged as bogus or expired
and a new session will be created with a new session ID pushed back to the browser instead.

The outcome of all this is that even if using a signed cookie for the session, that 64 characters
are in the string doesn't matter as one would only need to guess the second 32 characters
and simply supply that, with it being accepted as an unsigned cookie.

The question then becomes how concerned are you about the chances of someone guessing a 32
character string, that would make you want to use a 64 character string instead.

Thus, this issue certainly doesn't raise any problem that needs to be addressed at this point.
As such, am going to remove this from 3.3 task list. We can consider this again later as to
whether a check for a signed cookie is a good idea or not, or simply leave it as is and close
the issue.

> Tampering with signed cookies.
> ------------------------------
>                 Key: MODPYTHON-191
>                 URL: http://issues.apache.org/jira/browse/MODPYTHON-191
>             Project: mod_python
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.2.10
>            Reporter: Graham Dumpleton
> As reported by Andy Pearce in:
>   http://mail-archives.apache.org/mod_mbox/httpd-python-dev/200609.mbox/%3c44F824E2.4040304@jgassociates.ca%3e
> Andy Pearce wrote:
> > 
> > Hi,
> > 
> > I think I might have spotted a slight bug in Session.py. When the 
> > 'secret' parameter is supplied to use the SignedCookie class, it appears 
> > that __init__ of BaseSession doesn't check the return type of 
> > get_cookies().
> > 
> > If I understand the SignedCookie docs correctly, if the cookie value 
> > doesn't match its signature, it simply returns the contents as a Cookie 
> > rather than a SignedCookie (indicating that the user tampered with their 
> > cookie before sending it back).
> > 
> > However, there is no check in BaseSession's __init__ that the return of 
> > get_cookies() is a SignedCookie in the case that 'secret' is supplied.
> > 
> > Perhaps a minor point, but it would seem to make the option of using 
> > SignedCookies rather pointless, since the signature isn't being checked. 
> > Presumably if the cookie has been tampered with, your only safe option 
> > is to throw it away and generate a new one. I think this can be achieved 
> > by changing the lines:
> > 
> >     if cookies.has_key(session_cookie_name):
> >         self._sid = cookies[session_cookie_name].value
> > 
> > To something like:
> > 
> >     if cookies.has_key(session_cookie_name):
> >     if not secret or type(cookes[session_cookie_name]) \
> >            is Cookie.SignedCookie:
> >             self._sid = cookies[session_cookie_name].value
> > 
> > I'm fairly new to mod_python, so if I'm mistaken then my apologies, and 
> > a quick explanation of why would be very much appreciated! ^_^
> > 
> > Thanks,
> > 
> >     - Andy
> > 
> Is this correct and should the change suggested appropriate?

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