Re: IIS session cookies

["Kevin Spett" <kspett () spidynamics com>]

> From http://www.securiteam.com/windowsntfocus/6C00L003GA.html:

"LJALNFJCGLOICFEPIAPBFDEJ is a 32 character "munge" of the 32 bit session ID
(see later for how session ID is created)
Session ID is created from a random seed number that is generated when the
system starts up). The random seed is incremented every time a new session
starts. Note that the "munge" doesn't increment in the same way that the
Session ID does.
Since the 8 char string after ASPSESSIONID is a "munge" of the process ID it
will be (a) the same for all "In-process" applications (b) a different value
is shared for all "Medium isolation (pooled)" applications and (c) unique
for each Out-of-process application."

From
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnasp/html/
aspwsm.asp:

"The following steps are taken when generating ASP session cookies:
* Session ID values are 32-bit long integers.
* Each time the Web server is restarted, a random Session ID starting value
is selected.
* For each ASP session that is created, this Session ID value is
incremented.
* The 32-bit Session ID is mixed with random data and encrypted to generate
a 16-character cookie string. Later, when a cookie is received, the Session
ID can be restored from the 16-character cookie string (ASPSESSIONID).
* The encryption key used is randomly selected each time the Web server is
restarted."

I don't know for sure, but I'm guessing that they're using CryptGenRandom
for the PRNG, which uses mouse & keyboard events timing, system clock,
system time, system counter, memory status, free disk clusters, etc.  To my
knowledge, it's sufficiently "random" to make them unpredictable in
practical terms.

Hope that helps.



Kevin Spett
SPI Labs
http://www.spidynamics.com/


----- Original Message -----
From: "Cade Cairns" <cairnsc () securityfocus com>
To: "Kevin Spett" <kspett () spidynamics com>
Cc: <webappsec () securityfocus com>
Sent: Friday, December 06, 2002 2:48 AM
Subject: Re: IIS session cookies


> I'm curious whether the ASPSESSIONID value generated is predictable and if
> so, to what extent.
>
> Cade Cairns
> Symantec Corporation
>
> On Thu, 5 Dec 2002, Kevin Spett wrote:
>
> > What do you mean by "IIS session cookies"?  Do you mean the ASPSESSIONID
> > feature? And what do you mean by formed?  Are you talking about the PRNG
> > behind it, or how a developer can use them?
> >
> >
> > Kevin Spett
> > SPI Labs
> > http://www.spidynamics.com/
> >
> > ----- Original Message -----
> > From: "Cade Cairns" <cairnsc () securityfocus com>
> > To: <webappsec () securityfocus com>
> > Sent: Thursday, December 05, 2002 5:29 PM
> > Subject: IIS session cookies
> >
> >
> > > Hello webappsec,
> > >
> > > I'm looking for information on how IIS session cookies are formed
(that
> > > is, what data they consist of or how they are encoded, etc.)  Is
anyone
> > > aware of any papers or resources on the subject?
> > >
> > > Thanks,
> > >
> > > Cade Cairns
> > > Symantec Corporation