So you're trying to somehow prevent cookies from being stolen, after
they've been transmitted to the client. Are you talking about session
cookies, or about cookies that persist data (sorry, I'm still not
clear on the scope of the problem)?
If you're trying to solve the problem of stolen session ids, as stored
in cookies on the user's system, whether in memory or on disk, I'm not
really sure what you can do. I mean, there are the standard things of
marking it secure, so it shouldn't be sent without SSL; giving the
cookie a short lifetime, so that stealing it is only useful for a
short period of time; setting the various no-cache settings to keep it
in RAM (or the page file); and, if the problem space is small enough
(and the network in which the solution is provided is under strict
enough control) you may be able to rely on IP addresses and such, but
these (recommended) tacks only shrink the problem space to what you
already seem to be considering: the security of an untrusted client.
And they rely on the client behaving properly - it's always possible
for a client to intentionally or unintentionally ignore the server's
directives.
Protecting session cookies with cryptography seems to be trying to
allow a client to connect to a server and access that server's
resources, without actually letting the client know the secret (the
session ID) that identifies the client to the server. This would
require sending the session cookie to the client, but not actually
telling the client the raw session ID. As I mentioned, before,
without tightly binding the cookie to the client, in some other way,
the cookie can simply be replayed from another system.
The only reason I can see for encrypting a session cookie is if the
server doesn't trust the client - if the security of the client is not
sufficient to protect the cookie. But, if the security isn't enough
to protect the cookie, why would the security be strong enough to
protect the encrypted/signed cookie? And, if it's not enough to
protect the encrypted/signed cookie, why would it be enough to protect
any data used to tightly bind the cookie to the client?
If you're trying to solve persistent data, related to the user
account, but stored on the client system, that problem seems solvable
via cryptography. A key could be generated for each user. A
particular user's key could be used to encrypt/sign the data before
sending it to the client as a cookie. The key could then be used to
decrypt/check the data when received by the server. Of course, this
still requires the server to know which key to use, and still depends
upon the security of the session.
The big problem with this would be that cryptography is generally
pretty slow, and servers need to be very responsive. If every request
from every client requires the cookie be decrypted, reformed, and
re-encrypted, this could be a huge drain on the server. The
performance penalty would be highly dependent on the information being
stored in the cookie. Additionally, it may be simpler to just store
the data on the server - is there any reason why the information
cannot be stored server side?
I guess the problem still needs to be better defined. What is the
environment? Is it the Internet, a home network, or a corporate
network? What data is being protected? Is it just a shopping cart, or
is it something that has extra storage restrictions, such as credit
cards? What services does the server provide, and why can't the data
be stored on the server (of course, if this is the session cookie, the
answer would be self-evident)?
--
Aaron
On Dec 4, 2007 6:48 PM, Till Elsner <till.elsner@uni-
duesseldorf.de> wrote:
Hi Aaron,
the second problem you're mentioning is definitely the one i'm more
worrying about. In my eyes generating an ID the not (easily)
pedictable is a solvable problem, but a stolen cookie is another
thing. I think it would be okay for me to use short-living cookies,
but i wonder if this is the only secure way to protect users from
suffering from stolen cookies.
Till
Am 04.12.2007 um 23:13 schrieb Aaron Katz:
Ron,
It's unclear exactly what problem you're trying to solve.
Encrypting
& signing the cookie may stop tampering, but it won't prevent
stealing
the cookie and replaying it. So, you've basically got two types of
data that might be in the cookie: the session identifier, or the
state.
In either case, you have to decide whether or not to tie the
cookie to
the client. If you don't tie the cookie to the client, (or possibly
the current time, with a very short timeout, and automated
replacement), then nothing prevents replay of the cookie.
If you tie the cookie to the client, you've got the problem of
uniquely identifying the client. You've got to use stuff that's
available to the server, so that the server can confirm the cookie.
The obvious ideas would be IP address, user agent, and such, but
these
don't seem to work very well. The IP address can be "duplicated"
for
multiple clients (NAT), and can theoretically change at inopportune
times (DHCP). User agent agent isn't unique, and provides such
little
entropy (most people choose from the same small set of browsers (IE,
Fx, Mozilla, Opera, Lynx)) that the dictionary attack to discover it
is so easily overcome that it's probably not worth the effort.
The problem of uniquely tying the cookie to the client seems to rely
upon the problem of uniquely identifying the client. Uniquely
identifying the client is what the session cookie is trying to
solve.
To improve this, you might consider client certificates, but that
comes with its own problems.
If you're tying the cookie to the current time (directly, by time
stamp, or indirectly, if you choose to include the session
identifier
in a cookie that contains state), then you have to be sure that the
cookie doesn't contain anything that absolutely must live longer
than
the timeout. For example, some shopping carts retain their content,
between multiple sessions.
Also, in some cases, it may be entirely appropriate to allow
tampering
(I can't think of one off the top of my head, but it seems like a
reasonable assumption that this would be good, in some cases).
As implied, above, it would be easier to comment on your thoughts if
you identify the problem that you're trying to solve :)
--
Aaron
On Dec 4, 2007 1:47 PM, Ron <ronlists@skullsecurity.com> wrote:
Something I've recently been toying with is the idea of
encrypting/signing cookies with a private key on the server. The
the
cookies can't be read or tampered with by the user, nor can they be
stolen by cross-site attacks and the like.
This isn't something I've done a lot of work with, however, so I
may be
missing something obvious, and am open to comments on the idea.
Ron
Till Elsner wrote:
Hi, i'm investigating in web application security this time and
i'm
trying to find some information about session management with
cookies
and related security issues. Can anyone point me to tips on how
to make
cookie based sessions more secure and how to prevent session
hijacking?
How secure is session handling using cookies and what are the main
risks? Is anyone aware of good literature on that topic?
Thanks and have a nice day
Till
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Sponsored by: Watchfire
Methodologies & Tools for Web Application Security Assessment
With the rapid rise in the number and types of security threats,
web application security assessments should be considered a
crucial phase in the development of any web application. What
methodology should be followed? What tools can accelerate the
assessment process? Download this Whitepaper today!
https://www.watchfire.com/securearea/whitepapers.aspx?
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Sponsored by: Watchfire
Methodologies & Tools for Web Application Security Assessment
With the rapid rise in the number and types of security threats,
web application security assessments should be considered a crucial
phase in the development of any web application. What methodology
should be followed? What tools can accelerate the assessment
process? Download this Whitepaper today!
https://www.watchfire.com/securearea/whitepapers.aspx?
id=70170000000940F
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