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Vulnerabilities in digest-aa I-D

From: Allan M. Schiffman <ams@terisa.com>
Date: Sun, 31 Dec 1995 11:39:10 -0800
Message-Id: <v02130502ad0c8bd8de8e@[]>
To: www-security@ns1.rutgers.edu, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Cc: ekr@terisa.com
I've only spent about an hour looking at the most recent I-D (with some
help from Eric Rescorla), but we've found quite a few problems in that
time. I think that digest-aa should not proceed at least until these
problems are addressed.

There are five vulnerabilities which struck us immediately:

1) The gross structure of the digests allows for the exploitation of MD5
collisions. This is possibly not worth worrying about, since the best
attack we can come up with requires effort on the order of 2^64 operations.
In general, the sharing of long common prefixes between the digests and the
lack of secret or random material beyond the initial amount leads us to
suspect that there might be many other cryptoanalytic attacks we haven't
thought of.

2) The fine structure of the digests allows one to be substituted for
another. This allows for straightforward splicing and reflection attacks
which undercut the rationale for the protocol. This could be fixed by
insisting that each digest type have some sort of type-distinguishing data
or structure in them (there are three specified in the document: client
plain digests, client "message-digests" [sic], and server "message-digests"
[sic again]). Vulnerability to substitution is increased given the
one-sided and unstructured nature of the freshness material. One could
easily arrange that the client always provided freshness material, and
insist that freshness have structure that the either side can count on (say
that it must monotonically increase).

3) The "optional-ness" of the client message-digest and server
message-digests means that neither can be used for authentication given a
downgrade attack (the attacker removes the digest and substitutes
unauthenticated material).

4) The fact that no headers are included in the digesting process combined
with the fact that HTTP headers change the semantics of requests (and
replies) means that authenticated requests and replies can be transformed
by an attacker undetectably. For example, consider byte ranges where the
authorized request or only wants one portion of a document and the attacker
transforms the request into one for the entire document. This is difficult
to fix while retaining the spirit of the proposal.

5) There is no treatment of the security implications of retries and
multiple authorization headers. Absent this, I can imagine many flawed
implementation possibilities. Also, I think that it is assumed that this
mechanism works for proxy authentication, and if this is permitted, new
sorts of attacks are possible.

Outside of these immediate security vulnerabilities, I wonder about the
wisdom of wiring-in a single digest algorithm (the selection of MD5 could
easily be parameterized with no damage to the spec). I also wonder about
the wisdom of referencing Dave Kristol's extension mechanism (sounds like
what used to be called at PARC "error 33" -- making one risky project
dependent on another).

Given the above, here's an off-the-top-of-my-head attempt at addressing
these vulnerabilities, while retaining as much spirit of the design as
possible. It is an admittedly bad practice I am indulging in here -- this
is not a thought-out design, it's only meant to illustrate fixes.


1) The nonces are mandatory, and have the following structure:
host-id is the principal's DNS name or the "realm", I don't care.
tod is seconds since Unix epoch in hex.
discrim is a hex integer so that multiple nonces generated in a given
second monotonically increase.
I don't care what sep1 and sep2 are (slashes?).
This is so the principals can check for replay with finite memory.
Clients have nonces too.

2) The client auth header is:
Authorization: Digest
        cnonce="<client-nonce>",     -- must be fresh
        uri="<requested-uri>",       -- [Why was this in the original I-D?]
        opaque="<opaque>"            -- required if provided by server
        <client-digest> := H( H(A1) + CN + SN + OP + BI + H(H(A1) + A2) )
        <message-digest> := H( H(A1) + <client-digest> + H(H(A1) + CB) )
        A1 := 'MD5' + U + R + P
        A2 := <Method> + <requested-uri>
        BI := "cbody" | "no cbody"                      -- depending
        OP := "opaque" + <opaque> | "no opaque"         -- depending
        CB := "cbody" + <message-body> | "no cbody"     -- depending
        SN, CN -- server and client nonce values
        U -- username
        R -- realm
        P -- password
        <Method> -- entire request header line 0
        <requested-uri> -- uri sans proxy/routing

3) server response
When authorization succeeds, the Server MUST provide the following:

HTTP/1.1 200 OK
Authorization-Response: Digest
        <server-digest> := H( H(A1) + CD + SB + H(H(A1) + <Response>))
and with:
        A1 as above
        CD := <client-digest> -- from above
        SB := "sbody" + <message-body> | "no sbody" -- depending
        <Response> -- entire response header line 0

4) This mechanism must be outlawed for "Proxy-Authentication:" or it we need
to make the structure of A1 dependent on proxy vs. non-proxy use.

5) The headers that change the effect of a request or response such as:
        Range, Unless, If-Modified-Since
(I'm worried that there are others) must either be outlawed when using
Digest-Authentication, or these headers must be accounted-for in the
digests. Figuring-out which are the headers that have this property is a
surprisingly hard problem.

6) Multiple Authorization headers are forbidden.

7) Servers must either disregard the request line 0 URI (in favor of the
uri field of the authorization header) or reject requests where these are
not identical. Even better would be to drop the uri field from the
authorization header.


Happy Holidays,

Received on Sunday, 31 December 1995 11:37:49 UTC

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