Re: [Ace] ACE Framework Review

Stefanie Gerdes <gerdes@tzi.de> Wed, 24 October 2018 13:53 UTC

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From: Stefanie Gerdes <gerdes@tzi.de>
To: Ludwig Seitz <ludwig.seitz@ri.se>, ace@ietf.org
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Subject: Re: [Ace] ACE Framework Review
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Hi Ludwig,

On 10/22/2018 03:07 PM, Ludwig Seitz wrote:
> 
>> The minimal security requirements for the communication between two
>> communication partners should be listed (C-AS, RS-AS, C-RS,
>> respectively). Which pieces of information do they require prior to the
>> communication? How must the communication be secured? Which keying
>> material do they need to use? 
> 
> The first one I agree with. However the other questions seem more
> specific to the commSec solution to me, which would put them into the
> profiles.

In Appendix C, you specify that the security protocol between C and RS
must provide encryption, integrity and replay protection. It would be
useful to list similar requirements for the communication between C and
AS and RS and AS.

In section 5 you state "Nevertheless, it is REQUIRED that the data
exchanged with the AS is encrypted and integrity protected. Does that
refer to both the communication between the AS and C and between the AS
and RS? It would be useful to clarify that in the sentence.

The client may specify a key in the request to the AS. The AS must
ascertain that the client that sent the request is associated with this
key. Otherwise a client may request tokens for other clients. Although
the client cannot gain access with these tokens without the
corresponding keying material, it may use the token for DoS attacks on
the resource server's authorization information endpoint. Since the
tokens are valid, RS cannot throw them away.

> 
>> The framework should point out that all
>> claims that influence the security must stem from claimants that were
>> approved by the respective human being that is responsible for the
>> device, i.e., the requesting party for the client and the resource owner
>> for the AS and RS. Otherwise the solution is not secure.
>>
> 
> Are there claims that do not influence security?
> 
> The specification says (section 4):
> 
>    The consent of the resource owner, for giving a client access to a
>    protected resource, can be provided dynamically as in the traditional
>    OAuth flows, or it could be pre-configured by the resource owner as
>    authorization policies at the AS, which the AS evaluates when a token
>    request arrives.  The resource owner and the requesting party (i.e.,
>    client owner) are not shown in Figure 1.
> 
> I believe this is what you were looking for, if not please explain what
> is missing.

As I read it, this paragraph only addresses authorization information
that is meant for the server. The client may receive keying material for
RS from AS. In this case, C's owner, i.e., RqP, must authorize AS to
provide keying material to C.

> 
>> Validity of the access token (Token Expiration, Section 5.8.3): There
>> seem to be additional requirements here that are currently not
>> mentioned. It is not clear how RS determines the freshness of
>> introspection messages (and how fresh they are).
> 
> That would be up to the commSec protocol, also it is covered in the
> security considerations of RFC 7662. I will add a reference to those
> considerations in our security considerations section.
> 
>> Also, the token
>> introspection approach is particularly vulnerable to DoS attacks since
>> introspection messages must be exchanged to validate every token. 
> 
> This problem applies to regular OAuth introspection as well and is
> covered in the security considerations of RFC 7662.

It might be useful to mention that in the security considerations.

> 
>> The sequence number approach has the problem that an attacker may
>> withhold new tokens from RS. In this case, old tokens are infinitely
>> valid. This is particularly damaging because C may have a strong
>> interest to withhold new tokens and can easily do so by not relaying
>> them to RS. This approach therefore must not be used unless there is an
>> additional communication channel between RS and AS that regularly
>> informs RS which the most recent sequence number is.
>>
> This is a best effort approach when the RS has no internal sense of
> time. Also the idea was that several client could submit tokens with an
> ever increasing unique serial number maintained by the AS. Thus even if
> one client would withhold it's newer tokens to keep the old ones from
> expiring, the other clients would eventually submit enough new tokens
> (with higher sequence numbers) so that the token expires.
> 
> AFAIK the RS with no internal clock at all is a very rare occurrence,
> thus you can usually expect some form of wallclock time, and thus
> implement an expiration based on "time-since-you-received-this-token".

I don't think it is reasonable to require that the RS must have multiple
clients to ensure that the mechanism works. If you expect the RS to have
a wallclock time, you may want to omit the sequence number approach. If
the mechanism only works when additional requirements are met, these
requirements must be mentioned.

If the RS calculates the expiration of the token from the time it
received the token (an appraoch which currently does not seem to be
mentioned in the document?), the approach is vulnerable to attacks where
an attacker manages to withhold messages.

> 
>> Authorization of the AS on the RS side: in the ACE framework, RS checks
>> the integrity of the access token (e.g., section 4, section 6, Appendix
>> B: Resource Server). RS must also check that the access token stems from
>> an AS that is authorized by RO to provide the token. If RS accepts
>> tokens from authorization servers that are not approved by RO, the
>> solution is not secure. Introspection messages must also stem from an AS
>> that is approved by RO.
> 
> This is implicit, if the RS has the key that allows it to verify the
> integrity of the token that means that it is an AS approved by its RO.
> I will try to add a sentence to make that more explicit.
> 
>>
>> Access token validation: Section 5.8.1 should point out that RS must
>> check the integrity of the token and validate that it stems from AS that
>> is authorized by RO. 
> 
> We can expand on this phrase I think: "The RS receiving the token MUST
> verify the validity of the token."

That sounds like a good idea.

> 
> 
>> Also, it would be helpful if the section also
>> contained information how a server must react to missing fields, e.g.,
>> missing authenticity of the token or tokens from unknown authorization
>> servers.
>>
> This would be the following sentence in 5.8.1:
> "If the token is not valid, the RS MUST respond with a response code
> equivalent to the CoAP code 4.01 (Unauthorized)."

It would be useful to add when the RS can consider a token to be valid,
i.e., which requirements a token must meet to be valid.

> 
>> Confidentiality of access tokens:
>> In the access token, confidentiality is only demanded for symmetric
>> keys. The security considerations should point out that other data in
>> the access token may be confidential and then require protection as well.
>>
> This should be covered in RFC 6749  and RFC 6819 which are referenced in
> the security considerations.
> 
> 
>> Authorization Rules in Access Token: The framework should mention in
>> section 5.6.2 that the authorization rules that are presented to RS in
>> the scope of the access token must reflect RO's decisions.
> 
> That is already covered by the text in section 4 (see comment above).

Section 5.6.2 describes how AS specifies the response to the client.
I think it would help to avoid implementation errors if the section
describes that the authorization rules that the AS defines in the access
token must reflect RO's decisions.

> 
>>
>> Management of the authz-info resource:
>> * The authz-info resource is vulnerable to DoS attacks: clients may
>> (with or without intention) send large numbers of access tokens to RS. A
>> constrained RS may soon run out of memory/storage space if it needs to
>> store large numbers of tokens.
> 
> A RS is not expected to store large numbers of tokens. Section 5.8.1
> specifies:
> 
> "The RS MUST be prepared to store at least one access token for future
>    use."
> 
> Thus the RS can adapt the number of tokens it stores to its constrained
> storage resources.

I don't really understand how storing at least one access token helps to
mitigate the problem of DoS attacks.

> 
>> If introspection messages are used to
>> check the validity of access tokens, the use of the authz-info resource
>> amplifies the risk of DoS attacks since RS must send a message to its AS
>> for every received token. These problems should be mentioned in the
>> security considerations.
> 
> I will add a section describing the problem.
> 
> 
>> The preferred mitigation should be that the
>> authz-info resource is only used for protected communication. This does
>> obviously not work for the RPK mode. In the PSK mode, RS should be able
>> to decide that it uses the authz-info resource only for protected
>> updates.
> 
> I don't think that will help, since somehow the initial access token
> must get to the RS. So even when passing it through the DTLS handshake
> (as with PSK in the DTLS profile) the same DoS risk exists.

If the client is not able to finish the DTLS handshake, the RS can throw
the token away immediately. Also, DTLS has some protection against DoS
attacks. If a resource is used, clients can upload large numbers of
tokens to the resource, and no protection against DoS attacks is
available. If the tokens are valid, the RS must in the worst case store
them until they expire, even if the client never tries to communicate
with RS.

> 
>> * How must the authz-info resource be protected? E.g., an attacker that
>> is able to modify the resource may change or delete access tokens. The
>> framework leaves the protection of the resource to the profiles, but I
>> think some general advice can be offered by the framework.
> 
> I can add a paragraph on this in section 5.
> 
>>
>> Validity of the access information:
>> It seems that the people who wrote the framework did not consider that
>> the keying material is not only provided to RS but also to C.
>> How does C determine if the keying material it receives from AS is still
>> valid?
> 
> It is valid as long as you don't get a 4.01 (Unauthorized) from the RS.
> 
>> The access token response may comprise an expires_in
>> field that contains the validity period of the access token in seconds
>> (RFC 6749), but does not specify the validity period of the keying
>> material. But even if the validity periods are equal, C does not know
>> when the access token was generated. Also, the use of the expires_in
>> field is optional. Without protection, the access information is
>> infinitely valid. C may use outdated and maybe compromised keying
>> material to communicate with RS. The framework should provide
>> information how C determines if the keying material is still valid.
>> Also, C must check if the keying material it has for a communication
>> partner is still valid before sending a request.
>>
> 
> I would think that in most cases it is acceptable that the client just
> optimistically sends its request with the existing keying material and
> renews it if it gets back a 4.01.

You mean if the client communicates with a compromised RS, it just
continues to do so? I don't think that this is such a good idea. I think
the AS must check that the the keying material is valid at least as long
as the token and must then set the expires_in field to the remaining
lifetime of the token. Additionally, the AS should provide the time when
the token was issued in an iat field. A client that wants to communicate
with an RS must check if the keying material it has for the RS is still
valid.

> 
> Use cases where the security policy is such that this is unacceptable
> should use a specific profile that defines additional mechanisms for the
> client to check its tokens and associated keying material (e.g. by using
> client introspection).
> 
>> The framework should also mention that C must determine if AS is
>> authorized by its owner to provide keying material concerning RS to C.
> 
> If C is configured to communicate with RS, this implies that the owner
> of C accepts the AS in charge of RS. Otherwise the owner of C shouldn't
> have configured C to communicate with RS in the first place.

And how does C know that a certain AS is in charge of RS?

> 
>> It is not clear how C makes plain to AS with which RS it wants to
>> communicate. C may specify the intended audience in the token request
>> using the req_aud parameter. How is RS represented in this case?
>> C and AS must have a common understanding how RS is identified,
>> otherwise C may communicate with the wrong RS!
>> IP addresses may change and therefore are not suitable. If AS specifies
>> identifiers for RS, C must securely obtain this knowledge.
> 
> Identifying the RS (using the audience claim and the req_aud parameter)
> is application specific. Some applications may want to use public keys,
> others will want to use fixed identifiers that are manually assigned or
> otherwise generated.
> 
> The framework does not suggest IP addresses as a way to identify an RS.
> 
>> The framework must point out that AS must check if the keying material
>> that it provides to C in the access token response actually refers to
>> the RS that C wants to communicate with. The problem of mixups
>> concerning the RS must be mentioned in the security considerations.
>> Also, the framework should give advice how this problem may be addressed.
>>
> 
> I will add a paragraph describing the problem and giving some basic
> suggestions (such as public keys as identifiers).
> 
> 
>> Confidentiality of unauthorized requests:
>> The data that the client sends to the RS in an unauthorized resource
>> request may be confidential (e.g., because others must not know the
>> requested resources). The framework should mention in the security
>> considerations that the client must not send confidential data in the
>> unauthorized resource request.
> 
> I can add a sentence about this, although it seems very obvious.
> 
> /Ludwig
> 
> 

Viele Grüße
Steffi

-- 
Stefanie Gerdes			Tel: +49 421 218 63906
TZI Universität Bremen		E-Mail: gerdes@tzi.de
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