Re: [scim] SCIM Events Pull Request #20 - Unsecured Tokens

["Saxe, Dean" <[e-mail]>]

Thanks for raising this, Phil.

Intercepting proxies are in wide use, as is TLS termination that occurs in front of the targeted destination.  In both of these scenarios, unencrypted SETs may be observed by the TLS terminator, the data may be logged, and thus available to unauthorized parties.

I see two specific issues here.  First, disclosure of information is a risk with any sort of intercepting proxy or TLS termination if the SET is unencrypted.  If the content of the SET is not sufficiently sensitive as to be a risk if disclosed, this is not a problem.  However, for some data such as PII this is a challenge that needs to be solved.  Second, modification of the SET in flight is a risk that is sufficiently solved by signing.

Perhaps the focus should be on the security considerations section and the handling of unencrypted SETs when terminating TLS proxies may be in use.  The risk can be called out, a solution of using JWE proposed, and the attendant risks with JWEs as you outline below described.  Implementers must then make a decision about the handling of PII in SETs.

I’m going to reach out to some folks deep in the SSF world to get their feedback, as well.  I’ll post any relevant responses to the list.

Thanks,
-dhs

--
Dean H. Saxe, CIDPRO<https://idpro.org/cidpro/> (he/him)
Senior Security Engineer, AWS Identity Security Team | Amazon Web Services (AWS)
E: [e-mail]<mailto:[e-mail]> | M: 206-659-7293<tel:[phone]>

From: scim <[e-mail]> on behalf of Phillip Hunt <[e-mail]>
Date: Sunday, December 10, 2023 at 11:13 AM
To: SCIM WG <[e-mail]>
Subject: [EXTERNAL] [scim] SCIM Events Pull Request #20 - Unsecured Tokens


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I am just bringing to the group an issue that Dean raised in PR #20 (https://github.com/ietf-scim-wg/draft-ietf-scim-events/pull/20) on the SCIM Events Github site.

This change occures in Section 3.1 which provides advice on use of encryption and signing (original below).  Any new proposals from the group that address both Dean and my concerns would be greatly appreciated.  Please reply to this email thread so we can record the decision.

Dean’s request is to change the paragraph under “Unsecured" to:

Unsecured tokens SHALL NOT be used.
The justification in the PR was:

Updated language to prevent the use of unencrypted, unsigned tokens in this profile. The concept of "inside" and "outside" networks is no longer sufficient to consider whether transmitting over a TLS-only protocol is sufficient for securing such information.

Supporting such tokens is likely to lead to implementations that leak sensitive information and / or man in the middle interception/modification of SETs.

Based on this assumption (that TLS is unsuffient) we would have to also disallow the use of “signed” tokens as JWS does not provide confidentiality. Note that in many cases because SETs may be processed through gateways (e.g. aggregating events from a cluster), using JWE may become challenging as the iss and aud values would not be accessible.  Using JWE effectively forces point-to-point use only which again may cause the receiver to have to forward the unencrypted token on to its own internal endpoints.

It’s my belief that TLS is sufficient in most cases. MITM is not a threat except if administrative systems are compromised or faulty. To the contrary, RFC8935/8936 talks about exchanging events through fixed endpoints. I believe editorially it would be out of scope for this spec to imagine the number of subsequent poorly security or designed systems that might lead to a mis-configuration.

AFAIK, most OpenID Security Signals Framework implementations currently do not support signed or encrypted SETs (though i2gosignals does). These unencrypted systems are running in production now.

While I do generally agree using unencrypted tokens is a bad practice, saying SHALL NOT will likely be ignored. Editorially my goal is to give guidance that in the absence of SET signatures, what the alternative requirements should be.

I am proposing the following revised text for the “Unsecured Token” section:

Per Section 6 <xref target="RFC7519"/>, tokens MAY be generated with <tt>{"alg":"none"}</tt>.

This mode potentially speeds up processing by avoiding what would be redundant cryptographic
processing. An example would be replication scenarios within a cluster (e.g. Kubernetes).
Unencrypted tokens MUST be transferred over mutually-authenticated TLS layer encryption and
SHOULD only be used in restricted network environments.

Cheers,

Phil
[e-mail]

Original text:

3.1.  Security Event Token Signing and Encryption

   This specification uses Security Event Tokens as the message format
   for SCIM Events.  As SETs are based on JWT tokens [RFC7519], they can
   be transmitted insecurely, signed, or encrypted.  For more
   information see the JWT Cookbook specification [RFC7520] for
   examples.  The decision on whether to use JWS and JWE depends on
   operational considerations.  For each SCIM Feed relationship, it is
   up to deployers to decide on signing, encryption and algorithm
   requirements.  Deployers SHOULD be aware that too much emphasis on
   turning on every possible encryption feature may cause operational
   performance to suffer.  Deployers MUST weigh the security trade-offs
   of up-to-date SCIM services, vs. the potential information loss of an
   event.

   Unsecured
      Per Section 6 [RFC7519], tokens MAY be generated with
      {"alg":"none"}.  This mode speeds up processing and is best used
      in DBR scenarios.  Unencrypted tokens MUST be transferred over
      authenticated TLS layer encryption and SHOULD only be used in a
      restricted network environment.

   Signed
      JWS ([RFC7515]) signed SETs are useful when it is important to be
      able to verify the issuer of a SET as valid.  In addition, some



Hunt, et al.               Expires 24 May 2024                 [Page 27]
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Internet-Draft           draft-ietf-scim-events            November 2023


      systems MAY wish to validate the authenticity of the event in a
      review process which may occur at a later date.  While the content
      can be validated as originating from the correct issuer and is
      unmodified, the message contents remain unsecure.  Signed SETs
      MUST be transferred over encrypted transport.

   Encrypted
      JWE ([RFC7516]) are encrypted SETs and are useful when the
      transport mechanism is not fully secured (e.g. messages carried by
      a third party).  The use of JWEs ensures only the designated
      receiver can read the event and provides mutual authentication
      within the SET message itself.