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<ol class="chapter"><li class="chapter-item expanded "><a href="../../intro.html"><strong aria-hidden="true">1.</strong> Introduction to Kanidm</a></li><li class="chapter-item expanded "><a href="../../installing_the_server.html"><strong aria-hidden="true">2.</strong> Installing the Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../choosing_a_domain_name.html"><strong aria-hidden="true">2.1.</strong> Choosing a Domain Name</a></li><li class="chapter-item expanded "><a href="../../prepare_the_server.html"><strong aria-hidden="true">2.2.</strong> Preparing for your Deployment</a></li><li class="chapter-item expanded "><a href="../../server_configuration.html"><strong aria-hidden="true">2.3.</strong> Server Configuration and Install</a></li><li class="chapter-item expanded "><a href="../../security_hardening.html"><strong aria-hidden="true">2.4.</strong> Platform Security Hardening</a></li><li class="chapter-item expanded "><a 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Developers</li><li class="chapter-item expanded "><a href="../../DEVELOPER_README.html"><strong aria-hidden="true">17.</strong> Developer Guide</a></li><li class="chapter-item expanded "><a href="../../developers/faq.html"><strong aria-hidden="true">18.</strong> FAQ</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">19.</strong> Design Documents</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../developers/designs/access_profiles_rework_2022.html"><strong aria-hidden="true">19.1.</strong> Access Profiles 2022</a></li><li class="chapter-item expanded "><a href="../../developers/designs/access_profiles_and_security.html"><strong aria-hidden="true">19.2.</strong> Access Profiles Original</a></li><li class="chapter-item expanded "><a href="../../developers/designs/rest_interface.html"><strong aria-hidden="true">19.3.</strong> REST Interface</a></li><li class="chapter-item expanded "><a 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<main>
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<h1 id="oauth2-refresh-tokens"><a class="header" href="#oauth2-refresh-tokens">Oauth2 Refresh Tokens</a></h1>
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<p>Due to how Kanidm authentication sessions were originally implemented they had short session times
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(1 hour) due to the lack of privilege separation in tokens. Now with privilege separation being
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implemented session lengths have been extended to 8 hours with possible increases in the future.</p>
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<p>However, this leaves us with an issue with oauth2 - oauth2 access tokens are considered valid until
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their expiry and we should not issue tokens with a validity of 8 hours or longer since that would
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allow rogue users to have a long window of usage of the token before they were forced to re-auth. It
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also means that in the case that an account must be forcefully terminated then the user would retain
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access to applications for up to 8 hours or more.</p>
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<p>To prevent this, we need oauth2 tokens to "check in" periodically to re-afirm their session
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validity.</p>
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<p>This is performed with access tokens and refresh tokens. The access token has a short lifespan
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(proposed 15 minutes) and must be refreshed with Kanidm which can check the true session validity
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and if the session has been revoked. This creates a short window for revocation to propagate to
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oauth2 applications since each oauth2 application must periodically check in to keep their access
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token alive.</p>
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<h2 id="risks"><a class="header" href="#risks">Risks</a></h2>
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<p>Refresh tokens are presented to the relying server where they receive an access token and an
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optional new refresh token. Because of this, it could be possible to present a refresh token
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multiple times to proliferate extra refresh and access tokens away from the system. Preventing this
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is important to limit where the tokens are used and monitor and revoke them effectively.</p>
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<p>In addition, old refresh tokens should not be able to be used once exchanged, they should be "at
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most once". If this is not enforced then old refresh tokens can be used to gain access to sessions
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even if the associated access token was expired by many hours and it's refresh token was already
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used.</p>
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<p>This is supported by
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<a href="https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#section-2.2.2">draft oauth security topics section 2.2.2</a>
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and
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<a href="https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#refresh_token_protection">draft oauth security topics refresh token protection</a></p>
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<p>Refresh tokens must only be used by the client application associated. Kanidm strictly enforces this
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already with our client authorisation checks. This is discussed in
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<a href="https://www.rfc-editor.org/rfc/rfc6749#section-10.4">rfc6749 section 10.4</a>.</p>
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<h2 id="design"><a class="header" href="#design">Design</a></h2>
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<pre><code> ┌─────────────────────────────────────────┐
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│Kanidm │
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│ │
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│ ┌─────────┐ ┌─────────┐ │
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│ │ │ │ │ │
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│ │ │ │ │ │
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│ │ Session │ 3. Update │ Session │ │
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│ │ NIB 1 │─────NIB───────▶│ NIB 2 │ │
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│ │ │ │ │ │
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│ │ │ │ │ │
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│ │ │ │ │ │
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│ └─────────┘ └─────────┘ │
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│ │ │ │
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└───┼───────────────────────────┼─────────┘
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┌────┘ ▲ ┌────┘
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│ │ │
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│ │ │
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1. Issued │ 4. Issued
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│ │ │
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│ │ │
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│ │ │
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▼ │ ▼
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┌───────┐ │ ┌───────┐
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│ │ │ │ │
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│Access │ │ │Access │
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│ + │ │ │ + │
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│Refresh│──2. Refresh──┘ │Refresh│
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│ IAT 1 │ │ IAT 2 │
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│ │ │ │
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└───────┘ └───────┘
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</code></pre>
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<p>In this design we associate a "not issued before" (NIB) timestamp to our sessions. For a refresh
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token to be valid for issuance, the refresh tokens IAT must be greater than or equal to the NIB.</p>
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<p>In this example were the refresh token with IAT 1 re-used after the second token was issued, then
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this condition would fail as the NIB has advanced to 2. Since IAT 1 is not greater or equal to NIB 2
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then the refresh token <em>must</em> have previously been used for access token exchange.</p>
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<p>In a replicated environment this system is also stable and correct even if a session update is
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missed.</p>
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<pre><code> 2.
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┌───────────────────────Replicate────────────────┐
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│ │
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│ │
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┌───────┼─────────────────────────────────┐ ┌──────┼──────────────────────────────────┐
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│Kanidm │ │ │Kanidm│ │
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│ │ │ │ ▼ │
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│ ┌─────────┐ ┌─────────┐ │ │ ┌─────────┐ ┌─────────┐ │
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│ │ │ │ │ │ │ │ │ │ │ │
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│ │ │ │ │ │ │ │ │ │ │ │
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│ │ Session │ 4. Update │ Session │ │ │ │ Session │ 7. Update │ Session │ │
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│ │ NIB 1 │─────NIB───────▶│ NIB 2 │ │ │ │ NIB 1 │ ─────NIB───────▶│ NIB 3 │ │
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│ │ │ │ │ │ │ │ │ │ │ │
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│ │ │ │ │ │ │ │ │ │ │ │
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│ │ │ │ │ │ │ │ │ │ │ │
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│ └─────────┘ └─────────┘ │ │ └─────────┘ └─────────┘ │
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│ │ │ │ │ ▲ │ │
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└───┼───────────────────────────┼─────────┘ └──────┼────────────────────────┼─────────┘
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┌────┘ ▲ ┌────┘ │ ┌────┘
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│ │ │ │ │
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│ │ │ │ │
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1. Issued │ 5. Issued │ 8. Issued
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│ │ │ │ │
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│ │ │ │ │
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│ │ │ │ │
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▼ │ ▼ │ ▼
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┌───────┐ │ ┌───────┐ │ ┌───────┐
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│ │ │ │ │ │ │ │
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│Access │ │ │Access │ │ │Access │
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│ + │ │ │ + │ │ │ + │
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│Refresh│──3. Refresh──┘ │Refresh│ │ │Refresh│
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│ IAT 1 │ │ IAT 2 │─────6. Refresh──────────┘ │ IAT 3 │
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│ │ │ │ │ │
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└───────┘ └───────┘ └───────┘
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</code></pre>
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<p>In this example, we can see that the replication of the session with NIB 1 happens to the second
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Kanidm server, but the replication of session with NIB 2 has not occurred yet. If the token that was
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later issued with IAT 2 was presented to the second server it would still be valid and able to
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refresh since IAT 2 is greater or equal to NIB 1. This would also prompt the session to advance to
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NIB 3 such that when replication begun again, the session with NIB 3 would take precedence over the
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former NIB 2 session.</p>
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<p>While this allows a short window where a former access token could be used on the second replica,
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this infrastructure being behind load balancers and outside of an attackers influence significantly
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hinders the ability to attack this for very little gain.</p>
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<h2 id="attack-detection"><a class="header" href="#attack-detection">Attack Detection</a></h2>
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<p><a href="https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#section-4.14.2">draft oauth security topics section 4.14.2</a>
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specifically calls out that when refresh token re-use is detected then all tokens of the session
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should be canceled to cause a new authorisation code flow to be initiated.</p>
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<h2 id="inactive-refresh-tokens"><a class="header" href="#inactive-refresh-tokens">Inactive Refresh Tokens</a></h2>
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<p>Similar
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<a href="https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#section-4.14.2">draft oauth security topics section 4.14.2</a>
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also discusses that inactive tokens should be invalidated after a period of time. From the view of
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the refresh token this is performed by an internal exp field in the encrypted refresh token.</p>
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<p>From the servers side we will require a "not after" parameter that is updated on token activity.
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This will also require inactive session cleanup in the server which can be extended into the session
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consistency plugin that already exists.</p>
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<p>Since the act of refreshing a token is implied activity then we do not require other signaling
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mechanisms.</p>
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<h1 id="questions"><a class="header" href="#questions">Questions</a></h1>
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<p>Currently with authorisation code grants and sessions we issue these where the sessions are recorded
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in an async manner. For consistency I believe the same should be true here but is there a concern
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with the refresh being issued but a slight delay before it's recorded? I think given the nature of
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our future replication we already have to consider the async/eventual nature of things, so this
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doesn't impact that further, and may just cause client latency in the update process.</p>
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<p>However, we also don't want a situation where our async/delayed action queues become too full or
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