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Update refresh.md #546

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Update refresh.md #546

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1f3a5b2
OneTime tokens don't offer benefits
Nov 13, 2024
292c763
Rephrase
Nov 13, 2024
e1a65f7
Rename and rewrite "Refresh token security considerations" section
Nov 14, 2024
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19 changes: 7 additions & 12 deletions IdentityServer/v7/docs/content/tokens/refresh.md
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Expand Up @@ -47,12 +47,10 @@ var response = await client.RequestRefreshTokenAsync(new RefreshTokenRequest

The [Duende.AccessTokenManagement](https://github.com/DuendeSoftware/Duende.AccessTokenManagement/wiki) library can be used to automate refresh & access token lifetime management in ASP.NET Core.

## Refresh token security considerations
## Binding refresh tokens
Refresh tokens are a high-value target for attackers, because they typically have a much higher lifetime than access tokens.

It is recommended that a refresh token is either bound to the client via a client secret (for confidential/credentialed clients), or rotated for public clients.

The following techniques can be used to reduce the attack surface of refresh tokens.
It is recommended that a refresh token is either bound to the client via a client secret or using [Proof of Possession]({{< ref "/pop" >}}).

### Consent
We encourage you to request consent when a client requests a refresh token, as it not only makes the user aware of the action being taken, but also provides them with an opportunity to opt-out if they choose.
Expand All @@ -65,18 +63,15 @@ Refresh tokens usually have a much longer lifetime than access tokens. You can r
You can use the *AbsoluteRefreshTokenLifetime* and *SlidingRefreshTokenLifetime* client settings to fine tune this behavior.

### Rotation
The security of refresh tokens used by *public clients* can be improved by rotating the tokens on every use, because there is a chance that a stolen token will be unusable by the attacker. If a token is exfiltrated from some storage mechanism, a network trace, or log file, but the owner of the token uses it before the attacker, then the attack fails. However, this comes at the cost of reliability and database pressure and is only necessary for public clients (see [below](#avoid-rotation)). Rotation is configured with the *RefreshTokenUsage* client setting and, beginning in IdentityServer v7.0, is disabled by default.

When *RefreshTokenUsage* is configured for *OneTime* usage, rotation is enabled and refresh tokens can only be used once. When refresh tokens are used with *OneTime* usage configured, a new refresh token is included in the response along with the new access token. Each time the client application uses the refresh token, it must use the most recent refresh token. This chain of tokens will each appear as distinct token values to the client, but will have identical creation and expiration timestamps in the datastore.
Rotating the tokens on every use [has no security benefits](https://blog.duendesoftware.com/posts/20240405_refresh_token_reuse/) regardless of which type of client is used. And it comes with the cost of database pressure and reliability issues: Reusable refresh tokens are robust to network failures in a way that one time use tokens are not. If a one-time use refresh token is used to produce a new token, but the response containing the new refresh token is lost due to a network issue, the client application has no way to recover without the user logging in again. Reusable refresh tokens do not have this problem.

Beginning in version 6.3, the configuration option *DeleteOneTimeOnlyRefreshTokensOnUse* controls what happens to refresh tokens configured for OneTime usage. If the flag is on, then refresh tokens are deleted immediately on use. If the flag is off, the token is marked as consumed instead. Prior to version 6.3, OneTime usage refresh tokens are always marked as consumed.
Reusable tokens may have better performance in the [persisted grants store]({{< ref "/reference/stores/persisted_grant_store">}}). One-time use refresh tokens require additional records to be written to the store whenever a token is refreshed. Using reusable refresh tokens avoids those writes.

#### Confidential Clients Should Not Rotate Refresh Tokens {#avoid-rotation}
Confidential clients do not need one-time use refresh tokens because their tokens are bound to the authenticated client. One-time use tokens do not improve the security of confidential clients (see [OAuth 2.0 Security Best Current Practice](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#section-2.2.2) for more details).
Rotation is configured with the *RefreshTokenUsage* client setting and, beginning in IdentityServer v7.0, is disabled by default because of the reasons above.

Reusable refresh tokens are robust to network failures in a way that one time use tokens are not. If a one-time use refresh token is used to produce a new token, but the response containing the new refresh token is lost due to a network issue, the client application has no way to recover without the user logging in again. Reusable refresh tokens do not have this problem.
When *RefreshTokenUsage* is configured for *OneTime* usage, rotation is enabled and refresh tokens can only be used once. When refresh tokens are used with *OneTime* usage configured, a new refresh token is included in the response along with the new access token. Each time the client application uses the refresh token, it must use the most recent refresh token. This chain of tokens will each appear as distinct token values to the client, but will have identical creation and expiration timestamps in the datastore.

Reusable tokens may have better performance in the [persisted grants store]({{< ref "/reference/stores/persisted_grant_store">}}). One-time use refresh tokens require additional records to be written to the store whenever a token is refreshed. Using reusable refresh tokens avoids those writes.
Beginning in version 6.3, the configuration option *DeleteOneTimeOnlyRefreshTokensOnUse* controls what happens to refresh tokens configured for OneTime usage. If the flag is on, then refresh tokens are deleted immediately on use. If the flag is off, the token is marked as consumed instead. Prior to version 6.3, OneTime usage refresh tokens are always marked as consumed.

#### Accepting Consumed Tokens
To make one time use tokens more robust to network failures, you can customize the behavior of the *RefreshTokenService* such that consumed tokens can be used under certain circumstances, perhaps for a small length of time after they are consumed. To do so, create a subclass of the *DefaultRefreshTokenService* and override its *AcceptConsumedTokenAsync(RefreshToken refreshToken)* method. This method takes a consumed refresh token and returns a boolean flag that indicates if that token should be accepted, that is, allowed to be used to obtain an access token. The default implementation in the *DefaultRefreshTokenService* rejects all consumed tokens, but your customized implementation could create a time window where consumed tokens can be used.
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