Session tokens
When a user is authenticated in your application, Clerk generates a short-lived session token that you can use to authenticate requests to your backend. This token is a JSON Web Token (JWT) that contains information about the user and their session.
Read more about Clerk session tokens and how they work in the dedicated blog post.
Default claims
Every generated token has default claims that cannot be overridden by templates. Clerk's default claims include:
| Claim | Abbreviation expanded | Description | Example |
|---|---|---|---|
azp | authorized party | The Origin header that was included in the original Frontend API request made from the user. Most commonly, it will be the URL of the application. This claim could be omitted if, for privacy-related reasons, Origin is empty or null. | https://example.com |
exp | expiration time | The time after which the token will expire, as a Unix timestamp. Determined using the Token lifetime JWT template setting in the Clerk Dashboard. See RFC 7519 for more information. | 1713158400 |
fva | factor verification age | Each item represents the minutes that have passed since the last time a first or second factor, respectively, was verified. | [7, -1] which means it has been 7 minutes since the first factor was verified, and there either is not a second factor or the second factor has never been verified. |
iat | issued at | The time at which the token was issued as a Unix timestamp. See RFC 7519 for more information. | 1713158400 |
iss | issuer | The Frontend API URL of your instance. See RFC 7519 for more information. | https://clerk.your-site.com for a production instance, https://your-site.clerk.accounts.dev for a development instance |
nbf | not before | The time before which the token is considered invalid, as a Unix timestamp. Determined using the Allowed Clock Skew JWT template setting in the Clerk Dashboard. See RFC 7519 for more information. | 1713158400 |
sid | session ID | The ID of the current session. | sess_123 |
sub | subject | The ID of the current user of the session. See RFC 7519 for more information. | user_123 |
The act (actor) claim is only included if the user is impersonating another user. It's value is an object that contains the following properties:
| Claim | Abbreviation expanded | Description | Example |
|---|---|---|---|
iss | issuer | The referrer of the token. | https://dashboard.clerk.com |
sid | session ID | The session ID of the impersonated session. | sess_456 |
sub | subject | The ID of the impersonator. | user_456 |
The following claims are only included if the user is part of an organization:
| Claim | Abbreviation expanded | Description | Example |
|---|---|---|---|
org_id | organization ID | The ID of the active organization that the user belongs to. | org_123 |
org_permissions | organization permissions | The permissions of the user in the currently active organization. | ["org:sys_profile:manage", "org:sys_profile:delete"] |
org_slug | organization slug | The slug of the currently active organization that the user belongs to. | org-slug |
org_role | organization role | The role of the user in the currently active organization. | org:admin |
If you would like to add custom claims to your session token, you can customize it.
You can also create custom tokens using a JWT template.
Size limitations
The Clerk session token is stored in a cookie. All modern browsers limit the maximum size of a cookie to 4kb. Exceeding this limit can have adverse effects, including a possible infinite redirect loop for users who exceed this size in Next.js applications.
A session token with the default session claims won't run into this issue, as this configuration produces a cookie significantly smaller than 4kb. However, this limitation becomes relevant when implementing a custom session token. In this case, it's recommended to move particularly large claims out of the token and fetch these using a separate API call from your backend.
Claims to monitor for size limits:
user.organizationsuser.public_metadatauser.unsafe_metadataorg.public_metadataorg_membership.public_metadata
If you include any of these claims in your token, use caution to ensure the stored data doesn't exceed the size limit.
Validate session tokens
If you're using the middleware provided by our Clerk SDKs, validating session tokens is handled automatically in every request. If you're not using the middleware, you can still use the respective helpers provided by the SDKs to validate the tokens.
To learn how to manually verify a session token, refer to the dedicated guide.
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