User Receives "Malformed id_token_hint" Error During Logout Sequence

This article explains that the following error occurs when an end user attempts to log out:

Error when a user logs out: 'Malformed id_token_hint'

• User logout sequence
• OpenID Connect (OIDC)
• JSON Web Token (JWT)
• End-session endpoint requests

The error indicates that the value supplied for the id_token_hint parameter is syntactically or structurally invalid according to the JSON Web Token (JWT) format, which is required for an ID Token in OpenID Connect (OIDC). The Identity Provider (IdP) cannot parse or validate the token provided.

• The id_token_hint parameter, used in OpenID Connect (OIDC) end-session endpoint requests, must contain the ID Token that the Identity Provider (IdP) originally issued to the user when the user first signed in.

The error typically occurs for one of the following reasons:

• Incorrect Value: The application passes the wrong type of token, such as an Access Token or Refresh Token, or a non-token string instead of the actual ID Token.
• Improper Encoding, or Format: The token has been corrupted, truncated, or incorrectly manipulated before being sent, which breaks the required JSON Web Token (JWT) structure (three base64url-encoded parts separated by dots: header.payload.signature).
• Missing, or Empty Value: If the hint is required but an empty or null value is sent, it might occasionally trigger a parsing error.
• Client-Side Storage Issue: The ID Token was stored incorrectly or retrieved improperly from client-side storage, such as Session Storage, Local Storage, or cookies.

Perform the following steps to verify how the ID Token is handled by the application and resolve the issue:

1. Verify the Stored Token
   • Check Token Type: Ensure the application stores the ID Token specifically, which is usually returned alongside the Access Token during the initial authorization flow, and does not accidentally store the Access Token or Refresh Token for the logout process.
   • Inspect Storage: Use browser developer tools to inspect the storage, such as Local Storage or Session Storage, where the token is kept. Verify that the stored value is a complete JSON Web Token (JWT) (e.g., eyJ...) and has not been truncated or corrupted.
2. Verify the Logout Request
   • Inspect Parameter: Use network debugging tools, such as browser DevTools Network tab, to inspect the outgoing request to the Identity Provider's (IdP's) end-session endpoint.
   • Confirm Parameter Name: Ensure the parameter being used is correctly named id_token_hint, which is case-sensitive.
   • Confirm Value: Verify that the value of the id_token_hint parameter is the full, valid, unexpired ID Token retrieved from step 1.
3. Handle Token Expiration
   • Ensure that the application accounts for the possibility that the ID Token used for the hint might have expired.
   • It is generally safe to use an expired ID Token for the id_token_hint.