CVE-2026-59208: n8n: Cross-Issuer Token Exchange Account Binding via Subject-Only Identity Resolution

When an n8n instance is configured with more than one trusted token-exchange issuer, external identities are resolved to local accounts using only the JWT sub claim, ignoring the issuer (iss). As a result, two different issuers that emit the same subject value map to the same local account.

An attacker who can obtain a valid token from one trusted issuer with a sub matching a victim registered under a different issuer can authenticate as that victim and access their account.

This issue only affects instances where the token exchange feature is enabled and more than one trusted external issuer is configured.