OAuth mix-up attacks and RFC 9207: The issuer check that never made it to token exchange

In recent discussions on OAuth vulnerabilities, the token exchange flaw in n8n was highlighted, where a valid token from one issuer could be misused under a different issuer, exemplifying a mix-up attack. Mix-up attacks occur when OAuth clients, interacting with multiple authorization servers, become confused about which server response they receive, potentially allowing attackers to exploit the confusion and gain unauthorized access. Although RFC 9207, published in 2022, aims to address this by introducing the "iss" parameter to verify the authorization server's identity, it remains under-deployed and often overlooked in current implementations. This issue is particularly relevant as multi-authorization-server clients, such as those using MCP architectures, become more common, necessitating rigorous checks on issuer identity to prevent security breaches. The flaw lies in the failure to verify the specific issuer of a token, rather than just any trusted signer, a gap exploited in cases like CVE-2026-59208, where tokens were misapplied across different account namespaces. Proper implementation of RFC 9207 and issuer-specific verification is essential for securing OAuth systems, especially as they scale to accommodate more complex agent and tenant interactions.

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