H3-2026-1030¶
MCPJam Inspector Global Issuer Identity Impersonation Vulnerability
| Category | VULNERABILITY |
| Base Score | 9.6 |
Description¶
MCPJam, developer of the Inspector MCP debugging tool, ships a hosted mock identity provider that exposes a single global OAuth/OIDC issuer at /api/web/xaa. Remote attackers who supply any bearer token, including a guest or otherwise unprivileged token, can call the issuer's authenticate endpoint and receive a signed identity assertion (ID-JAG style id_token) for an arbitrary userId of their choosing. The root cause is that the mock IdP maintains one global issuer and one signing key shared across all tenants, with no verification that the caller belongs to the organization associated with the identity being asserted. Any authorization server or relying party that registers this global MCPJam issuer as trusted will accept the forged assertions as legitimate, without additional preconditions beyond network access to the endpoint.
Impact¶
Remote attackers who obtain a trust relationship with the global issuer can impersonate any user known to the mock IdP, enabling full account takeover against relying services. Downstream authorization servers that accept the forged assertions may grant unauthorized access to protected resources and APIs. Any organization relying on MCPJam's hosted mock IdP for cross-service identity trust is exposed until it adopts org-scoped issuers.