What happened
CWE-639 authorization-bypass-through-user-controlled-key in Microsoft MCP Gateway. The gateway trusts a key value the caller controls when making an authorization decision, so a low-privileged authenticated user can be treated as a higher-privileged principal. CVSS 8.8 (AV:N/AC:L/PR:L).
Why it matters
This is MCP infrastructure that brokers access to tools and resources on behalf of AI agents. An authz bypass on the gateway lets an attacker invoke MCP tools, read backend resources, or reach resources they were not granted, with network reach.
Attack vector
An authorized attacker supplies a user-controlled key value to an authorization decision, bypassing the intended authorization check and elevating privileges over the network.
Affected systems
Microsoft MCP Gateway (mcp-gateway) 1.0.0.0 (before 1.0.0)
Mitigation
Apply the Microsoft update per MSRC CVE-2026-109492 (msrc.microsoft.com/update-guide/vulnerability/CVE-2026-109492).