The vulnerability resides in the Streamable HTTP transport of obsidian‑tc, the server component that exposes Obsidian vaults. Until release 1.3.1, the transport does not validate the Host or Origin headers and lacks DNS‑rebinding protection, which is a form of improper authentication (CWE‑1385) and missing input validation (CWE‑346). The flaw allows an attacker to read, alter, or delete vault contents because the server accepts requests from any Origin when the local configuration sets auth.mode to none.

Systems running The‑40‑Thieves:obsidian‑tc with a Streamable HTTP transport older than v1.3.1, particularly those that enable the loopback configuration and allow auth.mode set to none, are affected.

The likely attack vector is a malicious web page served to a user of the vulnerable machine, which then issues cross‑origin requests to the localhost MCP endpoint. Inferred from the description, the attacker needs only a web browser on the victim device; no additional networking or privilege escalation steps are required. The CVSS score of 8.8 denotes high severity, and the absence of an EPSS value and KEV listing suggests that the exploitation likelihood is not well established but the potential impact is significant.