mcp-memory-service fixes a critical OAuth bypass: open client registration handed out read-write tokens without the API key

The mcp-memory-service project published a critical advisory, GHSA-5p27-64mv-pr73, on 2026-08-23 and released the fix in v11.8.2 the same morning. Rated 9.1 on CVSS 3.1 with a CVE requested, the finding is an unauthenticated authorization bypass through the server's own OAuth surface: an attacker could register a public client via /oauth/register, then replay the returned credentials against /oauth/token with grant_type=client_credentials and receive a read-write bearer token, bypassing the owner API key entirely.

Exposure required a specific but self-reinforcing configuration. MCP_OAUTH_ENABLED defaults to false, so a default install was never exposed; but once OAuth is enabled, open Dynamic Client Registration is the default, because MCP_DCR_REGISTRATION_KEY starts unset. The release notes are specific about the mechanics, describing four defects that lined up: the registration endpoint issued a client_secret even to public clients that registered with token_endpoint_auth_method=none, the client_credentials grant handler checked neither the client's registered grant types (RFC 6749 section 4.4) nor its authentication method, and the client store hashed an absent secret as the SHA-256 of the empty string, which the notes describe as not exploitable on its own since the comparison rejects empty input first. The fix copies the pattern the authorization_code grant already used, and public clients no longer receive a client_secret in their registration response at all.

Versions v10.20.0 through v11.8.1 are affected. Operators who cannot upgrade are directed to set MCP_DCR_REGISTRATION_KEY or disable OAuth. The report came privately from Sergio Rodriguez Jove with a working proof of concept.

This is the same project whose unauthenticated document API, CVE-2026-50027 at CVSS 9.8, reached NVD on 2026-08-14, and the advisory lands during an eventful stretch for the project's hosting: its v11.8.1 release notes state that development moved to Codeberg at the end of May 2026 and that the GitHub repository now operates as a mirror.