MCP Registry's GitHub OIDC tokens are replayable across registry deployments due to shared audienceGHSA-95c3-6vvw-4mrq

[SECURITY] registry_001 Vulnerability Report

While analyzing the code logic, an area that may lead to unintended behavior under specific conditions was discovered.

Overview

• Verified Version: c5c4b9e8890dd5754bee889b2f1417f4fe3b5ce5
• Vulnerability Type: Authentication bypass via cross-registry OIDC token replay
• Affected Location: cmd/publisher/commands/login.go:67-105,130-135,199-224; cmd/publisher/auth/github-oidc.go:24-38,58-75,108-165; internal/api/handlers/v0/auth/github_oidc.go:75-135,229-277,280-296
• Trigger Scenario: a workflow invokes mcp-publisher login github-oidc --registry <other-registry> (or equivalent publish flow) and the publisher still requests a GitHub Actions ID token with the shared audience mcp-registry; any other registry deployment running this code can replay that token to its own /v0/auth/github-oidc endpoint and mint a publish-capable registry JWT for the same GitHub owner namespace.

Root Cause

The client-side and server-side GitHub OIDC flow is bound only to a global audience string, not to the specific registry instance being targeted. On the client side, the publisher always appends audience=mcp-registry when requesting the GitHub Actions ID token, regardless of the selected --registry URL. On the server side, the exchange endpoint validates only that same fixed audience and then derives publish permissions directly from repository_owner. As a result, a token legitimately obtained while interacting with one registry deployment remains acceptable to any other deployment that shares the same code and audience string.

Source-to-Sink Chain

1. Source cmd/publisher/commands/login.go:67-105,130-135,199-224 parses the user-controlled --registry flag into flags.RegistryURL, creates a GitHubOIDCProvider, and calls authProvider.GetToken(ctx) for the chosen authentication method.
2. Propagation cmd/publisher/auth/github-oidc.go:24-38 obtains an OIDC token and immediately exchanges it against the selected registry URL. cmd/publisher/auth/github-oidc.go:58-75 builds exchangeURL := o.registryURL + "/v0/auth/github-oidc" and posts the GitHub token to whichever registry instance was selected. cmd/publisher/auth/github-oidc.go:108-165 constructs fullURL := requestURL + "&audience=mcp-registry" and therefore requests the same audience for every registry deployment.
3. Sink internal/api/handlers/v0/auth/github_oidc.go:75-135 validates only the shared audience value passed into ValidateToken. internal/api/handlers/v0/auth/github_oidc.go:254-277 calls h.validator.ValidateToken(ctx, oidcToken, "mcp-registry") and, on success, signs a new registry JWT. internal/api/handlers/v0/auth/github_oidc.go:280-296 converts claims.RepositoryOwner into the publish permission pattern io.github.<owner>/*, which is then embedded into the new registry JWT.

Exploitation Preconditions

1. The victim uses the GitHub Actions OIDC publishing path.
2. The victim workflow targets another registry deployment first, such as staging, self-hosted infrastructure, or an attacker-controlled registry URL.
3. The receiving registry deployment can observe the posted OIDC token and replay it before expiry to another registry deployment running the same shared audience configuration.

Risk

This breaks deployment isolation between registry instances. A token issued for one registry interaction can be replayed across trust boundaries, allowing one deployment to impersonate the same GitHub owner identity on another deployment.

Impact

An attacker-controlled or compromised registry deployment can mint a valid registry JWT on another deployment and inherit publish permissions for the victim GitHub owner namespace. In practical terms, this enables unauthorized publication or update actions for names such as io.github.<owner>/* on the victim registry instance.

Remediation

1. Replace the shared audience string with a registry-specific audience, such as a deployment-specific client ID or origin-derived identifier.
2. Ensure the publisher requests the audience that matches the exact registry instance it is targeting, and ensure the server validates that same instance-specific value.
3. Consider binding the exchange to additional deployment-specific claims so that a token captured by one registry cannot be replayed on another.
4. Add regression tests that cover cross-deployment replay attempts between different registry URLs.