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MCP Registry's GitHub OIDC tokens are replayable across registry deployments due to shared audience

Low severity GitHub Reviewed Published May 4, 2026 in modelcontextprotocol/registry • Updated May 15, 2026

Package

gomod github.com/modelcontextprotocol/registry (Go)

Affected versions

< 1.7.6

Patched versions

1.7.6

Description

[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.

References

@rdimitrov rdimitrov published to modelcontextprotocol/registry May 4, 2026
Published to the GitHub Advisory Database May 8, 2026
Reviewed May 8, 2026
Published by the National Vulnerability Database May 14, 2026
Last updated May 15, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(13th percentile)

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.

CVE ID

CVE-2026-44428

GHSA ID

GHSA-95c3-6vvw-4mrq

Source code

Credits

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