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FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities

High severity GitHub Reviewed Published Mar 31, 2026 in PrefectHQ/fastmcp • Updated Apr 6, 2026

Package

pip fastmcp (pip)

Affected versions

< 3.2.0

Patched versions

3.2.0

Description

Summary

While testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP OAuthProxy does not properly validate the user's consent upon receiving the authorization code from GitHub. In combination with GitHub’s behavior of skipping the consent page for previously authorized clients, this introduces a Confused Deputy vulnerability.

Technical Details

An adversary can initiate an authentication flow by connecting their malicious MCP client to a benign MCP server using the GitHubProvider OAuth integration. During this flow, the attacker consents to connect their client to the MCP server and, at that point, can capture the GitHub authorization URL they are redirected to after granting consent. The attacker can then lure a victim, who is already logged into GitHub and has previously connected an MCP client to the benign MCP server, to open this captured URL. As a result, the victim’s browser is immediately redirected to the OAuthProxy’s callback endpoint, which does not correctly enforce that this browser has just given consent. The OAuthProxy then redirects the victim’s browser to the malicious MCP client’s callback URL with a valid authorization code. The attacker can exchange this code for an access token to the benign MCP server associated with the victim’s GitHub account, potentially gaining unauthorized access to resources tied to that account.

Although this issue was verified in practice only for the GitHubProvider, a review of the source code, specifically the OAuthProxy._handle_idp_callback function, shows that the IdP callback handler does not verify whether the browser sending the state and code has previously consented to connecting the client to the server. As long as a valid state and code pair is provided, the OAuthProxy requests an access token from the IdP and then redirects the user-agent to the client’s callback URL with a new code and the corresponding state, allowing the client to retrieve the access token from the proxy. This pattern causes all OAuth integrations whose IdP allows skipping the consent page to be vulnerable to this attack.

Skipping the consent page is not, by itself, a vulnerability on the IdP side. Many providers legitimately skip consent for first-party or previously authorized clients with the same scopes. In this case, the core problem lies in the OAuthProxy callback handler not correctly verifying that the browser issuing the callback request is the same one that has just given the required consent.

Steps to reproduce

  1. Set up an MCP server using the GitHubProvider integration.
  2. Connect a benign MCP client to this MCP server.
  3. Configure your default browser to route all traffic through an interception proxy such as Burp Suite.
  4. In a private browsing window or a second browser, log into the GitHub account used in step 2.
  5. As the attacker, connect a new (malicious) MCP client to the MCP server from step 1.
  6. When the browser opens for the attacker’s client, enable interception in your proxy.
  7. In the browser, confirm the consent prompt.
  8. In the proxy, forward all requests up to the authorization request to the GitHub authorization server.
  9. Copy the authorization URL and drop the intercepted request.
  10. Simulate luring the victim onto the URL by opening this URL in the browser window opened in step 4.
  11. Observe that the malicious client receives a valid authorization code and gains access to the benign MCP server using the victim’s GitHub account.

In a more realistic scenario, the malicious client could be a public MCP client or a simple web server that logs the received authorization code or token, which the attacker then uses to obtain the access token and connect to the MCP server as the victim.

Recommendation

To mitigate this issue, the OAuthProxy should verify that the browser sending the authorization code has actually given consent for the corresponding client. This can be achieved by setting and validating a consent cookie or similar browser-bound state, as described in the mitigations section for this vulnerability in the MCP specification.

References

@jlowin jlowin published to PrefectHQ/fastmcp Mar 31, 2026
Published to the GitHub Advisory Database Mar 31, 2026
Reviewed Mar 31, 2026
Published by the National Vulnerability Database Apr 3, 2026
Last updated Apr 6, 2026

Severity

High

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 None
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
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:N/VC:N/VI:H/VA:N/SC:N/SI:N/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.
(15th percentile)

Weaknesses

Unintended Proxy or Intermediary ('Confused Deputy')

The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product's control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor. Learn more on MITRE.

CVE ID

CVE-2026-27124

GHSA ID

GHSA-rww4-4w9c-7733

Source code

Credits

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