{
    "document": {
        "category": "csaf_base",
        "csaf_version": "2.0",
        "distribution": {
            "tlp": {
                "label": "WHITE"
            }
        },
        "lang": "en",
        "notes": [
            {
                "category": "legal_disclaimer",
                "text": "The Netherlands Cyber Security Center (henceforth: NCSC-NL) maintains this portal to enhance access to its information and vulnerabilities. The use of this information is subject to the following terms and conditions:\n\nThe vulnerabilities disclosed in this portal are gathered by NCSC-NL from a variety of open sources, which the user can retrieve from other platforms. NCSC-NL makes every reasonable effort to ensure that the content of this portal is kept up to date, and that it is accurate and complete. Nevertheless, NCSC-NL cannot entirely rule out the possibility of errors, and therefore cannot give any warranty in respect of its completeness, accuracy or real-time keeping up-to-date. NCSC-NL does not control nor guarantee the accuracy, relevance, timeliness or completeness of information obtained from these external sources. The vulnerabilities disclosed in this portal are intended solely for the convenience of professional parties to take appropriate measures to manage the risks posed to the cybersecurity. No rights can be derived from the information provided therein.\n\nNCSC-NL and the Kingdom of the Netherlands assume no legal liability or responsibility for any damage resulting from either the use or inability of use of the vulnerabilities disclosed in this portal. This includes damage resulting from the inaccuracy of incompleteness of the information contained in it.\nThe information on this page is subject to Dutch law. All disputes related to or arising from the use of this portal regarding the disclosure of vulnerabilities will be submitted to the competent court in The Hague. This choice of means also applies to the court in summary proceedings."
            }
        ],
        "publisher": {
            "category": "coordinator",
            "contact_details": "cert@ncsc.nl",
            "name": "National Cyber Security Centre",
            "namespace": "https://www.ncsc.nl/"
        },
        "title": "CVE-2026-27124",
        "tracking": {
            "current_release_date": "2026-04-03T17:40:01.006214Z",
            "generator": {
                "date": "2026-02-17T15:00:00Z",
                "engine": {
                    "name": "V.E.L.M.A",
                    "version": "1.7"
                }
            },
            "id": "CVE-2026-27124",
            "initial_release_date": "2026-03-31T23:07:45.165933Z",
            "revision_history": [
                {
                    "date": "2026-03-31T23:07:45.165933Z",
                    "number": "1",
                    "summary": "CVE created.| Source created.| CVE status created. (valid)| Description created for source.| References created (3).| CWES updated (1)."
                },
                {
                    "date": "2026-03-31T23:07:54.662172Z",
                    "number": "2",
                    "summary": "NCSC Score created."
                },
                {
                    "date": "2026-04-03T15:39:30.035078Z",
                    "number": "3",
                    "summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| Products created (1).| References created (1).| CWES updated (1)."
                },
                {
                    "date": "2026-04-03T15:39:32.474331Z",
                    "number": "4",
                    "summary": "NCSC Score updated."
                },
                {
                    "date": "2026-04-03T16:49:59.481440Z",
                    "number": "5",
                    "summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (1).| CWES updated (1)."
                },
                {
                    "date": "2026-04-03T16:50:01.530280Z",
                    "number": "6",
                    "summary": "NCSC Score updated."
                },
                {
                    "date": "2026-04-03T17:39:08.354062Z",
                    "number": "7",
                    "summary": "Unknown change."
                }
            ],
            "status": "interim",
            "version": "7"
        }
    },
    "product_tree": {
        "branches": [
            {
                "branches": [
                    {
                        "branches": [
                            {
                                "category": "product_version_range",
                                "name": "vers:unknown/<3.2.0",
                                "product": {
                                    "name": "vers:unknown/<3.2.0",
                                    "product_id": "CSAFPID-5992235"
                                }
                            }
                        ],
                        "category": "product_name",
                        "name": "fastmcp"
                    }
                ],
                "category": "vendor",
                "name": "jlowin"
            }
        ]
    },
    "vulnerabilities": [
        {
            "cve": "CVE-2026-27124",
            "cwe": {
                "id": "CWE-441",
                "name": "Unintended Proxy or Intermediary ('Confused Deputy')"
            },
            "notes": [
                {
                    "category": "description",
                    "text": "## Summary\nWhile 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.\n\n## Technical Details\nAn 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.\n\nAlthough this issue was verified in practice only for the *GitHubProvider*, a review of the source code, specifically the `OAuthProxy._handle_idp_callback` [function](https://github.com/jlowin/fastmcp/blob/ee5f465a82350e1c5a56c4a2b47cfdc4cd736e76/src/fastmcp/server/auth/oauth_proxy.py#L1762), 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.\n\nSkipping 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.\n\n## Steps to reproduce\n1. Set up an MCP server using the *GitHubProvider* integration.\n2. Connect a benign MCP client to this MCP server.\n3. Configure your default browser to route all traffic through an interception proxy such as Burp Suite.\n4. In a private browsing window or a second browser, log into the GitHub account used in step 2.\n5. As the attacker, connect a new (malicious) MCP client to the MCP server from step 1.\n6. When the browser opens for the attacker’s client, enable interception in your proxy.\n7. In the browser, confirm the consent prompt.\n8. In the proxy, forward all requests up to the authorization request to the GitHub authorization server.\n9. Copy the authorization URL and drop the intercepted request.\n10. Simulate luring the victim onto the URL by opening this URL in the browser window opened in step 4.\n11. Observe that the malicious client receives a valid authorization code and gains access to the benign MCP server using the victim’s GitHub account.\n\nIn 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.\n\n## Recommendation\n\nTo 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](https://mcp.mintlify.app/specification/2025-11-25/basic/security_best_practices#mitigation) for this vulnerability in the MCP specification.",
                    "title": "github - https://api.github.com/advisories/GHSA-rww4-4w9c-7733"
                },
                {
                    "category": "description",
                    "text": "FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, 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. This issue has been patched in version 3.2.0.",
                    "title": "cveprojectv5 - https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/27xxx/CVE-2026-27124.json"
                },
                {
                    "category": "description",
                    "text": "FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, 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. This issue has been patched in version 3.2.0.",
                    "title": "nvd - https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-27124"
                },
                {
                    "category": "other",
                    "text": "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",
                    "title": "CVSSV4"
                },
                {
                    "category": "other",
                    "text": "8.2",
                    "title": "CVSSV4 base score"
                },
                {
                    "category": "other",
                    "text": "3.8",
                    "title": "NCSC Score"
                },
                {
                    "category": "other",
                    "text": "The value of the most recent CVSS (V3) score, There is cwe data available from source Nvd",
                    "title": "NCSC Score top decreasing factors"
                }
            ],
            "product_status": {
                "known_affected": [
                    "CSAFPID-5992235"
                ]
            },
            "references": [
                {
                    "category": "external",
                    "summary": "Source - github",
                    "url": "https://api.github.com/advisories/GHSA-rww4-4w9c-7733"
                },
                {
                    "category": "external",
                    "summary": "Source - cveprojectv5",
                    "url": "https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/27xxx/CVE-2026-27124.json"
                },
                {
                    "category": "external",
                    "summary": "Source - nvd",
                    "url": "https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-27124"
                },
                {
                    "category": "external",
                    "summary": "Reference - github",
                    "url": "https://github.com/advisories/GHSA-rww4-4w9c-7733"
                },
                {
                    "category": "external",
                    "summary": "Reference - cveprojectv5; github; nvd",
                    "url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733"
                },
                {
                    "category": "external",
                    "summary": "Reference - github",
                    "url": "https://github.com/jlowin/fastmcp/security/advisories/GHSA-rww4-4w9c-7733"
                }
            ],
            "title": "CVE-2026-27124"
        }
    ]
}