CVE-2026-32871

# PoC Concept for FastMCP CVE-2026-32871 # This demonstrates how a malicious path parameter can trigger directory traversal import requests # Target scenario: An OpenAPI spec defines an endpoint like /api/v1/users/{user_id} # The FastMCP provider is configured to forward requests to http://internal-backend/api/v1/ malicious_user_id = "../../admin/settings" # Path traversal payload # The vulnerable _build_url() method constructs the URL like this: # base = "http://internal-backend/api/v1/" # template = "/users/{user_id}" # result = urljoin(base, template.replace("{user_id}", malicious_user_id)) # Expected: http://internal-backend/api/v1/users/../../admin/settings # Resolved by urljoin: http://internal-backend/admin/settings # This accesses an arbitrary endpoint with the backend's credentials. print(f"Sending request with malicious parameter: {malicious_user_id}") # In a real exploit, this would be sent via the MCP client interface.

{"cve": {"id": "CVE-2026-32871", "sourceIdentifier": "[email protected]", "published": "2026-04-02T15:16:38.740", "lastModified": "2026-04-10T15:58:07.330", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "FastMCP is a Pythonic way to build MCP servers and clients. Prior to version 3.2.0, the OpenAPIProvider in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications. The RequestDirector class is responsible for constructing HTTP requests to the backend service. A vulnerability exists in the _build_url() method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{user_id}), the system directly substitutes parameter values into the URL template string without URL-encoding. Subsequently, urllib.parse.urljoin() resolves the final URL. Since urljoin() interprets ../ sequences as directory traversal, an attacker controlling a path parameter can perform path traversal attacks to escape the intended API prefix and access arbitrary backend endpoints. This results in authenticated SSRF, as requests are sent with the authorization headers configured in the MCP provider. This issue has been patched in version 3.2.0."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X", "baseScore": 10.0, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "HIGH", "vulnIntegrityImpact": "HIGH", "vulnAvailabilityImpact": "HIGH", "subConfidentialityImpact": "HIGH", "subIntegrityImpact": "HIGH", "subAvailabilityImpact": "HIGH", "exploitMaturity": "NOT_DEFINED", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H", "baseScore": 10.0, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 6.0}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-918"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*", "versionEndExcluding": "3.2.0", "matchCriteriaId": "4247F36F-7D37-47DF-A475-E607F1BAD799"}]}]}], "references": [{"url": "https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/PrefectHQ/fastmcp/pull/3507", "source": "[email protected]", "tags": ["Issue Tracking", "Patch"]}, {"url": "https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0", "source": "[email protected]", "tags": ["Product", "Release Notes"]}, {"url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767", "source": "[email protected]", "tags": ["Exploit", "Mitigation", "Vendor Advisory"]}, {"url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "tags": ["Exploit", "Mitigation", "Vendor Advisory"]}]}}