CVE-2025-62800

FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.

CVSS v3 6.1 MEDIUM

6.1^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 2.8 / Impact : 2.7

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction REQUIRED

Confidentiality Impact LOW

Integrity Impact LOW

Availability Impact NONE

Scope CHANGED

References

Link	Resource
https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc	Exploit Third Party Advisory
https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc	Exploit Third Party Advisory

Configurations

Configuration 1 (hide)

cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*

History

08 Oct 2026, 11:10

Type	Values Removed	Values Added
Summary		(es) FastMCP es el framework estándar para construir aplicaciones MCP. Las versiones anteriores a la 2.13.0 tienen una vulnerabilidad de cross-site scripting reflejado en la página de callback del cliente OAuth (oauth_callback.py) donde valores controlados por el usuario sin escapar son insertados en el HTML generado, permitiendo la ejecución arbitraria de JavaScript en el origen del servidor de callback. El problema está solucionado en la versión 2.13.0.

Information

Published : 2025-10-28 22:15

Updated : 2026-10-08 11:10

NVD link : CVE-2025-62800

Mitre link : CVE-2025-62800

CVE.ORG link : CVE-2025-62800

JSON object : View

Products Affected

jlowin

fastmcp

CWE

CWE-79

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

{"id": "CVE-2025-62800", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2025-62800", "role": "CISA Coordinator", "options": [{"exploitation": "poc"}, {"automatable": "no"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2025-10-29T15:52:40.720132Z"}}], "cvssMetricV31": [{"type": "Primary", "source": "nvd@nist.gov", "cvssData": {"scope": "CHANGED", "version": "3.1", "baseScore": 6.1, "attackVector": "NETWORK", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N", "integrityImpact": "LOW", "userInteraction": "REQUIRED", "attackComplexity": "LOW", "availabilityImpact": "NONE", "privilegesRequired": "NONE", "confidentialityImpact": "LOW"}, "impactScore": 2.7, "exploitabilityScore": 2.8}], "cvssMetricV40": [{"type": "Secondary", "source": "security-advisories@github.com", "cvssData": {"Safety": "NOT_DEFINED", "version": "4.0", "Recovery": "NOT_DEFINED", "baseScore": 5.3, "Automatable": "NOT_DEFINED", "attackVector": "NETWORK", "baseSeverity": "MEDIUM", "valueDensity": "NOT_DEFINED", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/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", "exploitMaturity": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED", "userInteraction": "PASSIVE", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "subIntegrityImpact": "LOW", "vulnIntegrityImpact": "NONE", "integrityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "vulnAvailabilityImpact": "NONE", "availabilityRequirement": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "subConfidentialityImpact": "LOW", "vulnConfidentialityImpact": "NONE", "confidentialityRequirement": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED"}}]}, "affected": [{"source": "security-advisories@github.com", "affectedData": [{"vendor": "jlowin", "product": "fastmcp", "versions": [{"status": "affected", "version": "< 2.13.0"}]}]}], "published": "2025-10-28T22:15:36.983", "references": [{"url": "https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc", "tags": ["Exploit", "Third Party Advisory"], "source": "security-advisories@github.com"}, {"url": "https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc", "tags": ["Exploit", "Third Party Advisory"], "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "security-advisories@github.com", "description": [{"lang": "en", "value": "CWE-79"}]}], "descriptions": [{"lang": "en", "value": "FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0."}, {"lang": "es", "value": "FastMCP es el framework est\u00e1ndar para construir aplicaciones MCP. Las versiones anteriores a la 2.13.0 tienen una vulnerabilidad de cross-site scripting reflejado en la p\u00e1gina de callback del cliente OAuth (oauth_callback.py) donde valores controlados por el usuario sin escapar son insertados en el HTML generado, permitiendo la ejecuci\u00f3n arbitraria de JavaScript en el origen del servidor de callback. El problema est\u00e1 solucionado en la versi\u00f3n 2.13.0."}], "lastModified": "2026-10-08T11:10:00.250", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "938DF638-7486-4DDB-A950-1A16D8FCCEA1", "versionEndExcluding": "2.13.0"}], "operator": "OR"}]}], "sourceIdentifier": "security-advisories@github.com"}