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Vulnerability Details :

CVE-2026-27124

Summary
Assigner-GitHub_M
Assigner Org ID-a0819718-46f1-4df5-94e2-005712e83aaa
Published At-03 Apr, 2026 | 15:22
Updated At-03 Apr, 2026 | 16:44
Rejected At-
Credits

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

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.

Vendors
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Not available
Products
-
Metrics (CVSS)
VersionBase scoreBase severityVector
Weaknesses
Attack Patterns
Solution/Workaround
References
HyperlinkResource Type
EPSS History
Score
Latest Score
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N/A
No data available for selected date range
Percentile
Latest Percentile
-
N/A
No data available for selected date range
Stakeholder-Specific Vulnerability Categorization (SSVC)
▼Common Vulnerabilities and Exposures (CVE)
cve.org
Assigner:GitHub_M
Assigner Org ID:a0819718-46f1-4df5-94e2-005712e83aaa
Published At:03 Apr, 2026 | 15:22
Updated At:03 Apr, 2026 | 16:44
Rejected At:
▼CVE Numbering Authority (CNA)
FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities

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.

Affected Products
Vendor
jlowin
Product
fastmcp
Versions
Affected
  • < 3.2.0
Problem Types
TypeCWE IDDescription
CWECWE-441CWE-441: Unintended Proxy or Intermediary ('Confused Deputy')
Type: CWE
CWE ID: CWE-441
Description: CWE-441: Unintended Proxy or Intermediary ('Confused Deputy')
Metrics
VersionBase scoreBase severityVector
4.08.2HIGH
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
Version: 4.0
Base score: 8.2
Base severity: HIGH
Vector:
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
Metrics Other Info
Impacts
CAPEC IDDescription
Solutions
Configurations
Workarounds
Exploits
Credits
Timeline
EventDate
Replaced By
Rejected Reason
References
HyperlinkResource
https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
x_refsource_CONFIRM
Hyperlink: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
Resource:
x_refsource_CONFIRM
▼Authorized Data Publishers (ADP)
CISA ADP Vulnrichment
Affected Products
Metrics
VersionBase scoreBase severityVector
Metrics Other Info
Impacts
CAPEC IDDescription
Solutions
Configurations
Workarounds
Exploits
Credits
Timeline
EventDate
Replaced By
Rejected Reason
References
HyperlinkResource
https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
exploit
Hyperlink: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
Resource:
exploit
Information is not available yet
▼National Vulnerability Database (NVD)
nvd.nist.gov
Source:security-advisories@github.com
Published At:03 Apr, 2026 | 16:16
Updated At:22 Apr, 2026 | 14:37

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.

CISA Catalog
Date AddedDue DateVulnerability NameRequired Action
N/A
Date Added: N/A
Due Date: N/A
Vulnerability Name: N/A
Required Action: N/A
Metrics
TypeVersionBase scoreBase severityVector
Secondary4.08.2HIGH
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
Primary3.16.1MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Type: Secondary
Version: 4.0
Base score: 8.2
Base severity: HIGH
Vector:
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
Type: Primary
Version: 3.1
Base score: 6.1
Base severity: MEDIUM
Vector:
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CPE Matches
jlowin
jlowin
>>fastmcp>>Versions before 3.2.0(exclusive)
cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*
Weaknesses
CWE IDTypeSource
CWE-441Secondarysecurity-advisories@github.com
CWE ID: CWE-441
Type: Secondary
Source: security-advisories@github.com
Evaluator Description
Evaluator Impact

Evaluator Solution

Vendor Statements
References
HyperlinkSourceResource
https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733security-advisories@github.com
Exploit
Mitigation
Vendor Advisory
https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733134c704f-9b21-4f2e-91b3-4a467353bcc0
Exploit
Mitigation
Vendor Advisory
Hyperlink: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
Source: security-advisories@github.com
Resource:
Exploit
Mitigation
Vendor Advisory
Hyperlink: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-rww4-4w9c-7733
Source: 134c704f-9b21-4f2e-91b3-4a467353bcc0
Resource:
Exploit
Mitigation
Vendor Advisory

Change History

0
Information is not available yet

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1Records found
CVE-2025-62800
Matching Score-8
Assigner-GitHub, Inc.
ShareView Details
Matching Score-8
Assigner-GitHub, Inc.
CVSS Score-5.3||MEDIUM
EPSS-0.04% / 12.38%
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Published-28 Oct, 2025 | 21:34
Updated-07 Nov, 2025 | 01:49
Rejected-Not Available
Known To Be Used In Ransomware Campaigns?-Not Available
KEV Added-Not Available
KEV Action Due Date-Not Available
FastMCP vulnerable to reflected XSS in client's callback page

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.

Action-Not Available
Vendor-jlowinjlowin
Product-fastmcpfastmcp
CWE ID-CWE-79
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Details not found