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Public proof of conceptFix available

CVE-2025-62800

A public proof of concept exists; no exploitation attested.

  • Published2025-10-28
  • Fixed in2.13.0
  • WeaknessCWE-79

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC
  1. 2025-10-2822:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2025-62800?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2025-62800?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Jlowin fastmcp
BranchAffectedFixed in
2.13< 2.13.02.13.0

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

  1. Update Jlowin fastmcp to 2.13.0

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionpoc2025-10-29
  • OpenCVESSVC decisionpoc—
  • CNACVSS 4.05.3read 2026-10-23
  • NVDCVSS 3.16.1read 2026-10-23
  • CNAweakness (CWE)CWE-79read 2026-10-23
  • FIRST EPSSEPSS score0.3 %2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CVSS 4.0 vector
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
Published
2025-10-28
Last Modified
2026-10-08
NVD status
Analyzed

Frequently asked questions

What is 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.

Is CVE-2025-62800 actively exploited?

A proof-of-concept exploit exists for CVE-2025-62800, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2025-62800?

CVE-2025-62800 has a CVSS v3 base score of 6.1 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

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