CVE-2025-62800 json
FastMCP vulnerable to reflected XSS in client's callback page
published last yr. modified last mo. also EUVD-2025-36568 , GHSA-mxxr-jv3v-6pgc , PYSEC-2026-1364
- cvss
- 6.1
- version 3.1, NVD
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.3%
- EPSS 0.3% chance of exploitation in 30 days, percentile 20%
- public exploits
- 0
- none known
Summary
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.
Patch / fix
- pypi fastmcp 2.13.0
- jlowin fastmcp 2.13.0
- github.com/jlowin/fastmcp@2a20f54 commit
- github.com/jlowin/fastmcp#2090 pull request
Is CVE-2025-62800 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.
Check with FencerCISA SSVC
- Exploitation
- Proof of concept is public
- Automatable
- no
- Technical impact
- partial
- Assessed
- last yr.
Exploit prediction
0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-09).
Exploits and checks
- github.com https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc referenced PoC, tagged "exploit" by a source, unverified
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 4.0 | 5.3 | 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 |
| CNA | 4.0 | 5.3 | 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 |
| NVD | 3.1 | 6.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | fastmcp | <2.13.0 |
2.13.0 |
Affected products
- jlowin fastmcp
Classification
- Weakness
-
- CWE-79 Improper Neutralization of Input During Web Page Generation
References
-
github.com
/jlowin/fastmcp/pull/2090
web
-
github.com
/jlowin/fastmcp/commit/2a20f54617a37213ed83894a8c2f0ac38a2e83a3
web
-
github.com
/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc
exploit, web, third-party-advisory
-
nvd.nist.gov
/vuln/detail/CVE-2025-62800
advisory
-
github.com
/jlowin/fastmcp
package
-
pypi.org
/project/fastmcp
package
-
github.com
/advisories/GHSA-mxxr-jv3v-6pgc
advisory
Sources
- CVE List CVE-2025-62800
- GitHub Advisory GHSA-mxxr-jv3v-6pgc
- NVD CVE-2025-62800
- OSV PYSEC-2026-1364
Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.
CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.