CVE-2026-126401 json
Fleet before 4.86.2 Unauthenticated DoS via SSO Callback XML Canonicalization
published 17d ago modified 2d ago also EUVD-2026-125164 , GHSA-mjrh-3v6h-8g5p
Unreviewed: the maintainers' own advisory
Published by the maintainers on their GitHub repository fleetdm/fleet. GitHub has not reviewed it into the GitHub Advisory Database, so the affected packages and versions are the maintainers' word. Read the full advisory on GitHub.
- cvss
- 7.5
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.4%
- EPSS 0.4% chance of exploitation in 30 days, percentile 30%
- public exploits
- 0
- none known
Summary
Fleet before 4.86.2 contains a denial of service vulnerability that allows unauthenticated attackers to exhaust server memory through quadratic XML canonicalization of unsigned SAML responses. Attackers can send a single crafted SAML response with deeply nested elements declaring new namespace prefixes to POST /api/v1/fleet/sso/callback, crashing the Fleet server.
Patch / fix
- fleetdm fleet 4.86.2
Is CVE-2026-126401 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.
Check with FencerExploit prediction
0.36% chance of exploitation in the next 30 days, higher than 30% of all scored CVEs (FIRST EPSS, 2026-11-14).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 4.0 | 8.7 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N |
| CNA | 3.1 | 7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| CNA | 4.0 | 8.7 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/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 |
| NVD | 3.1 | 7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
Affected products
- fleetdm fleet
Classification
- Weakness
-
- CWE-407 Inefficient Algorithmic Complexity
References
-
github.com
/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p
vendor-advisory, advisory
-
vulncheck.com
/advisories/fleet-before-4.86.2-unauthenticated-dos-via-sso-callback-xml-canonicalization
third-party-advisory
Sources
- CVE List CVE-2026-126401
- NVD CVE-2026-126401
- Maintainer advisory GHSA-mjrh-3v6h-8g5p
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.
The maintainers of fleetdm/fleet published their own advisory on GitHub. Reviewed sources take precedence over it.