CVE
Fleet unauthenticated denial of service in SSO callback
Affects Fleet servers before 4.86.2 where the SSO callback endpoint is reachable over the network. Fixed in 4.86.2.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 4.86.2
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 4.86.2.
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.51% · 44th percentile
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 26/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Who should check
Teams running Fleet (fleetdm) servers before 4.86.2 that expose the SSO callback endpoint over the network.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.7/10CVSS 8.7
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
- Would score about 31 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 4.86.2.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-126401 (Fleet before 4.86.2 Unauthenticated DoS via SSO Callback XML Canonicalization), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-126401 (entry last updated 2026-11-13):
Affected:
- fleetdm fleet: fixed in 4.86.2
Fixed versions: 4.86.2.
Conditions (all must be true for the bug to be hit):
- Fleet versions before 4.86.2 (affected versions < 4.86.2). (source: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p)
- The SSO callback endpoint must be reachable, so SSO callback processing of SAML responses is exercised. (source: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126401 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126401 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p
- https://www.vulncheck.com/advisories/fleet-before-4.86.2-unauthenticated-dos-via-sso-callback-xml-canonicalization
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-126401
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126401"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- fleetdm fleet
fixed in 4.86.2
Evidence 4
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126401
Is CVE-2026-126401 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A single unauthenticated request to POST /api/v1/fleet/sso/callback can exhaust the server's memory and crash the Fleet server. This causes a denial of service for the whole instance, with no data exposure or privilege escalation.Source: github.com
Which versions are affected by CVE-2026-126401?
fleet: before 4.86.2.Source: cve.now feed data
Am I affected, and how do I check?
Fleet versions before 4.86.2 are affected. The issue involves SSO callback processing of SAML responses, so check whether the Fleet server version is below 4.86.2 and whether the SSO callback endpoint is reachable from the network.Source: github.com
How do I fix CVE-2026-126401?
Upgrade Fleet to version 4.86.2 or later.Source: github.com