CVE-2026-126401
Received Received - Intake

Fleet SAML Response Denial of Service Vulnerability

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: VulnCheck

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.

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Meta Information

Published
2026-11-13
Last Modified
2026-11-13
Generated
2026-11-13
AI Q&A
2026-11-13
EPSS Evaluated
N/A
NVD
CVE-2026-126401
EUVD
EUVD-2026-125164

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
fleet fleet to 4.86.2 (exc)
fleetdm fleet to 4.86.2 (exc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-407 An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

Attack-Flow Graph

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Executive Summary

This is an unauthenticated denial of service (DoS) vulnerability in Fleet software versions before 4.86.2. It allows attackers to crash the Fleet server by sending a single malicious SAML response with deeply nested elements and new namespace prefixes to the endpoint POST /api/v1/fleet/sso/callback. The attack exploits inefficient XML canonicalization, consuming excessive server memory and causing a crash.

Detection Guidance

Monitor for unusual memory consumption or server crashes when processing SAML responses. Check logs for POST requests to /api/v1/fleet/sso/callback with large or deeply nested XML payloads. Use network traffic analysis tools to inspect SAML response structures for excessive nesting or namespace declarations.

Impact Analysis

This vulnerability can disrupt service availability by crashing the Fleet server, preventing all users from accessing the system. Since no authentication is required, any attacker can exploit it remotely. The attack consumes server resources, leading to downtime and potential loss of functionality for all users relying on the Fleet server.

Mitigation Strategies

Upgrade Fleet to version 4.86.2 or later to patch the vulnerability. If immediate patching is not possible, block the vulnerable endpoint POST /api/v1/fleet/sso/callback at a reverse proxy or load balancer if SSO is not actively used.

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