SAML authentication vulnerability due to stdlib XML parsing

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

not scored

EPSS

• CVE-2020-26276: 2.20% estimated exploitation activity in the next 30 days; percentile 82.49 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Due to issues in Go's standard library XML parsing, a valid SAML response may be mutated by an attacker to modify the trusted document. This can result in allowing unverified logins from a SAML IdP. Users that configure Fleet with SSO login may be vulnerable to this issue. Patches This issue is patched in 3.5.1 using https://github.com/mattermost/xml-roundtrip-validator. Workarounds If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet. References See https://mattermost.com/blog/coordinated-disclosure-go-xml-vulnerabilities/ for more information about the underlying vulnerabilities. For more information If you have any questions or comments about this advisory: Email us at security@fleetdm.com Join #fleet in osquery Slack

Published by the source

11 Feb 2022, 23:59 UTC

Last modified by the source

1 Feb 2023, 05:05 UTC

Source last checked

26 Oct 2026, 14:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-w3wf-cfx3-6gcx

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# Patch Next advisory brief: CVE-2020-26276

Advisory: SAML authentication vulnerability due to stdlib XML parsing
IDs: CVE-2020-26276, GHSA-w3wf-cfx3-6gcx
Record: https://patchnext.com/advisories/cve-2020-26276 (revision 1, source checked 26 Oct 2026, 14:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/fleetdm/fleet/v4 3.5.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/fleetdm/fleet/v4: affected < 3.5.1; fixed in 3.5.1

## Suggested inspection (read-only)
- Go github.com/fleetdm/fleet/v4: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/fleetdm/fleet/v4.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w3wf-cfx3-6gcx (GitHub-reviewed advisory GHSA-w3wf-cfx3-6gcx lists affected ranges: Go github.com/fleetdm/fleet/v4 >=0 <3.5.1)

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