CVE-2020-26276
SAML authentication vulnerability in Fleet
Fleet is an open source osquery manager. In Fleet before version 3.5.1, 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. This issue is patched in 3.5.1. The fix was made using https://github.com/mattermost/xml-roundtrip-validator If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- High
- Integrity
- High
- Availability
- High
Exploit likelihood
What each source reports
CVE List Score used
SAML authentication vulnerability in Fleet
Fleet is an open source osquery manager. In Fleet before version 3.5.1, 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. This issue is patched in 3.5.1. The fix was made using https://github.com/mattermost/xml-roundtrip-validator If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet.
- Record
CVE-2020-26276- CVSS 3.1
- 10.0 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H - Weakness
- CWE-290
- Updated
- 2026-11-02
EUVDEUVD-2022-1220
Fleet is an open source osquery manager. In Fleet before version 3.5.1, 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. This issue is patched in 3.5.1. The fix was made using https://github.com/mattermost/xml-roundtrip-validator If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet.
- Record
EUVD-2022-1220- CVSS 3.1
- 10.0 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-w3wf-cfx3-6gcx No score
SAML authentication vulnerability due to stdlib XML parsing
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 [email protected]
- Join #fleet in osquery Slack
- Record
GHSA-w3wf-cfx3-6gcx- Weakness
- CWE-290
- Updated
- 2026-11-02
NVD
Fleet is an open source osquery manager. In Fleet before version 3.5.1, 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. This issue is patched in 3.5.1. The fix was made using https://github.com/mattermost/xml-roundtrip-validator If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet.
- Record
CVE-2020-26276- CVSS 3.1
- 10.0 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H - CVSS 3.1
- 9.8 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - CVSS 2.0
- 6.8 Medium
AV:N/AC:M/Au:N/C:P/I:P/A:P - Weakness
- CWE-290
- Updated
- 2026-11-02
OSV
Fleet is an open source osquery manager. In Fleet before version 3.5.1, 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. This issue is patched in 3.5.1. The fix was made using https://github.com/mattermost/xml-roundtrip-validator If upgrade to 3.5.1 is not possible, users should disable SSO authentication in Fleet.
- Record
CVE-2020-26276- CVSS 3.1
- 9.8 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - Updated
- 2026-11-02
OSVGHSA-w3wf-cfx3-6gcx No score
SAML authentication vulnerability due to stdlib XML parsing
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 [email protected]
- Join #fleet in osquery Slack
- Record
GHSA-w3wf-cfx3-6gcx- Weakness
- CWE-290
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
github.com/fleetdm/fleet/v4Go |
before 3.5.1 | 3.5.1 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2020-26276 is a critical vulnerability (CVSS 10.0) in Fleetdm Fleet, published December 17, 2020. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 2.20%, the 82nd percentile. Fixed in github.com/fleetdm/fleet/v4 3.5.1.
References
github.com/fleetdm/fleet/security/advisories/GHSA-w3wf-cfx3-6gcx
mattermost.com/blog/coordinated-disclosure-go-xml-vulnerabilities
nvd.nist.gov/vuln/detail/CVE-2020-26276