How severe is CVE-2020-26276?

CVE-2020-26276 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2020-26276?

CVE-2020-26276 affects the following packages: github.com/fleetdm/fleet/v4 (Go). Ecosystems affected: Go.

How do I fix CVE-2020-26276?

Update github.com/fleetdm/fleet/v4 to 3.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26276 is resolved across your whole dependency graph.

How do I detect CVE-2020-26276 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/fleetdm/fleet/v4, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2020-26276 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2020-26276 actively exploited in the wild?

No public exploit code has been indexed for CVE-2020-26276 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2020-26276?

CVE-2020-26276 has an EPSS (Exploit Prediction Scoring System) score of 2.2%, placing it in the 82th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2020-26276?

CVE-2020-26276 is classified as CWE-290 (CWE-290). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2020-26276 published, and has it been updated?

CVE-2020-26276 was published on December 17, 2020 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

CRITICAL severity

CVE-2020-26276 — fleet

CRITICALFix: fleetdm/fleet@57812a5

CVE-2020-26276 is a critical-severity (CVSS 9.8) CWE-290 vulnerability in github.com/fleetdm/fleet/v4. A fix is available for github.com/fleetdm/fleet/v4 — see the affected versions and patch details below.

SAML authentication vulnerability due to stdlib XML parsing

Also known asGHSA-w3wf-cfx3-6gcx

Published

Dec 17, 2020

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs82th percentile — riskier than 82% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-26276 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐹github.com/fleetdm/fleet/v4

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/fleetdm/fleet/v4`	all versions	3.5.1`go get github.com/fleetdm/fleet/v4@v3.5.1`

Affected Products

1 product · 1 configurations

Application

fleetfleetdm

< 3.5.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/fleetdm/fleet/v4, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/fleetdm/fleet/v4 to 3.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26276 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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 i