CVE-2026-23518: Fleet has a JWT signature bypass vulnerability in Azure AD MDM enrollment

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.3%, percentile 16: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 16% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes github.com/fleetdm/fleet (Go) >= 4.78.0, < 4.78.3, fixed in 4.78.3; github.com/fleetdm/fleet (Go) >= 4.77.0, < 4.77.1, fixed in 4.77.1; github.com/fleetdm/fleet (Go) >= 4.76.0, < 4.76.2, fixed in 4.76.2; github.com/fleetdm/fleet (Go) >= 4.75.0, < 4.75.2, fixed in 4.75.2. Check your lockfile, including transitive dependencies.

What it is

Fleet is open source device management software. In versions prior to 4.78.3, 4.77.1, 4.76.2, 4.75.2, and 4.53.3, a vulnerability in Fleet's Windows MDM enrollment flow could allow an attacker to submit forged authentication tokens that are not properly validated. Because JWT signatures were not verified, Fleet could accept attacker-controlled identity claims, enabling enrollment of unauthorized devices under arbitrary Azure AD user identities. Versions 4.78.3, 4.77.1, 4.76.2, 4.75.2, and 4.53.3 fix the issue. If an immediate upgrade is not possible, affected Fleet users should temporarily disable Windows MDM.

From the GitHub advisory

Summary

A vulnerability in Fleet’s Windows MDM enrollment flow could allow an attacker to submit forged authentication tokens that are not properly validated. Because JWT signatures were not verified, Fleet could accept attacker-controlled identity claims, enabling enrollment of unauthorized devices under arbitrary Azure AD user identities.

Impact

If Windows MDM is enabled, an attacker can enroll rogue devices by submitting a forged JWT containing arbitrary identity claims. Due to missing JWT signature verification, Fleet accepts these claims without validating that the token was issued by Azure AD, allowing enrollment under any Azure AD user identity.

Patches

Weakness type: CWE-347.

Product: github.com/fleetdm/fleet (Go) · Assigned by: GitHub_M · Published: 2026-01-21

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.25% probability of exploitation activity in the next 30 days; percentile 16.

Timeline

• 2026-01-20 — GitHub-reviewed advisory GHSA-63m5-974w-448v published (critical)
• 2026-01-21 — CVE record published by GitHub_M
• 2026-01-22 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-63m5-974w-448v github.com
2. CVE.org record CVE-2026-23518 cve.org
3. NVD entry CVE-2026-23518 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. pkg.go.dev pkg.go.dev

Critical Go advisories · Today’s Security Digest