CVE-2026-54998-exp

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Overview

CVE-2026-54998 is an Incorrect Authorization vulnerability (CWE-863) in Microsoft Exchange Online. It allows an authenticated attacker with low-level user privileges to elevate their access rights beyond the intended authorization scope over the network.

The vulnerability was published on July 3, 2026. CVSS v3.1 score: 8.8 (High).

Affected Versions

• Microsoft Exchange Online (all versions prior to the patch released on July 3, 2026)
• Microsoft 365 environments using Exchange Online

Patch Available: Yes (released July 3, 2026 via GitHub Advisory GHSA-phr2-vr74-whpx).

Technical Breakdown (Root Cause)

The root cause is improper authorization checks in Exchange Online. An authenticated user with standard/low privileges can manipulate requests to gain higher-level permissions, allowing actions beyond their assigned scope (e.g., accessing other users' mailboxes, administrative functions, etc.).

Attack Chain

1. Attacker logs in with a standard user account.
2. Sends specially crafted requests to Exchange Online APIs.
3. Bypasses authorization controls to elevate privileges.
4. Performs unauthorized actions (data access, configuration changes, etc.).

PoC

Exploit - href

Detection Guidance

• Monitor Exchange Online audit logs for suspicious privilege escalation attempts.
• Look for abnormal permission usage by low-privileged accounts.
• Enable Microsoft 365 Defender alerts for anomalous access patterns.

Mitigation

• Apply the official patch immediately (July 3, 2026 update).
• Enforce the principle of least privilege for all Exchange Online accounts.
• Implement network-level access controls and conditional access policies.
• Regularly review and monitor user permissions and audit logs.