CVE-2026-54998 — Exchange Online
HIGHCVE-2026-54998 is a high-severity (CVSS 8.8) CWE-863 vulnerability in microsoft exchange online. 1 public exploit reference exists, so weaponization risk is real. No vendor fix is recorded yet; mitigation options are listed below.
Incorrect authorization in Microsoft Exchange Online allows an authorized attacker to elevate privileges over a network.
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-54998.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-54998 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Incorrect authorization in Microsoft Exchange Online allows an authorized attacker to elevate privileges over a network.
Affected Products
exchange onlinemicrosoftResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft exchange online deployment and check each version against the affected-products list above.
Remediation status
No patch has shipped for CVE-2026-54998 yet — track the microsoft exchange online advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
Frequently Asked Questions
Is CVE-2026-54998 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.