Microsoft Security Update Guide · Exchange
Microsoft Exchange Server Elevation of Privilege Vulnerability
Published
What does this mean?
- Elevation of Privilege. An attacker can gain more rights than they should have, for example admin rights.
- It can be exploited over the network (including from the internet if the service is reachable there), with an ordinary user account and without any user action.
- CVSS 8.8 out of 10: high. Microsoft rates it Important.
- No attacks reported; Microsoft expects exploitation to be less likely.
- What should you do? Install the security update for the version you run (or a later cumulative update):
- Microsoft Exchange Server 2016 Cumulative Update 23: KB5128898
15.01.2507.077 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5128897
15.02.1544.050 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5128896
15.02.1748.055 - Microsoft Exchange Server Subscription Edition RTM: KB5128895
15.02.2562.056
- Microsoft Exchange Server 2016 Cumulative Update 23: KB5128898
Incorrect authorization in Microsoft Exchange Server allows an authorized attacker to elevate privileges over a network.
How do I check if I am affected?
Exchange Server: check and update
- Check your server versions in the Exchange Management Shell:
Get-ExchangeServer | Format-List Name,AdminDisplayVersion - Microsoft's Exchange Health Checker (CSS-Exchange on GitHub) shows which updates are missing per server.
- Install the security update (SU) for the CU you run. Microsoft updates Exchange Online.
What kind of flaw is this?
- CWE-863: Incorrect Authorization
The check whether a user may do something does not work correctly.
Key facts
Released
13 okt 2026
Impact
Elevation of Privilege
Max severity
Important
Weakness
CVSS 3.1
8.8 / 7.7 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:CPublicly disclosed
No
Exploited
No
Exploitability assessment
Exploitation Less Likely
Customer action required
Yes
Links
CVSS metrics
| Base score metrics | |
|---|---|
| Attack Vector | Network |
| Attack Complexity | Low |
| Privileges Required | Low |
| User Interaction | None |
| Scope | Unchanged |
| Confidentiality | High |
| Integrity | High |
| Availability | High |
| Temporal score metrics | |
| Exploit Code Maturity | Unproven |
| Remediation Level | Official Fix |
| Report Confidence | Confirmed |
Affected products and updates
| Product | Update | Build |
|---|---|---|
| Microsoft Exchange Server 2016 Cumulative Update 23 | KB5128898 | 15.01.2507.077 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5128897 | 15.02.1544.050 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5128896 | 15.02.1748.055 |
| Microsoft Exchange Server Subscription Edition RTM | KB5128895 · Security Update | 15.02.2562.056 |
FAQ
- How could an attacker exploit this vulnerability?
- An authenticated attacker with low privileges could poison a shared cache used by the Exchange Control Panel, causing a malicious mail add-in to be installed in the context of another user's mailbox. Successful exploitation could allow the attacker to gain elevated privileges in the affected mailbox context. User interaction is not required.
Revisions
| Version | Date | Note |
|---|---|---|
| 1 | 13 okt 2026 | Information published. |
This is in the weekly digest for week 42: everything that changed in Microsoft 365 this week. →
View at Microsoft → ← All security advisories
Source: Microsoft Security Update Guide. 365Fanatic is not affiliated with Microsoft.
