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 8.1 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: KB5121611
15.01.2507.073 - Microsoft Exchange Server Subscription Edition RTM: KB5121608
15.02.2562.049 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5121610
15.02.1544.046 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5121609
15.02.1748.051
- Microsoft Exchange Server 2016 Cumulative Update 23: KB5121611
Missing 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-862: Missing Authorization
An action is missing the check whether the user may perform it.
Key facts
Released
8 sep 2026
Impact
Elevation of Privilege
Max severity
Important
Weakness
CVSS 3.1
8.1 / 7.1 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N/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 | None |
| 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 | KB5121611 | 15.01.2507.073 |
| Microsoft Exchange Server Subscription Edition RTM | KB5121608 · Security Update | 15.02.2562.049 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5121610 | 15.02.1544.046 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5121609 | 15.02.1748.051 |
FAQ
- How could an attacker exploit this vulnerability?
- An attacker with a low-privilege user account and an assigned mailbox, could exploit weaknesses in how Exchange validates requests and identity tokens to impersonate another user. By doing so, the attacker could then access mailboxes through Exchange services as if they were that user.
- What privileges could be gained by an attacker who successfully exploited the vulnerability?
- The attacker would be able to take over the mailboxes of all Exchange users, attackers can send emails, read emails, download attachments.
Revisions
| Version | Date | Note |
|---|---|---|
| 1 | 8 sep 2026 | Information published. |
This is in the weekly digest for week 37 · What changed this week? →
View at Microsoft → ← All security advisories
Source: Microsoft Security Update Guide. 365Fanatic is not affiliated with Microsoft.
