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 yet, but Microsoft expects exploitation to be likely.
- What should you do? Install the security update for the version you run (or a later cumulative update):
- Microsoft Exchange Server Subscription Edition RTM: KB5129955
15.02.2562.053 - Microsoft Exchange Server 2016 Cumulative Update 23: KB5129958
15.01.2507.075 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5129956
15.02.1748.053 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5129957
15.02.1544.048
- Microsoft Exchange Server Subscription Edition RTM: KB5129955
Weak authorization in Microsoft Exchange Server allows an authenticated attacker to elevate privileges over a network.
Show NCSC advisory (1)
NCSC-2026-0419 · v1.00Likelihood M · Impact H
Kwetsbaarheden verholpen in Microsoft Excange ServerMicrosoft heeft kwetsbaarheden verholpen in Exchange. Een kwaadwillende kan de kwetsbaarheden misbruiken om zich voor te doen als andere gebruiker, zich verhoogde rechten toe te kennen en willekeurige code uit te voeren met rechten van het proces.
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.
Key facts
Released
2 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 More 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 Subscription Edition RTM | KB5129955 · Security Update | 15.02.2562.053 |
| Microsoft Exchange Server 2016 Cumulative Update 23 | KB5129958 | 15.01.2507.075 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5129956 | 15.02.1748.053 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5129957 | 15.02.1544.048 |
FAQ
- Is Exchange Online affected, and do Exchange Online customers need to take action?
- Microsoft has already deployed a related service-side fix to Exchange Online. Exchange Online customers do not need to take any action to receive the fix. Customers using affected on-premises Microsoft Exchange Server products should install the applicable security updates listed in the Security Updates table. Where can I find more information? Please see the Exchange blog for more information.
- What privileges could be gained by an attacker who successfully exploited this vulnerability?
- An authenticated attacker who successfully exploited this vulnerability could gain unauthorized access to other users' mailboxes within the same organization and read email messages and attachments. The vulnerability does not allow access across tenant boundaries.
Revisions
| Version | Date | Note |
|---|---|---|
| 1 | 2 okt 2026 | Information published. |
This is in the weekly digest for week 40 · What changed this week? →
View at Microsoft → ← All security advisories
Source: Microsoft Security Update Guide. 365Fanatic is not affiliated with Microsoft.
