Microsoft Security Update Guide · Exchange
Microsoft Exchange Server Spoofing Vulnerability
Published
What does this mean?
- Spoofing. An attacker can pretend to be another user or source, for example through a misleading link or page.
- It can be exploited over the network (including from the internet if the service is reachable there), without an account and if a user takes part, for example by opening a link or file.
- CVSS 9.3 out of 10: critical. 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 2016 Cumulative Update 23: KB5128898
15.01.2507.077 - 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 2019 Cumulative Update 14: KB5128897
15.02.1544.050
- Microsoft Exchange Server 2016 Cumulative Update 23: KB5128898
Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing 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-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Input is not cleaned before it appears in a web page (cross-site scripting). An attacker can run script in another user's browser.
Key facts
Released
13 okt 2026
Impact
Spoofing
Max severity
Important
Weakness
CVSS 3.1
9.3 / 8.1 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N/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 | None |
| User Interaction | Required |
| Scope | Changed |
| 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 | KB5128898 | 15.01.2507.077 |
| 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 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5128897 | 15.02.1544.050 |
FAQ
- How could an attacker exploit this vulnerability?
- An attacker could send a specially crafted link to a user and persuade them to click it, with no authentication required. If a user with administrative privileges clicks the link, successful exploitation could allow the attacker to perform actions using that user's privileges, potentially allowing access to or modification of organizational data.
- According to the CVSS metric, user interaction is required (UI:R). What interaction would the user have to do?
- Exploitation of the vulnerability requires that a user to open a specially crafted link. An attacker would have no way to force users to click the link. Instead, an attacker would have to convince users to click the link through social engineering.
- According to the CVSS metric, a successful exploitation could lead to a scope change (S:C). What does this mean for this vulnerability?
- The vulnerability is in the web server, but the malicious scripts execute in the victim’s browser on their machine.
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.
