Microsoft Security Update Guide · Exchange
Microsoft Exchange Server Spoofing Vulnerability
Published · updated 14 jul 2026
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 8.1 out of 10: high. Microsoft rates it Critical.
- Already exploited in the wild. Microsoft reports attacks. Treat the update as a priority.
- What should you do? Install the security update for the version you run (or a later cumulative update):
- Microsoft Exchange Server Subscription Edition RTM: KB5094139
15.02.2562.043 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5094140
15.02.1748.046 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5094142
15.02.1544.041 - Microsoft Exchange Server 2016 Cumulative Update 23: KB5094144
15.01.2507.069
- Microsoft Exchange Server Subscription Edition RTM: KB5094139
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
14 mei 2026
Last revised
14 jul 2026 (v2.1)
Impact
Spoofing
Max severity
Critical
Weakness
CVSS 3.1
8.1 / 7.5 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N/E:F/RL:O/RC:CPublicly disclosed
No
Exploited
Yes
Exploitability assessment
Exploitation Detected
Customer action required
Yes
Links
CVSS metrics
| Base score metrics | |
|---|---|
| Attack Vector | Network |
| Attack Complexity | Low |
| Privileges Required | None |
| User Interaction | Required |
| Scope | Unchanged |
| Confidentiality | High |
| Integrity | High |
| Availability | None |
| Temporal score metrics | |
| Exploit Code Maturity | Functional |
| Remediation Level | Official Fix |
| Report Confidence | Confirmed |
Affected products and updates
| Product | Update | Build |
|---|---|---|
| Microsoft Exchange Server Subscription Edition RTM | KB5094139 · Security Update | 15.02.2562.043 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5094140 | 15.02.1748.046 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5094142 | 15.02.1544.041 |
| Microsoft Exchange Server 2016 Cumulative Update 23 | KB5094144 | 15.01.2507.069 |
FAQ
- How could an attacker exploit this vulnerability?
- An attacker could exploit this issue by sending a specially crafted email to a user. If the user opens the email in Outlook Web Access and certain interaction conditions are met, arbitrary JavaScript can be executed in the browser context. How do I protect my Exchange Server from this vulnerability? The Exchange Emergency Mitigation Service will provide mitigation automatically, and is on by default. If it is not already enabled on your Exchange Server, you need to enable Exchange Emergency Mitigation Service. You can find more information and instruction in the Exchange blog here. Am I protected from this vulnerability if I am running Internet Explorer or Edge with Internet Explorer Mode? No, because Content Security Policy (CSP) is not supported by Internet Explorer nor Microsoft Edge using Internet Explorer Mode. To stay protected, please make sure to not use Internet Explorer (Mode)…
Revisions
| Version | Date | Note |
|---|---|---|
| 2.1 | 14 jul 2026 | Updated FAQ information. This is an informational change only. |
| 2 | 9 jun 2026 | Added links to June 2026 Exchange Server security updates. Microsoft recommends installing this updates as soon as possible. |
| 1.1 | 18 mei 2026 | Updated FAQ information. This is an informational change only. |
| 1 | 14 mei 2026 | Information published. |
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Source: Microsoft Security Update Guide. 365Fanatic is not affiliated with Microsoft.
