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 9.6 out of 10: critical. Microsoft rates it Critical.
- 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: KB5103212
15.02.2562.045 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5103213
15.02.1748.048 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5103214
15.02.1544.043 - Microsoft Exchange Server 2016 Cumulative Update 23: KB5103215
15.01.2507.071
- Microsoft Exchange Server Subscription Edition RTM: KB5103212
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 jul 2026
Impact
Spoofing
Max severity
Critical
Weakness
CVSS 3.1
9.6 / 8.3 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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 | None |
| User Interaction | Required |
| Scope | Changed |
| 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 | KB5103212 · Security Update | 15.02.2562.045 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5103213 | 15.02.1748.048 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5103214 | 15.02.1544.043 |
| Microsoft Exchange Server 2016 Cumulative Update 23 | KB5103215 | 15.01.2507.071 |
FAQ
- Microsoft recommends installing the July 2026 Security Updates for your version of Exchange Server to be protected from this vulnerability. Customers who have installed the July 2026 Security Updates on all affected Exchange servers are fully protected and may safely remove the CVE-2026-42897 mitigation (which is similar in nature and mitigates CVE-2026-55008 also). Microsoft recommends keeping the Emergency Mitigation (EM) Service enabled to receive future emergency mitigations and security protections. For additional information, please see the Exchange blog post.
- How could an attacker exploit this vulnerability?
- An attacker could exploit this issue by sending a specifically crafted malicious email to the user. If the user opens this email, in Outlook Web Access, and certain conditions are met, arbitrary execution of JavaScript would occur in the browser context.
Revisions
| Version | Date | Note |
|---|---|---|
| 1 | 14 jul 2026 | Information published. |
What changed in Microsoft 365 this week? →
View at Microsoft → ← All security advisories
Source: Microsoft Security Update Guide. 365Fanatic is not affiliated with Microsoft.
