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), with an ordinary user account and without any user action.
- CVSS 6.5 out of 10: medium. 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 Subscription Edition RTM: KB5121608
15.02.2562.049 - Microsoft Exchange Server 2016 Cumulative Update 23: KB5121611
15.01.2507.073 - Microsoft Exchange Server 2019 Cumulative Update 15: KB5121609
15.02.1748.051 - Microsoft Exchange Server 2019 Cumulative Update 14: KB5121610
15.02.1544.046
- Microsoft Exchange Server Subscription Edition RTM: KB5121608
Server-side request forgery (ssrf) in Microsoft Exchange Server allows an authorized 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-918: Server-Side Request Forgery (SSRF)
The server can be made to send requests to other systems on the attacker's behalf (server-side request forgery).
Key facts
Released
8 sep 2026
Impact
Spoofing
Max severity
Important
Weakness
CVSS 3.1
6.5 / 5.7 (base / temporal)
Vector string
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/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 | None |
| 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 Subscription Edition RTM | KB5121608 · Security Update | 15.02.2562.049 |
| Microsoft Exchange Server 2016 Cumulative Update 23 | KB5121611 | 15.01.2507.073 |
| Microsoft Exchange Server 2019 Cumulative Update 15 | KB5121609 | 15.02.1748.051 |
| Microsoft Exchange Server 2019 Cumulative Update 14 | KB5121610 | 15.02.1544.046 |
FAQ
- What is the nature of the spoofing?
- An authenticated attacker could cause an affected Exchange server to send HTTP requests to internal or loopback systems by submitting a specially crafted internet calendar subscription. Successful exploitation could expose sensitive information returned as valid calendar content from resources reachable by the server.
- According to the CVSS metric, privileges required is low (PR:L). What does that mean for this vulnerability?
- The attacker must be authenticated using valid Exchange user credentials.
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.
