Microsoft Exchange Server 2016 Cumulative…: Improper Authentication
CVE-2025-53786 is a high-severity improper authentication vulnerability in Microsoft Exchange Server 2016 Cumulative Update 23 from Microsoft, with a CVSS 3.1 base score of 8.0 out of 10. It can be exploited remotely over the network by an attacker with administrator privileges, without any action from a user. The record lists version 15.01.2507.055 as the first without the flaw, so updating Microsoft Exchange Server 2016 Cumulative Update 23 to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 8.03% chance of exploitation within 30 days, which is in the top 5.13% of all scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Microsoft Exchange Server 2016 Cumulative Update 23 to 15.01.2507.055 or a later version.
- Update Microsoft Exchange Server 2019 Cumulative Update 14 to 15.02.1544.025 or a later version.
- Update Microsoft Exchange Server 2019 Cumulative Update 15 to 15.02.1748.024 or a later version.
- Read the advisory on msrc.microsoft.com for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 23 | 15.01.0.0 up to, but not including, 15.01.2507.055 | 15.01.2507.055 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 14 | 15.02.0.0 up to, but not including, 15.02.1544.025 | 15.02.1544.025 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 15 | 15.02.0.0 up to, but not including, 15.02.1748.024 | 15.02.1748.024 |
| Microsoft | Microsoft Exchange Server Subscription Edition RTM | 15.02.0.0 up to, but not including, 15.02.2562.017 | 15.02.2562.017 |
| Microsoft | Exchange Server | 2016; 2019 | Not stated |
| Microsoft | Exchange Server Subscription Edition | before 15.02.2562.017 | 15.02.2562.017 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- HighSuccess depends on conditions outside the attacker's control.
- Privileges required
- HighAdministrator-level access is required first.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H. Scored by CNA (microsoft.com).
Official description
On April 18th 2025, Microsoft announced Exchange Server Security Changes for Hybrid Deployments and accompanying non-security Hot Fix. Microsoft made these changes in the general interest of improving the security of hybrid Exchange deployments. Following further investigation, Microsoft identified specific security implications tied to the guidance and configuration steps outlined in the April announcement. Microsoft is issuing CVE-2025-53786 to document a vulnerability that is addressed by taking the steps documented with the April 18th announcement. Microsoft strongly recommends reading the information, installing the April 2025 (or later) Hot Fix and implementing the changes in your Exchange Server and hybrid environment.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Authentication
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.
How developers prevent this class of flaw
- Use an authentication framework or library such as the OWASP ESAPI Authentication feature.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Other vulnerabilities in Microsoft Exchange Server 2016 Cumulative Update 23
- 9.3 Critical severity
- 9.3 Critical severity
- 8.8 High severity
- 9.6 Critical severity
- 8.1 High severity
- 6.1 Medium severity
Recent Improper Authentication vulnerabilities
- 7.5 High severity
- 8.1 High severity
- 10.0 Critical severity
- 9.8 Critical severity
- 7.0 High severity
- 9.8 Critical severity
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