Microsoft Exchange Server Deserialization of Untrusted Data
CVE-2023-21529 is a high-severity deserialization of untrusted data vulnerability in Microsoft Exchange Server 2013 Cumulative Update 23 from Microsoft, with a CVSS 3.1 base score of 8.8 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. The record lists version 15.00.1497.047 as the first without the flaw, so updating Microsoft Exchange Server 2013 Cumulative Update 23 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on April 13, 2026, which means attacks have been observed in the wild, including in ransomware campaigns.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft Exchange Server 2013 Cumulative Update 23 to 15.00.1497.047 or a later version.
- Update Microsoft Exchange Server 2016 Cumulative Update 23 to 15.01.2507.021 or a later version.
- Update Microsoft Exchange Server 2019 Cumulative Update 11 to 15.02.0986.041 or a later version.
- CISA's required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- 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 2013 Cumulative Update 23 | 15.00.0 up to, but not including, 15.00.1497.047 | 15.00.1497.047 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 23 | 15.01.0 up to, but not including, 15.01.2507.021 | 15.01.2507.021 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 11 | 15.02.0 up to, but not including, 15.02.0986.041 | 15.02.0986.041 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 12 | 15.02.0 up to, but not including, 15.02.1118.025 | 15.02.1118.025 |
| Microsoft | Exchange Server | 2013; 2016; 2019 | Not stated |
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
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- 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:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (microsoft.com).
Official description
Microsoft Exchange Server Remote Code Execution Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Deserialization of Untrusted Data
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
How developers prevent this class of flaw
- If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.
- When deserializing data, populate a new object rather than just deserializing. The result is that the data flows through safe input validation and that the functions are safe.
- Explicitly define a final object() to prevent deserialization.
More Deserialization of Untrusted Data vulnerabilities (CWE-502)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Deadline for US federal agencies to remediate, set by CISA.
- Record last changed materially (score, affected products, status or references).
References
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-21529US Government Resource
Other references
Other vulnerabilities in Microsoft Exchange Server 2013 Cumulative Update 23
- 9.8 Critical severity
- 8.0 High severity
- 8.8 High severity
- 9.8 Critical severity
- 9.8 Critical severity
- 7.5 High severity
Recent Deserialization of Untrusted Data vulnerabilities
- 8.8 High severity
- 9.8 Critical severity
- 8.8 High severity
- 7.2 High severity
- 9.8 Critical severity
- 9.8 Critical severity
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