Security Digest · Vulnerability record · Known exploited
CVE-2023-21529: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
Exploited in the wild (CISA KEV). Microsoft Exchange Server Remote Code Execution Vulnerability
CISA KEV · added 2026-04-13 CVSS 8.8 High EPSS 59.3% · p99
Should you act?
Exploited: patch now if you run it
CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Yes, if you run Microsoft Exchange Server. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-04-13, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-04-27; treat that as the outer limit, not the target. CISA also lists it as known to be used in ransomware campaigns.
- EPSS 59.3%, percentile 99: FIRST’s model estimates a 59.3% chance of exploitation activity in the next 30 days, higher than 99% of all scored CVEs (very high). EPSS is a probability, not a verdict.
- Needs an account (low (any account)). Exposure depends on who can log in.
What it is
Microsoft Exchange Server Remote Code Execution Vulnerability
CISA’s summary
Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.
Weakness type: CWE-502.
Product: Microsoft Exchange Server · Assigned by: microsoft · Published: 2023-02-14
Affected and fixed versions
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 < 15.00.1497.047 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 23 | 15.01.0 < 15.01.2507.021 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 11 | 15.02.0 < 15.02.0986.041 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 12 | 15.02.0 < 15.02.1118.025 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| microsoft | 3.1 | 8.8 High | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: low (any account)
- User interaction: none
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: added 2026-04-13; federal due date 2026-04-27. Known ransomware use: Known.
- CISA 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.
- CISA SSVC assessment: exploitation active, automatable no, technical impact total.
- FIRST EPSS (model of 2026-10-21): 59.29% probability of exploitation activity in the next 30 days; percentile 99.
Timeline
- — CVE record published by microsoft
- — Added to CISA’s Known Exploited Vulnerabilities catalog
- — CISA remediation deadline for US federal agencies
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- Microsoft Exchange Server Remote Code Execution Vulnerability msrc.microsoft.com
- microsoft.com microsoft.com
Vulnerability records
- CVE.org record CVE-2023-21529 cve.org
- NVD entry CVE-2023-21529 nvd.nist.gov
Government
All exploited Microsoft vulnerabilities · Today’s Security Digest