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Response snapshot Oct 26, 2026, 22:56 UTC

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VulnerabilityCVE-2023-21529Known exploitation

Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability

Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.

At a glance

Exploited
Yes: on CISA's Known Exploited list
Severity
8.8 High (CVSS 3.1)
Chance of exploitation
59.3% in the next 30 days
CISA federal deadline
Apr 27, 2026: passed
Sources
2 independent sources

What to do

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 CISA KEV entry CVE-2023-21529 (opens a new tab)

CISA federal deadline

Federal due date passed

Applies to covered US federal agencies; it is not your organization's deadline.

Added to the catalog Known ransomware use: Known
CISA catalog (opens a new tab)

Scores and how they were measured

CVSS base severity

8.8

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

59.3%

Scored Percentile 99.1%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    CISA KEV entry CVE-2023-21529 (opens a new tab)

    The CVE-2023-21529 catalog entry establishes known exploitation, date added, required action, and the federal due date; not zero-day status.

    Published Checked

  2. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2023-21529, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

  3. Primary source

    CVE Program CNA microsoft (opens a new tab)

    The published CVE-2023-21529 document's CNA supplies this CVSS 3.1 base score, vector, and authoritative source update date.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, April 13). Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (CVE-2023-21529). https://kevwire.com/record/CVE-2023-21529

MLA

"Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (CVE-2023-21529)." KEVWire, 13 Apr. 2026, kevwire.com/record/CVE-2023-21529.

Chicago

KEVWire. "Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (CVE-2023-21529)." Last modified April 13, 2026. https://kevwire.com/record/CVE-2023-21529.

Evidence summary (plain text)

Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (Apr 13, 2026).
Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.
CVSS 8.8 (version 3.1; evidence as of Aug 19, 2026; https://cveawg.mitre.org/api/cve/CVE-2023-21529).
EPSS 59.3% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2023-21529&date=2026-10-26).
Sources:
CISA KEV entry CVE-2023-21529: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2023-21529&date=2026-10-26
CVE Program CNA microsoft: https://cveawg.mitre.org/api/cve/CVE-2023-21529
Record ID: CVE-2023-21529.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Documented by a cited source.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
CISA federal due date: Apr 27, 2026. This is a deadline for covered US federal agencies, not the reader's SLA.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 20:16 UTC. Ingestion is not an independent editorial investigation.

CISA added this entry on 2026-04-13. This record's publication date is the catalog addition date, not necessarily the initial vulnerability disclosure. KEV means known exploitation, not necessarily a zero-day. CISA due dates apply to covered US federal agencies, not your organizational SLA.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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Record details

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