CVE Intelligence
CISA KEV High Insecure deserialization

CVE-2023-21529 — Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Published Feb 14, 2023 · Updated Oct 13, 2026 · Sources: CISA Known Exploited Vulnerabilities, NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: microsoft

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

CISA KEV · Apr 13, 2026 R used by ransomware (CISA)
How the score was calculated
  • Confirmed exploitation (CISA KEV)+35
  • Used in ransomware campaigns (CISA)+10
  • EPSS 59.3% (30-day exploitation probability)+15
  • Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Confirmed
CISA KEV, listed Apr 13, 2026
EPSS FIRST
59.29%
ahead of 99% of scored CVEs · Oct 14, 2026
Affects
Microsoft
Exchange Server and 4 more
CVSS base
8.8
High · 3 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
Active
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Total
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

CISA required action CISA

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

US federal deadline: Apr 27, 2026 — passed 171 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Microsoft Exchange Server, Microsoft Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Microsoft Exchange Server 2016 Cumulative Update 23…).
  2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
  3. Apply the fixed version or the patch documented under Remediation.
  4. EPSS ahead of 99% of scored CVEs: prioritize it in the patch queue.
  5. Search available logs for exploit probes matching the vulnerability class (insecure deserialization), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Microsoft Exchange Server version 2013-cumulative_update_23version 2016-cumulative_update_23version 2019-cumulative_update_11version 2019-cumulative_update_12 Vulnerable NIST National Vulnerability Database
Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 ≥ 15.00.0 and < 15.00.1497.047 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2016 Cumulative Update 23 ≥ 15.01.0 and < 15.01.2507.021 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 11 ≥ 15.02.0 and < 15.02.0986.041 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 12 ≥ 15.02.0 and < 15.02.1118.025 Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
8.8 CVSS 3.1 High 2.8 5.9 secure@microsoft.com via NIST National Vulnerability Database
8.8 CVSS 3.1 High — — GitHub, Inc. via GitHub Security Advisories
8.8 CVSS 3.1 High — — microsoft via CVE Program (cve.org) e CISA ADP
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (score shown at the top: secure@microsoft.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-502 · Deserialization of Untrusted Data

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

Listed
Apr 13, 2026
Federal deadline
Apr 27, 2026
passed 171 day(s) ago
Ransomware use
Known

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

EPSS over time FIRST

Oct 12, 2026 → Oct 14, 2026 · 3 reading(s) · max 59.29%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
  • Monitor serialized objects in requests and unexpected processes spawned by the runtime.
  • Hunt retroactively in logs from before Apr 13, 2026 (KEV listing): exploitation may have started earlier.
web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)application logs

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · microsoft/microsoft exchange server 2013 cumulative update 23, microsoft/microsoft exchange server 2016 cumulative updat
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 13, 2026
  • Fix documented
    Oct 13, 2026 · NIST National Vulnerability Database
  • Affected products added · microsoft/exchange server
    Oct 13, 2026 · NIST National Vulnerability Database
  • CVSS changed · 8.8
    Oct 13, 2026 · NIST National Vulnerability Database
  • Source status changed · → Analyzed
    Oct 13, 2026 · NIST National Vulnerability Database
  • Added to CISA KEV (detected)
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • Record created in this database
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • Added to CISA KEV catalog
    Apr 12, 2026 · CISA
  • 2023
  • CVE published
    Feb 14, 2023 · microsoft

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 13, 2026 – Oct 13, 2026
  • 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

6 on the record

Elsewhere on this site

Microsoft
every CVE for this vendor
2026-04
what else CISA listed that month
Insecure deserialization
same class
CWE-502
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-87696 Microsoft Exchange Server Elevation of Privilege Vulnerability 1d
  • CVE-2026-87695 Microsoft Exchange Server Elevation of Privilege Vulnerability 1d
  • CVE-2026-87692 Microsoft Exchange Server Elevation of Privilege Vulnerability 1d
  • CVE-2026-77910 Microsoft Exchange Server Elevation of Privilege Vulnerability 1d
  • CVE-2026-70571 Microsoft Exchange Server Remote Code Execution Vulnerability 1d
  • CVE-2026-70279 Microsoft Exchange Server Spoofing Vulnerability 1d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2023-21529 is "Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability", rated High. It affects Exchange Server (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 8.8 describes technical severity. EPSS 59.29% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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