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CVE-2023-21529

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Microsoft Exchange Server. For every flaw being used in attacks, see Exploited flaws; for Microsoft’s, see Microsoft flaws being used in attacks.

CVE-2023-21529 is a security flaw in Microsoft Exchange Server, Microsoft’s email server, which companies run for their staff’s mail. Someone who already has a way in can use Microsoft Exchange Server to run their own code. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 13 Apr 2026, and says ransomware gangs have used it.[1]

Microsoft Exchange Server is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 15.00.1497.047, 15.01.2507.021, 15.02.0986.041 and 15.02.1118.025.

Description

CISA calls it a deserialization of untrusted data in Microsoft Exchange Server. In its words: “Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.”[1]

The underlying weakness is deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.[2]

It scores 8.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]

Affected versions

NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]

ProductFixed in
Microsoft Exchange Server 2013 Cumulative Update 2315.00.1497.047
Microsoft Exchange Server 2016 Cumulative Update 2315.01.2507.021
Microsoft Exchange Server 2019 Cumulative Update 1115.02.0986.041
Microsoft Exchange Server 2019 Cumulative Update 1215.02.1118.025

Most families won’t have Microsoft Exchange Server; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 13 Apr 2026, 1,154 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 27 Apr 2026 to fix it.[1] CISA says ransomware gangs have used it.

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 59%. That’s in the top 1% of the flaws it scores.[4]

CISA assesses it as not automatable: each attack takes some work.

CISA doesn’t say who was attacked, or how.

Fix

Microsoft has published a fix or advice.[3] 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.”[1]

  1. Check whether you use it.Microsoft Exchange Server is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 15.00.1497.047, 15.01.2507.021, 15.02.0986.041 and 15.02.1118.025), or not in use. CISA gave US government agencies until 27 Apr 2026 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 8.8.[2]
CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 59%.[4]

See also

Other Microsoft Exchange Server flaws

Other Microsoft flaws

  • CVE-2026-96951, Microsoft Windows: command injection ·
  • CVE-2026-65660, Microsoft SharePoint: code injection ·
  • CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
  • CVE-2026-81963, Microsoft Windows: link following ·
  • CVE-2019-1068, Microsoft SQL Server: remote code execution ·

Other deserialization of untrusted data flaws

  • CVE-2021-23758, Ajax.NET Professional: deserialization of untrusted data ·
  • CVE-2026-63077, JetBrains TeamCity: deserialization of untrusted data ·
  • CVE-2026-45247, Mirasvit Full Page Cache Warmer: full page cache warmer deserialization of untrusted data ·
  • CVE-2026-20131, Cisco Secure Firewall Management Center (FMC): software and cisco security cloud control (SCC) firewall management deserialization of untrusted data ·
  • CVE-2025-26399, SolarWinds Web Help Desk: deserialization of untrusted data ·

Added the same day

Every Microsoft flaw on CISA’s list · Everything added in April 2026

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability”. Known Exploited Vulnerabilities Catalog, added 13 Apr 2026.
  2. ^ National Institute of Standards and Technology. “CVE-2023-21529 Detail”. National Vulnerability Database, published 14 Feb 2023.
  3. ^ Microsoft. Advisories and fixes: msrc.microsoft.com.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.593, percentile 99.1, 31 Oct 2026.
  5. CVE Program. “CVE-2023-21529”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.