Authenticated Deserialization RCE in Microsoft Exchange Server (CVE-2023-21529)
CISA: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
CVSS 3.1
8.8high
EPSS
62%p99
Published
()
KEV added
AI analysis
CVE-2023-21529 is a deserialization-of-untrusted-data flaw (CWE-502) in on-premises Microsoft Exchange Server that allows remote code execution. Per its CVSS vector, an attacker with valid low-privileged credentials (PR:L) sends crafted untrusted serialized data to the server over the network, requiring no user interaction. Successful exploitation yields code execution on the Exchange server, exposing mail stores and providing a foothold for lateral movement, and related coverage ties it to the fast-moving Storm-1175 ransomware operation, with ransomware use listed as known in CISA's KEV entry. Organizations running self-hosted Exchange Server are in scope; the source data does not list specific affected builds, but the fix shipped in Microsoft's February 2023 Patch Tuesday updates. The flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2026-04-13, EPSS estimates a 62.1% probability of exploitation within 30 days (99th percentile), and no public proof-of-concept is known.
What to do: Apply the February 2023 Exchange Server security updates to every on-premises Exchange server; U.S. federal agencies must mitigate or patch per BOD 22-01 following the KEV listing. Because exploitation requires authenticated low-privilege access, inventory exposed OWA/ECP endpoints, review and rotate credentials, and hunt for compromise indicators (unusual processes, webshells, unexpected mailbox activity) given known ransomware use.
Affected
Microsoft Exchange Server
—
Estimated exposure
mass≈50,000–100,000 internet-exposed on-prem Exchange servers; on-prem Exchange plausibly hosts 1M+ users worldwide — Public internet scans have repeatedly counted tens of thousands of exposed Exchange OWA/ECP endpoints, while total on-prem Exchange deployments across hundreds of thousands of organizations serve millions of mailboxes; this is an estimate,…
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
CISA Known Exploited Vulnerability
Affected
Microsoft Exchange Server
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.
Microsoft reports China-linked ransomware group Storm-1175 switched from Medusa to a new C++ strain, StormEncryptor, likely exploiting N-able flaw CVE-2026-18577.
Microsoft Threat Intelligence reports that the financially motivated, China-linked group Storm-1175 began deploying a new ransomware strain called StormEncryptor on August 2, 2026, replacing its previous Medusa ransomware. StormEncryptor is written in C++, appends the .encrypted extension to files, and drops a !!!README_FIRST!!!.txt ransom note in each scanned directory. Microsoft assesses the group is likely exploiting CVE-2026-18577, an authentication bypass in N-able disclosed on August 2, 2026 and added to CISA's Known Exploited Vulnerabilities catalog the next day. Since 2023, Storm-1175 has exploited more than 16 vulnerabilities in products including Microsoft Exchange, Ivanti, ConnectWise ScreenConnect, JetBrains TeamCity, SimpleHelp, CrushFTP, and GoAnywhere MFT, often moving from initial access to data theft and ransomware deployment within days.