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Actively exploitedKnown ransomware use

CVE-2020-0688 Microsoft Exchange Server Validation Key Remote Code Execution Vulnerability

Microsoft Exchange Server Validation Key fails to properly create unique keys at install time, allowing for remote code execution.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 3 Nov 2021.
  • CISA lists it as known to be used in ransomware campaigns.
  • Rated High severity (CVSS 8.8, scored by CISA).
  • This product type is often exposed to the internet, which is where attackers scan for it first.
  • It has been in the catalogue for 1,829 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

A remote code execution vulnerability exists in Microsoft Exchange software when the software fails to properly handle objects in memory, aka 'Microsoft Exchange Memory Corruption Vulnerability'.

Affected Versions

ProductVersions
Microsoft Microsoft Exchange Server 2013Cumulative Update 23
Microsoft Microsoft Exchange Server 2019 Cumulative Update 3unspecified
Microsoft Microsoft Exchange Server 2016 Cumulative Update 14unspecified
Microsoft Microsoft Exchange Server 2016 Cumulative Update 15unspecified
Microsoft Microsoft Exchange Server 2019 Cumulative Update 4unspecified
Microsoft Microsoft Exchange Server 2010 Service Pack 3 Update Rollup 30unspecified

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Microsoft Exchange Server. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 3 May 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2020-0688
  3. CVE record: CVE-2020-0688
  4. FIRST EPSS
  5. portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-0688
  6. zerodayinitiative.com/advisories/ZDI-20-258/
  7. packetstormsecurity.com/files/156592/Microsoft-Exchange-2019-15.2.221.12-Remote-Code-Execution.html
  8. packetstormsecurity.com/files/156620/Exchange-Control-Panel-Viewstate-Deserialization.html