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Medium 6.6Known exploited (CISA KEV)Published May 11, 2021
CVE-2021-31207Microsoft Exchange Server Security Feature Bypass Vulnerability
Updated under an hour ago ()
Does this affect you?
It affects Microsoft Exchange Server 2013 Cumulative Update 23. Add what you run to find out whether that includes you.
CISA lists it as known exploited since Nov 3, 2021; federal agencies had to remediate by Nov 17, 2021, and it is known to be used in ransomware. Treat that date as your deadline.
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Microsoft Exchange Server Security Feature Bypass Vulnerability
- CVSS
- 6.6 MEDIUM (v3.1, CNA)CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
- Weakness (CWE)
- CWE-434
- Known exploited
- Added to CISA KEV Nov 3, 2021; federal remediation due Nov 17, 2021; known ransomware useApply updates per vendor instructions.
- EPSS (exploitation in the next 30 days)
- 99.78% (percentile 100)
- Assigned by
- microsoft
- NVD analysis
- Analyzed
- Record last updated
- Oct 21, 2025
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 microsoft:exchange_server | from 15.00.0 before 15.00.1497.018 | CNA record |
microsoft exchange_server microsoft:exchange_server | 2013; 2016; 2016; 2019; 2019 | NVD |
References
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207x-refsource-misc, patch, vendor-advisory
- https://www.zerodayinitiative.com/advisories/ZDI-21-819/x-refsource-misc, third-party-advisory, vdb-entry
- http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.htmlx-refsource-misc, exploit, third-party-advisory, vdb-entry
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31207us-government-resource
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.