ProxyLogon — Microsoft Exchange Server 2013 Cumulative Update 21 Vulnerability
CVE-2021-26855
Microsoft Exchange Server Remote Code Execution Vulnerability
EPSS: 0.943 (99.9th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C.
Affected products
- Microsoft Exchange Server 2013 Cumulative Update 21 — versions 15.00.0
- Microsoft Exchange Server 2013 Cumulative Update 22 — versions 15.00.0
- Microsoft Exchange Server 2013 Cumulative Update 23 — versions 15.00.0
- Microsoft Exchange Server 2016 Cumulative Update 10 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 11 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 12 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 13 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 14 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 15 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 16 — versions 15.01.0
CISA KEV (Known Exploited Vulnerabilities)
This CVE is on the CISA KEV catalog, added on . CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.
BOD 22-01 due date: .
Required action: Apply updates per vendor instructions.
Known ransomware campaign use: yes.
Public proof-of-concept exploits
References
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855 (x_refsource_MISC)
- packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-Fil… (x_refsource_MISC)
- packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-… (x_refsource_MISC)
- packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Em… (x_refsource_MISC)
- packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.ht… (x_refsource_MISC)
Frequently asked questions
- What is CVE-2021-26855?
- CVE-2021-26855 is a critical-severity vulnerability in Microsoft Exchange Server 2013 Cumulative Update 21. CVSS score: 9.1/10. Published 2021-03-02.
- How severe is CVE-2021-26855?
- Critical severity. CVSS v3 base score is 9.1 out of 10.
- Is CVE-2021-26855 known to be exploited?
- Yes. CVE-2021-26855 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 235 public proof-of-concept repositories are indexed.