ProxyLogon — Microsoft Exchange Server SSRF
CVE-2021-26855
Microsoft Exchange Server Remote Code Execution Vulnerability
Vulnerability class: SSRF (Server-Side Request Forgery)
Published · last modified .
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.
EPSS exploit prediction
EPSS: 1.000 (100.0th percentile), scored .
Very high probability of exploitation in the next 30 days — among the top 0.1% most-likely-to-be-exploited CVEs. 100th percentile — 100.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 1.000 · newest: 1.000 · change: 0.000
Affected products
- Microsoft Exchange Server — versions 2013, 2016, 2019
- 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
Weakness classification (CWE)
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
- af854a3a-2127-422b-91ae-364da2661108 (Patch, Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
- af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
- af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
- af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
- 134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
- msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855 (vendor-advisory, Patch, patch, Vendor Advisory)
Frequently asked questions
- What is CVE-2021-26855?
- CVE-2021-26855 is a critical-severity vulnerability in Microsoft Exchange Server, classified under Server-Side Request Forgery (SSRF). CVSS score: 9.1/10. Published 2021-03-03.
- 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. 238 public proof-of-concept repositories are indexed.