ProxyShell — Microsoft Exchange Server Arbitrary file upload
CVE-2021-31207
Microsoft Exchange Server Security Feature Bypass Vulnerability
Vulnerability class: Unrestricted File Upload
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.6 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.998 (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: 0.998 · newest: 0.998 · change: 0.000
Affected products
- Microsoft Exchange Server — versions 2013, 2016, 2019
- Microsoft Exchange Server 2013 Cumulative Update 23 — versions 15.00.0
- Microsoft Exchange Server 2016 Cumulative Update 19 — versions 15.01.0
- Microsoft Exchange Server 2016 Cumulative Update 20 — versions 15.01.0
- Microsoft Exchange Server 2019 Cumulative Update 8 — versions 15.02.0
- Microsoft Exchange Server 2019 Cumulative Update 9 — versions 15.02.0
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
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207 (Patch, Vendor Advisory)
- www.zerodayinitiative.com/advisories/ZDI-21-819/ (VDB Entry, Third Party Advisory)
- packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-… (Exploit, VDB Entry, Third Party Advisory)
- 134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
Frequently asked questions
- What is CVE-2021-31207?
- CVE-2021-31207 is a medium-severity vulnerability in Microsoft Exchange Server, classified under Unrestricted Upload of File with Dangerous Type. CVSS score: 6.6/10. Published 2021-05-11.
- How severe is CVE-2021-31207?
- Medium severity. CVSS v3 base score is 6.6 out of 10.
- Is CVE-2021-31207 known to be exploited?
- Yes. CVE-2021-31207 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 56 public proof-of-concept repositories are indexed.