CVE-2021-34473
CriticalExploitedPublic exploitMicrosoft Exchange Server Remote Code Execution Vulnerability
Published Sep 14, 2022 · Updated Aug 10, 2026 · Also known as Proxy Shell, ProxyShell
Key Facts
Patch immediately- CVE-2021-34473 is a critical-severity vulnerability (CVSS 9.8 v3.1) affecting Exchange Server (Microsoft).
- It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Aug 30, 2021 and is known to be used in ransomware campaigns. Public exploit code is available.
- EPSS estimates a 100% probability of exploitation in the next 30 days.
- Fixed versions are available, including Exchange Server 15.00.1497.015, Exchange Server 15.01.2176.012, Exchange Server 15.01.2242.008.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
9.8
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
100%
Higher than 99.9% of all scored CVEs
Exploited in the wild
Yes
4 catalog listings, used by ransomware
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2021-34473 brief as a PDF
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Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| CISA KEV | Nov 3, 2021 | Nov 17, 2021 | Known |
| Industry KEV feed | Aug 30, 2021 | Nov 17, 2021 | Known |
| CIRCL KEV | Nov 3, 2021 | Nov 17, 2021 | Known |
| ENISA EUVD | Nov 3, 2021 | n/a | Not reported |
Required action: Apply updates per vendor instructions.
Affected Products
5 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| Exchange ServerMicrosoft | 2013-cumulative_update_23, 2016-cumulative_update_19, 2016-cumulative_update_20, 2019-cumulative_update_8, 2019-cumulative_update_9 | 15.00.1497.015 |
Patched Releases
Exchange Server
15.00.1497.015, 15.01.2176.012, 15.01.2242.008, 15.02.0792.013, 15.02.0858.010
Timeline
Added to Industry KEV feed
Aug 30, 2021
Added to CISA KEV
Nov 3, 2021
Added to CIRCL KEV
Nov 3, 2021
Added to ENISA EUVD
Nov 3, 2021
Publicly disclosed
Sep 14, 2022
Last updated
Aug 10, 2026
Weakness Type
- CWE-918 Server-Side Request Forgery (SSRF)
References
- advisoryhttps://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-34700
- advisoryhttps://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-34726
- advisoryhttps://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-34727
- advisoryhttps://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-34730
- advisoryhttps://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-34731
FAQ
Common Questions.
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected: Exchange Server (Microsoft) versions 2013-cumulative_update_23, 2016-cumulative_update_19, 2016-cumulative_update_20, 2019-cumulative_update_8, 2019-cumulative_update_9. The full list is in the CVE brief.
It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Aug 30, 2021 and is known to be used in ransomware campaigns.
Yes. It was added on Nov 3, 2021 with a federal remediation due date of Nov 17, 2021.
Public exploit code is available.
Fixed versions are available, including Exchange Server 15.00.1497.015, Exchange Server 15.01.2176.012, Exchange Server 15.01.2242.008.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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