What is CVE-2021-34473?
Microsoft Exchange Server Remote Code Execution Vulnerability
The record classifies CVE-2021-34473 under weakness class CWE-918. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity. 5 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 1933 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 9.1 — CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N - EPSS (FIRST)
- 99.9% probability of exploitation in the next 30 days, higher than 99.9% of all scored CVEs
- CISA KEV
- Listed since 2021-11-03, federal remediation deadline 2021-11-17 — used in known ransomware campaigns
- Threadlinqs priority
- 10/10 — CISA lists it as used in ransomware, which Threadlinqs scores at the maximum
- Published
- 2021-07-14, last modified 2026-08-10
Is CVE-2021-34473 being exploited?
CISA added CVE-2021-34473 to the Known Exploited Vulnerabilities catalog on 2021-11-03, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2021-11-17 under BOD 22-01. CISA flags the vulnerability as one used in known ransomware campaigns. Weaponised exploit code for CVE-2021-34473 is publicly available. 10 public proof-of-concept repositories are tracked for this identifier. A ProjectDiscovery Nuclei detection template exists (http/cves/2021/CVE-2021-34473.yaml). It currently carries a trending score of 50 in the Threadlinqs vulnerability feed.
- horizon3ai/proxyshell (github)
- cyberheartmi9/Proxyshell-Scanner (github)
- kh4sh3i/ProxyShell (github)
- p2-98/CVE-2021-34473 (github)
- je6k/CVE-2021-34473-Exchange-ProxyShell (github)
- RaouzRouik/CVE-2021-34473-scanner (github)
- ipsBruno/CVE-2021-34473-NMAP-SCANNER (github)
- learningsurface/ProxyShell-CVE-2021-34473.py (github)
Affected products and versions
- Microsoft: Exchange Server 2013, Exchange Server 2016, Exchange Server 2019
Showing 3 of 5 recorded product entries.
How to fix CVE-2021-34473
The record marks a vendor fix as available for CVE-2021-34473. Patch reference: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34473. Vendor advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34473. Because CVE-2021-34473 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2021-11-17. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2021-34473
4 tracked threats in the Threadlinqs corpus reference CVE-2021-34473, either in the campaign’s CVE list or as an indicator on the campaign record.
- FamousSparrow APT Targets Azerbaijani Oil & Gas Sector via ProxyShell/ProxyNotShell Exchange Exploitation — CRITICAL · 2026-08-23
- BlackCat/ALPHV Ransomware Abuses Azure Storage Account Keys via Sphynx Encryptor to Mass-Encrypt Cloud Storage — HIGH · 2026-07-26
- FamousSparrow APT Targets Azerbaijani Oil & Gas Industry via Exchange ProxyShell/ProxyNotShell (Deed RAT, Terndoor, Mofu Loader) — CRITICAL · 2026-06-10
- UAT-8099 (China) BadIIS Malware Campaign Targeting IIS Servers for SEO Fraud — HIGH · 2026-02-02
Sources
Enriched from CVE.org, FIRST EPSS, CISA KEV, public proof-of-concept repositories, ProjectDiscovery Nuclei. Last verified by Threadlinqs on .