CVE-2021-34473 — Exchange Server
CRITICALCVE-2021-34473 is a critical-severity (CVSS 9.1) Server-Side Request Forgery (SSRF) vulnerability in microsoft exchange server. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) and has been used in ransomware campaigns — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.
Microsoft Exchange Server Remote Code Execution Vulnerability
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 — including use in ransomware campaigns. Federal agencies were required to remediate by 2021-11-17.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-34473.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-34473 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 394,415 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected Products
exchange servermicrosoftResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft exchange server deployment and check each version against the affected-products list above.
Remediation status
No patch has shipped for CVE-2021-34473 yet — track the microsoft exchange server advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
How to detect CVE-2021-34473
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2021-34473 -u https://target- Template
- Exchange Server - Remote Code Execution
- Severity
- critical
- Impact
- Successful exploitation of this vulnerability could allow an attacker to execute arbitrary code on the affected Exchange Server, potentially leading to a complete compromise of the system.
- Remediation
- Apply Microsoft Exchange Server 2019 Cumulative Update 9 or upgrade to the latest version.
Template by ProjectDiscovery nuclei-templates (arcc, intx0x80, dwisiswant0), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2021-34473 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.