CVE-2021-31207 — Exchange Server
MEDIUMCVE-2021-31207 is a medium-severity (CVSS 6.6) Unrestricted File Upload 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 Security Feature Bypass 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.
- 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-2021-31207.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-31207 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 395,249 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Microsoft Exchange Server Security Feature Bypass 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 Security Feature Bypass Vulnerability
Microsoft Exchange Server Security Feature Bypass Vulnerability
Microsoft Exchange Server Security Feature Bypass Vulnerability
Microsoft Exchange Server Security Feature Bypass 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-2021-31207 yet — track the microsoft exchange server advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
Frequently Asked Questions
Is CVE-2021-31207 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.