critical Known exploitedREV. 01
CVE-2021-34473Microsoft Exchange Server — ProxyShell Remote Code Execution
A request-forwarding flaw in Microsoft Exchange Server lets an unauthenticated attacker reach internal services and run code on the mail server. It is the entry point of the exploit chain published as ProxyShell.
Read the risk
THE VITAL STATS- CVSS score
- 9.8/ 10 CVSS v3.1
- EPSS probability
- 100% Likelihood of exploitation
- Attack complexity
- Low Conditions needed to exploit
- Known exploited
- Yes — CISA KEV
- CVE published
- 2021-07-14
- Added to KEV
- 2021-11-03
- Exploit published
- Yes
- Confidence
- medium
- Kill chain
- initial access
- MITRE ATT&CK
- T1190
- Severity
- critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Attacker needsNetwork access to an unpatched on-premises Exchange Server. No credentials needed.
Behind the card
4 REFERENCES- 01CVE recordNVD recordnvd.nist.gov
- 02Vendor advisoryMicrosoft security update guideportal.msrc.microsoft.com
- 03ExploitPublic ProxyShell exploitpacketstormsecurity.com
- 04CISACISA Known Exploited Vulnerabilities entrycisa.gov
Field-level provenanceTRACE THE SOURCES +
Which source supports each field, when it was retrieved, and who extracted it.
| Fields | Source | Retrieved | Extractor | Confidence |
|---|---|---|---|---|
| title, summary, cvss, severity, exploitation.attack_complexity, exploitation.exploit_available, remediation.patch_available, references | nvd.nist.gov | 2026-08-07 | claude-code | medium |
| exploitation.known_exploited, exploitation.prerequisites, recommended_action | cisa.gov | 2026-08-07 | claude-code | high |
| epss | api.first.org | 2026-08-07 | claude-code | high |
Printing history
THE RECORD- r1Initial base card
First printing. This card has not been reprinted since publication.