medium Known exploitedREV. 01
CVE-2021-31207ProxyShell — Exchange Upload Turns Access Into a Web Shell
Microsoft Exchange accepts a file where it should not, the last link of the ProxyShell chain that leaves a lasting foothold on the mail server.
Read the risk
THE VITAL STATS- CVSS score
- 6.6/ 10 CVSS v3.1
- EPSS probability
- 100% Likelihood of exploitation
- Attack complexity
- High Conditions needed to exploit
- Known exploited
- Yes — CISA KEV
- CVE published
- 2021-05-11
- Added to KEV
- 2021-11-03
- Exploit published
- Yes
- Confidence
- high
- Kill chain
- persistence
- MITRE ATT&CK
- T1190, T1505.003
- Severity
- medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
Attacker needsAn elevated session, reached through the earlier links of the chain
Behind the card
5 REFERENCES- 01CVE recordNVD recordnvd.nist.gov
- 02CISACISA Known Exploited Vulnerabilities catalogcisa.gov
- 03Vendor advisoryportal.msrc.microsoft.comportal.msrc.microsoft.com
- 04Exploitpacketstormsecurity.compacketstormsecurity.com
- 05Writeupzerodayinitiative.comzerodayinitiative.com
Field-level provenanceTRACE THE SOURCES +
Which source supports each field, when it was retrieved, and who extracted it.
| Fields | Source | Retrieved | Extractor | Confidence |
|---|---|---|---|---|
| cvss, exploitation.attack_complexity, exploitation.exploit_available, references, published_at | nvd.nist.gov | 2026-08-07 | scrty-crds-pipeline/0.1 | high |
| exploitation.known_exploited, exploitation.kev_added | cisa.gov | 2026-08-07 | scrty-crds-pipeline/0.1 | high |
| epss | api.first.org | 2026-08-07 | scrty-crds-pipeline/0.1 | high |
| title, summary, severity, remediation, mitre_attack, kill_chain, recommended_action, confidence, tags, exploitation.prerequisites | nvd.nist.gov | 2026-08-07 | claude-code | high |
Printing history
THE RECORD- r1Initial base card
First printing. This card has not been reprinted since publication.