critical Known exploitedREV. 01
CVE-2021-26855ProxyLogon — Exchange Server-Side Request Forgery
Microsoft Exchange accepts a request and forwards it onward on the caller's behalf, letting an unauthenticated attacker act as the server and reach mailboxes — the opening move of the ProxyLogon chain.
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
- 2021-03-03
- Added to KEV
- 2021-11-03
- Exploit published
- Yes
- Confidence
- high
- 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 the Exchange web endpoint
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
- 05Exploitpacketstormsecurity.compacketstormsecurity.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.