Vulnerability

CVE-2021-26855

Critical· 9.1KEVEXPLOITCVSS9.1EPSS94%

.rules3/5

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Threat signal

100/100

Severe

Severity

critical

CVSS

9.1

EPSS

94%

Active

KEV

CISA KEV — actively exploited

Ransomware: Known1502d overdue

AffectedMicrosoft — Exchange Server

Added to KEV

2021-11-03

Federal patch due (BOD 22-01)

2022-05-03

Required action (CISA verbatim)

Apply updates per vendor instructions.

Vendor advisory + references

https://nvd.nist.gov/vuln/detail/CVE-2021-26855

CISA description: “Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.”

CVSS

v3.1Open in FIRST calculator →

9.1/ 10CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack VectorNetworkExploitable remotely over the internet — no local access needed.
Attack ComplexityLowNo special conditions — exploit works reliably.
Privileges RequiredNoneUnauthenticated — no credentials needed.
User InteractionNoneFully automated — no victim action needed.
ScopeUnchangedImpact contained to the vulnerable component.
Confidentiality ImpactHighTotal disclosure of sensitive data.
Integrity ImpactHighTotal compromise of integrity — full data modification possible.
Availability ImpactNoneNo availability impact.

EPSS

Exploit prediction · 30-day horizonFIRST.org →

94.3%

probability of exploitation in next 30 days

p100

higher than 100% of all CVEs

3-day trend↑ 0pp

Statistically about to be weaponized — top-tier triage urgency.

Vendor VEX

No VEX statements published for CVE-2021-26855. Vendors publish VEX (Vulnerability Exploitability eXchange) to assert per-product whether a CVE is actually exploitable in their distribution.

SSVCAct

Active exploitation with total impact on essential/support systems

exploitation: active
automatable: yes
impact: total
mission: support

Lifecycle

2021-11-03Added to CISA KEVransomware: Known
2021-11-03Public exploit observedexploitdb, metasploit
2021-11-03Published
2022-05-03Federal patch due (BOD 22-01)
2026-06-12Last modified1 source

No source-event history recorded yet. New revisions will appear here as ingestors re-fetch this CVE.