Threats / Microsoft / CVE-2021-27065
CVE-2021-27065
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-09-19
Microsoft Exchange Server vulnerability
Microsoft Exchange Server remote code execution vulnerability enabling unauthenticated attackers to execute arbitrary code. Part of the ProxyLogon exploit chain actively exploited in ransomware campaigns.
Verdict
Today item, not a backlog item.
Critical remote code execution in Exchange Server with high exploitation prevalence. Actively weaponized in ransomware operations. Immediate patching required for all affected Exchange deployments.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.99876 (verify live)6Exploit Weaponized · public PoC7CVSS 7.83CISA SSVC Exploitation: active · Automatable: yes · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Fully weaponized — public exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
67 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.99876 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.8 (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C) · Affected: Microsoft, Exchange Server. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-39 CWE-39 — weakness family: Path traversal / file.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: yes · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 246 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a functional/weaponized public exploit). Catalog entries: Metasploit module. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyHAFNIUM State-sponsored (PRC)
CISA's AA21-062A and the Microsoft Threat Intelligence Center attribute the ProxyLogon Exchange exploitation chain to HAFNIUM (now tracked as Silk Typhoon), a state-sponsored group assessed to operate out of the PRC, naming both the group and the Exchange CVEs.16
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I gain unauthenticated access to the Exchange Server through the vulnerability without requiring valid credentials.
Business
Attackers bypass authentication controls, eliminating the first security perimeter and gaining direct code execution capability.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I execute arbitrary code with Exchange Server process privileges to establish persistence and lateral movement.
Business
Compromised Exchange Server becomes a beachhead for enterprise-wide network infiltration and data exfiltration.
3
Lateral reach — past segmentation narrative 3
Attacker
I deploy ransomware payloads across the organization using the compromised Exchange infrastructure.
Business
Critical business systems are encrypted and held for ransom, causing operational shutdown and financial extortion losses.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05
Coverage & confidence
— what we know, and what we don’tEstablished (cited)
Coverage gaps — stated, not hidden
Found an error? Propose a correction.
A correction is a cited counter-claim — it must cite a source,
gets reviewed by a second human, and is never auto-applied. See the
correction convention and our identity
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