Threats / Microsoft / CVE-2021-33766
CVE-2021-33766
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-09-19
Microsoft Exchange Server vulnerability
Microsoft Exchange Server authentication bypass allows unauthenticated attackers to disclose email traffic. Actively exploited in the wild with high exploit probability.
Verdict
Today item — known-exploited.
An unauthenticated remote attacker can bypass authentication controls on Exchange Server to intercept and read email communications. This information disclosure vulnerability has been actively exploited and poses immediate risk to email confidentiality.
CISA KEV Yes · 2022-01-185EPSS 0.98136 (verify live)6Exploit Weaponized · public PoC7CVSS 7.33CISA SSVC Exploitation: active · Automatable: yes · Impact: partial4
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
5 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 2022-01-18).
Probability (EPSS)
EPSS 0.98136 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/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-287 Improper Authentication — weakness family: Authentication.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: partialCISA'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 188 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 public proof-of-concept / detection template). Catalog entries: nuclei template. 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 urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I send a crafted request to the Exchange Server without valid credentials.
Business
Attackers gain unauthorized access to email systems, bypassing authentication controls.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I intercept and exfiltrate email traffic and sensitive communications from the target organization.
Business
Confidential business communications, trade secrets, and personal data are exposed to unauthorized parties.
3
Lateral reach — past segmentation narrative 3
Attacker
I use stolen email content for espionage, blackmail, or competitive intelligence.
Business
Organization suffers reputational damage, regulatory penalties, and loss of competitive advantage.
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
Disclosure & credit2
Catalogued by microsoftCNA
Credited with finding itLE XUAN TUYEN - VNPT ISC working with Trend Micro Zero Day Initiativefinder
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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