Threats / Microsoft / CVE-2021-34473
CVE-2021-34473
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-10
Microsoft Exchange Server vulnerability
Microsoft Exchange Server remote code execution vulnerability exploited in ransomware campaigns. SSRF vector enables unauthenticated attackers to execute arbitrary code with system privileges.
Verdict
Today item, not a backlog item.
Critical vulnerability actively exploited in the wild for ransomware deployment. High EPSS score reflects widespread exploitation. Organizations running vulnerable Exchange versions face immediate compromise risk without patching.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.99999 (verify live)6Exploit Weaponized · public PoC7CVSS 9.13CISA 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
962 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 0.99999 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 9.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/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-918 Server-Side Request Forgery (SSRF).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 112 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, 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 urgencyAPT40 State-sponsored (PRC)
The CISA-led joint advisory AA24-190A names APT40 (tracked in ATT&CK as Leviathan) as a PRC Ministry of State Security group that rapidly weaponizes newly public vulnerabilities, naming the ProxyShell Exchange chain, Log4Shell, and Atlassian Confluence CVEs alongside the group.16
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious request exploiting the SSRF flaw to bypass authentication and reach internal Exchange services.
Business
Attacker gains unauthenticated access to email infrastructure, bypassing perimeter defenses.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I execute arbitrary code on the Exchange Server with system-level privileges through the vulnerability.
Business
Attacker achieves remote code execution and establishes persistent access to critical messaging systems.
3
Lateral reach — past segmentation narrative 3
Attacker
I deploy ransomware payloads across the organization's network from the compromised Exchange Server.
Business
Organization experiences widespread encryption of files, email systems, and backups, triggering operational shutdown.
4
Data at risk — exfiltration narrative 4
Attacker
I exfiltrate sensitive data and demand ransom payment while threatening public disclosure.
Business
Organization faces financial extortion, regulatory penalties, reputational damage, and potential data breach notifications.
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 itNo finder named in the public CVE record — the work behind this find is unattributed.
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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