Threats / Microsoft / CVE-2021-42321
CVE-2021-42321
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft Exchange vulnerability
Microsoft Exchange vulnerability allowing authenticated attackers to execute remote code through improper validation of cmdlet arguments, exploited in ransomware campaigns.
Verdict
Today item, not a backlog item.
An authenticated attacker can bypass argument validation in Exchange cmdlets to achieve remote code execution. This vulnerability has been actively exploited in the wild for ransomware deployment, representing a critical post-authentication compromise vector.
CISA KEV Yes · 2021-11-175Ransomware use Flagged5EPSS 0.90388 (verify live)6Exploit Public PoC7CVSS 8.83CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Public proof-of-concept exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
11 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 2021-11-17), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.90388 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 8.8 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C) · Affected: Microsoft, Exchange. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-184 Incomplete Blocklist, CWE-502 Deserialization of Untrusted Data — weakness family: Injection.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: no · 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 7 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 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 gain initial access through compromised credentials or prior vulnerability chain to authenticate to Exchange.
Business
Attacker establishes foothold within email infrastructure, bypassing perimeter controls.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I craft malicious cmdlet arguments that bypass validation checks to inject and execute arbitrary code on the Exchange server.
Business
Attacker achieves code execution with Exchange service privileges, enabling lateral movement and data access.
3
Lateral reach — past segmentation narrative 3
Attacker
I deploy ransomware or additional payloads across the compromised infrastructure.
Business
Organization experiences operational shutdown, data encryption, and extortion demands affecting business continuity.
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