Threats / Microsoft / CVE-2021-26857
CVE-2021-26857
· 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 through the ProxyLogon exploit chain, actively exploited in ransomware campaigns.
Verdict
Today item, not a backlog item.
Deserialization flaw (CWE-502) in Exchange Server allows unauthenticated remote code execution. Actively exploited in the wild for ransomware deployment. Critical severity due to widespread Exchange deployments and demonstrated ransomware campaign activity.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.95762 (verify live)6Exploit Weaponized · public PoC7CVSS 7.83CISA SSVC Exploitation: active · Automatable: no · 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
50 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 0.95762 — 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-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 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.
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.13
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I exploit the deserialization vulnerability to achieve unauthenticated remote code execution on exposed Exchange servers.
Business
Attackers gain initial compromise of email infrastructure without requiring valid credentials, enabling lateral movement and data exfiltration.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I chain this vulnerability with other ProxyLogon exploits to escalate privileges and establish persistent access within the organization.
Business
Compromised Exchange servers become staging points for ransomware deployment across the enterprise network.
3
Lateral reach — past segmentation narrative 3
Attacker
I deploy ransomware payloads from the compromised Exchange infrastructure to encrypt critical business systems.
Business
Widespread operational disruption, data encryption, and financial extortion demands targeting organizations reliant on email and file services.
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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