Threats / Microsoft / CVE-2021-26858
CVE-2021-26858
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft Exchange Server vulnerability
Microsoft Exchange Server remote code execution vulnerability exploited in the wild as part of the ProxyLogon attack chain, enabling unauthorized system compromise.
Verdict
Today item, not a backlog item.
Critical remote code execution in Exchange Server with active exploitation and ransomware deployment. Immediate patching required for all affected installations.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.89509 (verify live)6CVSS 7.83CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
49 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 0.89509 — 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.
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.12
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
Gain unauthenticated access to Exchange Server through pre-authentication vulnerability.
Business
Attacker establishes initial foothold without valid credentials, bypassing primary security controls.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
Execute arbitrary code on the Exchange Server with system privileges.
Business
Complete compromise of email infrastructure and underlying server, enabling data theft and lateral movement.
3
Lateral reach — past segmentation narrative 3
Attacker
Deploy ransomware or persistence mechanisms across the compromised environment.
Business
Operational disruption, data encryption, extortion demands, and potential regulatory violations from data exposure.
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