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CVE-2021-26855

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS 9.1 (CRITICAL) KEV: KNOWN CATALOG

Attention and evidence

HYPE SCORE
0.0/10
REALITY · THREAT PRIORITY
8.9/10
General review priority; not probability

Evidence confidence: Exploitation recorded

Why this priority?
  • Recorded exploitation baseline: +7.00
  • Technical severity: CVSS 9.1: +1.36
  • Fresh EPSS: 100.00%: +0.50

General review priority, not probability, victim count or asset-specific risk. A range covers missing CVSS/EPSS inputs within the current evidence state.

Sources: 26 documents and public datasets.

Hype components

Headline sample (25%)
0/100
Press, 30 days (60%)
0/100
Grounded branding (15%)
0/100
HYPE TOTAL0/100

Sources and documents

Barracuda Blog
Press · 03 Sep 2026, 07:00 · Threat Spotlight: Unpatched software vulnerabilities - Barracuda Blog
Barracuda Networks Blog
Press · 09 Dec 2025, 08:00 · Barracuda detects increased probing for Microsoft Exchange vulnerabilities - Barracuda Networks Blog
Barracuda Blog
Press · 05 Dec 2025, 08:00 · Cybersecurity Threat Advisory: Global Microsoft Exchange attack - Barracuda Blog
Rescana
Press · 10 Oct 2024, 07:00 · Mitigating CVE-2021-26855: Critical SSRF Vulnerability in Microsoft Exchange Server - Rescana
Cisco Blogs
Press · 27 Sep 2022, 07:00 · Threat Trends: Vulnerabilities - Cisco Blogs
See all 26 source documents
Qualys
Press · 06 May 2022, 07:00 · CISA Alert: Top 15 Routinely Exploited Vulnerabilities - Qualys
www.trendmicro.com
Press · 19 Nov 2021, 08:00 · Squirrelwaffle Exploits ProxyShell and ProxyLogon to Hijack Email Chains - www.trendmicro.com
Check Point Research
Press · 26 Sep 2021, 07:00 · 27th September – Threat Intelligence Report - Check Point Research
Arctic Wolf
Press · 03 Sep 2021, 07:00 · ProxyToken: Authentication Bypass Vulnerability in On-Premises Microsoft Exchange - Arctic Wolf
Qualys
Press · 28 Jul 2021, 07:00 · CISA Alert: Top Routinely Exploited Vulnerabilities - Qualys
Microsoft
Press · 18 Mar 2021, 07:00 · Automatic on-premises Exchange Server mitigation now in Microsoft Defender Antivirus - Microsoft
Palo Alto Networks
Press · 16 Mar 2021, 07:00 · Accelerate SecOps Investigation and Response to the Microsoft Exchange Server Attack with Cortex XSOAR - Palo Alto Networks
Help Net Security
Press · 15 Mar 2021, 07:00 · As attacks on Exchange servers escalate, Microsoft investigates potential PoC exploit leak - Help Net Security
Cyberint
Press · 14 Mar 2021, 08:00 · HAFNIUM – Microsoft Exchange Server Vulnerability - Cyberint
Unit 42
Press · 11 Mar 2021, 08:00 · Microsoft Exchange Server Attack Timeline: Discoveries and Mitigations - Unit 42
WeLiveSecurity
Press · 10 Mar 2021, 08:00 · Exchange servers under siege from at least 10 APT groups - WeLiveSecurity
Morphisec
Press · 10 Mar 2021, 08:00 · How to Protect Yourself From the Microsoft Exchange Vulnerabilities - Morphisec
Unit 42
Press · 09 Mar 2021, 08:00 · Remediation Steps for the Microsoft Exchange Server Vulnerabilities - Unit 42
The Hacker News
Press · 08 Mar 2021, 08:00 · Microsoft Exchange Cyber Attack — What Do We Know So Far? - The Hacker News
Cloudflare Blog
Press · 07 Mar 2021, 08:00 · Protecting against recently disclosed Microsoft Exchange Server vulnerabilities: CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065 - Cloudflare Blog
Cisco Talos Blog
Press · 04 Mar 2021, 08:00 · Threat Advisory: HAFNIUM and Microsoft Exchange zero-day - Cisco Talos Blog
Securelist
Press · 04 Mar 2021, 08:00 · Zero-day vulnerabilities in Microsoft Exchange Server - Securelist
Unit 42
Press · 03 Mar 2021, 08:00 · Threat Assessment: Active Exploitation of Four Zero-Day Vulnerabilities in Microsoft Exchange Server - Unit 42
Qualys
Press · 03 Mar 2021, 08:00 · Microsoft Exchange Server Zero-Days (ProxyLogon) – Automatically Discover, Prioritize and Remediate Using Qualys VMDR - Qualys
Microsoft
Press · 02 Mar 2021, 08:00 · HAFNIUM targeting Exchange Servers with 0-day exploits - Microsoft
The Record from Recorded Future News
Press · 01 Mar 2021, 08:00 · Microsoft: Chinese APT Targeted Exchange Servers With Four Zero-Days - The Record from Recorded Future News

Score evolution

● Hype   ● Reality

Earlier evidence-support scores are retained in history; this chart shows only the current priority definition.

First record: hype 0.0 · reality 8.9 (2026-11-02 15:12). The curve appears once the score changes.

Public exploit evidence

13

Public exploit entries / repo candidates

See 13 repos with metrics
RepositoryStarsForksLanguageUpdated
Flangvik/SharpProxyLogon24741C#31 Mar 2021, 11:57
herwonowr/exprolog18539Python19 Oct 2021, 07:07
hackerschoice/CVE-2021-268555932Python11 Mar 2021, 20:58
alt3kx/CVE-2021-26855_PoC5329Python12 Mar 2021, 16:19
ZephrFish/Exch-CVE-2021-268552815Python31 Jul 2026, 00:59
pussycat0x/CVE-2021-26855-SSRF2412Python10 Mar 2021, 07:41
srvaccount/CVE-2021-26855-PoC1719Go09 Mar 2021, 21:59
thau0x01/poc_proxylogon81Python10 Feb 2022, 15:20
Yt1g3r/CVE-2021-26855_SSRF46Python11 Mar 2021, 03:28
KotSec/CVE-2021-26855-Scanner32Go12 Mar 2021, 12:55
1342486672/Flangvik00—07 Jun 2022, 02:09
timb-machine-mirrors/testanull-CVE-2021-26855_read_poc.txt00—04 Jan 2024, 22:48
yaoxiaoangry3/Flangvik00—30 Oct 2021, 11:31
Public exploit evidence
Evidence context and dated references

Reality is general threat/review priority, combining observed exploitation, CVSS severity and fresh EPSS. Evidence confidence is separate. This is not calibrated risk, a victim count or an asset-specific assessment. A range reflects missing numeric inputs.

Evidence in context

Confidence in exploitation evidence
Exploitation recorded
Priority basis
Recorded exploitation; CVSS 9.1; EPSS 100.0%
Attention, last 30 days
0 distinct story titles; 0 press documents; 0 publisher domains.
Technical potential
network access, no prior privileges, no user interaction.
Observed activity
Exploitation recorded in CISA KEV.
Known scale
Not quantified.
Evidence confidence basis
Trusted exploitation confirmation

Score evaluated: 2026-11-05 13:58:24.835925+00:00 · EPSS snapshot: 2026-11-05T12:03:04Z

Source event timeline
First press mention
01 Mar 2021, 08:00
Published
02 Mar 2021, 23:55
Added to the CISA KEV catalog
03 Nov 2021, 00:00
Latest press mention
03 Sep 2026, 07:00
Score calculated
05 Nov 2026, 13:58
PoCs on GitHub
13 repositories
Source: CVE List V5 · CISA KEV · FIRST EPSS · GitHub · RSSVulnRadar