CVE-2021-26855 · ProxyLogon
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Microsoft Exchange Server Remote Code Execution Vulnerability
rank 70 = 40 + 19 + 0 + 5 + 6
rank@v1 · act now
Someone is already using it
CISA KEV catalog · public proof-of-concept · FIRST EPSS
EPSS 0.99996 · CISA KEV listed · without this subject the record ranks 30
What it costs you if it fires
NVD CVSS base score, then the CNA's own
CVSS 7.5, from NVD · without this subject the record ranks 51
It is getting worse, not staying still
FIRST EPSS series, rescorings and bad loads removed
no persisting rise in 365 days · without this subject the record ranks 70
How much of the world it touches
NVD CPE configurations
3 products in the CPE ranges · without this subject the record ranks 65
Someone else already set the date
CISA KEV remediation due date
CISA remediation date held · without this subject the record ranks 64
$4.40M = $4.99M incident cost x 0.8814887365328109 exposure x 0.99996 likelihood
udv@v1 · unaudited · wrong for you until you say what you run
$4.99M is the cost of a breach that happened, from IBM Cost of a Data Breach Report 2026, global average. It is not the value of any asset of yours. That average already covers breaches of every severity, so scaling it by exposure understates a partial-impact record. That is an assumption, not a measurement. Your own asset value goes in the decision record.
Hindsight · the state
Foresight · where to check next
This record is one instance of CWE-918, server-side request forgery (ssrf). The index holds 3591 prior instances of this class, 23 KEV-listed. The nearest of its KEV-listed records are drawn beside this. taxonomy@v1
Sources disagree here.
Both shown, dated. We don't pick winners. disagreement-rules@v1
Running Microsoft Exchange Server 2013 Cumulative…? Check your version
A deterministic range check against the ranges this record publishes. It tells you where a version sits in the record, not whether your deployment is exploitable. No account needed.
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Harm, if it fires from the vector and the weakness
Written for the ticket you're about to file.
CISA KEV & Federal Remediation Deadline from the record and CISA
The fix, and how long it's been waiting.
Exploit Status & Known PoCs a ladder, not a score
Current rung: Ransomware campaign use known, 2021-11-03.
Affected & Fixed Versions as published
A rung lights from a row in the index, never from an assessment. "Held sources" is the true scope of the claim: this index reads the CVE record, NVD, CISA KEV, FIRST's EPSS and public proof-of-concept repositories, and it does not observe attacks.
What it is stated by the CNA
ProxyLogon is the common name for CVE-2021-26855. Microsoft Exchange Server Remote Code Execution Vulnerability cve-aliases@2026-08-27
NVD's analysis a second authority
- severity
- 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P
- weakness
- CWE-918
- affected versions
- microsoft exchange server 2013; microsoft exchange server 2016; microsoft exchange server 2019
- also scored by
- [email protected] 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N · [email protected] 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
The CNA scored it 9.1 and NVD 7.5. Both are shown because both were published; we do not pick one.
check it yourself msrc.microsoft.com receipt
Dated events
Movement the material steps
Show all 136 observations
136 observations held. A highlighted step crossed a promotion threshold. A step marked with a scoring model moved because the instrument was replaced, not because the vulnerability changed. receipt
The full biography
Listed by CISA 246 days after it published and the record had been public for 5 years, 6 months when this index was last written.
No material movement in 15 days as of the last ingest this index recorded.
The pattern, elsewhere
This record is one instance of CWE-918, server-side request forgery (ssrf). The index holds 3591 prior instances of this class, 23 KEV-listed. The nearest of its KEV-listed records are drawn beside this. [email protected]
"Matches n of m" counts the attributes the two records can both be compared on: the CVSS metrics they share a version of, any weakness class beyond this one, and the vendor. fit-rules@v1
3591 records in this class held. The six above are drawn from the ones CISA has listed as exploited, ranked by how much of the record matches this one. That is why every one of them is KEV-listed: the pool is, so the colour on those spokes says nothing the pool has not already said.
No curated product neighbourhood is held for this product, so the records above are the nearest this index can name. The neighbourhood table covers a small set of products and is extended by hand.
Siblings and hubs
Records related to this one
Browse the index from here
These are index-wide views, not findings about CVE-2021-26855.
What usually comes next
Everything above needs no account. What is below is work: assembling it, scoping it to your estate, watching it, and handing it to someone else.