CISA KEV · catalog date Nov 3, 2021 · first observed Jul 19, 2026
Evidence dossier
CVE-2021-26855
Microsoft Exchange Server Remote Code Execution Vulnerability
Exploited in the wild (CISA KEV since Nov 3, 2021). microsoft reports CVSS 3.1 9.1. EPSS estimates 100.0% exploit likelihood as of Jul 18, 2026.
As of Aug 27, 2026
Normalized restatement
Microsoft Exchange Server Remote Code Execution Vulnerability
- State
- PUBLISHED
- Published
- Mar 2, 2021
- Updated
- Oct 21, 2025
- Evidence coverage
- 92%
Evidence chronology
What was known when
- Source dateSource date omittedFirst observed by CASCACISA-ADPOriginal evidence ↗
Record text: CISA ADP Vulnrichment
Inspect raw assertion
- Field
container- Value
- CISA ADP Vulnrichment
- Source dateSource date omittedFirst observed by CASCACVE ProgramOriginal evidence ↗
Record text: CVE Program Container
Inspect raw assertion
- Field
container- Value
- CVE Program Container
- Source dateSource date omittedFirst observed by CASCAmicrosoftOriginal evidence ↗
Record text: Microsoft Exchange Server Remote Code Execution Vulnerability
Inspect raw assertion
- Field
container- Value
- Microsoft Exchange Server Remote Code Execution Vulnerability
- Source dateFirst observed by CASCACISA KEVOriginal evidence ↗
Exploitation cataloged: Microsoft Exchange Server Remote Code Execution Vulnerability
Inspect raw assertion
- Field
observed_exploitation- Value
- Microsoft Exchange Server Remote Code Execution Vulnerability
- Source dateFirst observed by CASCAFIRST EPSSOriginal evidence ↗
EPSS score: 100% probability · 99.99th percentile
Inspect raw assertion
- Field
model_probability- Value
- Probability 0.999990000000; percentile 0.999910000000
FIRST EPSS · score date Jul 18, 2026 · 100th percentile · first observed Jul 19, 2026
microsoft · CVSS 3.1 · first observed Jul 19, 2026 · same-version scores align in this snapshot
Evidence detail
Source limits and decisions
Why each evidence state has this value
casca-unknown-reasons-v1Eligible evidence is present for this bounded claim.
- Revision
- evidence-policy-v1.1.0
- Cutoff
- Aug 27, 2026
- Resolution
- None
Eligible evidence is present for this bounded claim.
- Revision
- evidence-policy-v1.1.0
- Cutoff
- Aug 27, 2026
- Resolution
- None
Eligible evidence is present for this bounded claim.
- Revision
- casca-direct-cvss-eligibility-v1
- Cutoff
- Aug 27, 2026
- Resolution
- None
The cited source assertion is retained while canonical product linkage remains open.
- Revision
- casca-factor-d-obligations-v1
- Cutoff
- Aug 27, 2026
- Resolution
- Resolve identity
Source comparison
Who said what
CISA ADP Vulnrichment
Inspect raw assertion
- Field
container- Value
- CISA ADP Vulnrichment
CVE Program Container
Inspect raw assertion
- Field
container- Value
- CVE Program Container
Microsoft Exchange Server Remote Code Execution Vulnerability
Inspect raw assertion
- Field
container- Value
- Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Inspect raw assertion
- Field
observed_exploitation- Value
- Microsoft Exchange Server Remote Code Execution Vulnerability
100% probability · 99.99th percentile
Inspect raw assertion
- Field
model_probability- Value
- Probability 0.999990000000; percentile 0.999910000000
Applicability
Cited product scope
Grouped from 1 configuration nodes in this exact snapshot. Visual grouping is navigational; asset exposure and root cause require cited evidence.
Identity source boundaries
- Cpe dictionary1,775,266 records · observed through 2026-07-21T06:45:29.809Z
27d65b0f-b718-4b4f-bb79-c47c68d09dfa - Cpe match643,502 records · observed through 2026-07-21T08:13:17.697Z
955dae73-7302-438b-aee1-058d7cc5d48e
25 scope groups
Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.00.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.00.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.00.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.01.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]Canonical linkage remains open; the cited source assertion is retained below.
Inspect raw assertion
[{"status": "affected", "version": "15.02.0", "lessThan": "publication", "versionType": "custom"}]product-d87daca995d075bf30565cc429e7242a47683272975068df1d327ec1d8ff609cLinked exactInspect raw assertions
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 10
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
079e1e3f-ff25-4b0d-ac98-191d6455a014
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 5
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
ccf101be-27fd-4e2d-a694-c606bd3d1ed7
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 14
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
69af19dc-3d65-49a8-a85f-511085cdf27b
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 9
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
4830d6a9-af74-480c-8f69-8648cd619980
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 12
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
28fca0e8-7d27-4746-9731-91b834ca3e64
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 17
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
ee320413-d2c9-4b28-89bf-361b44a3f0ff
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 7
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
55284cf7-0d04-4216-83fe-4b1f9ca94207
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 3
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
63e362cb-cf75-4b7e-a4b1-d6d84afcbb68
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_7:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 22
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
36ce5c6d-9a04-41f5-ae7c-265779833649
cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 2
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
da166f2a-d83b-4d50-ad0b-668d813e0585
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 19
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
73b3b3fe-7e85-4b86-a983-2c410ffef4b8
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 21
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
d4ab3c25-cea8-4d66-aee4-953c8b17911a
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 20
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
8a9fb275-7f17-48b2-b528-be89309d2af5
cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_21:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 0
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
b560f8fd-068e-4a16-a37f-a62dce88fcf2
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 18
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
104f96dc-e280-4e0a-8586-b043b55888c2
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 4
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
9be04790-85a2-4078-88ce-1787bc5172e7
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 23
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
44ecf39a-1de1-4870-a494-06a53494338d
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 8
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
ca2ce223-aa49-49e6-ac32-59270eff55ad
cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_22:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 1
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
751fd35f-2ecd-4b75-9589-988cc6ad3058
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 11
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
29805ec7-6403-44b9-91ec-109c087e98eb
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 16
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
051de6c4-7456-4c42-bc51-253208aadb4e
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 6
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
4df5bdb5-205d-4b64-a49a-0152afcf4a13
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 13
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
075e907f-af2f-4c31-86c7-51972be412a1
cpe:2.3:a:microsoft:exchange_server:2019:-:*:*:*:*:*:*- Official link
- Linked exact
- Role
- Vulnerable target
- Configuration
- 0 · node/0 · match 15
- Logic
- OR
- Version bounds
- No explicit bounds
- Match ID
40d8a6db-9225-4a3f-ad76-192f6cccf002
Affected-product evidence
Accepted scope and product mapping
1 canonical links · 1 source-reported links
vendor-01299a5a765aea70faa05f2d06e0c26a5d498fd34553e868d89e8a43b31864db · product-d87daca995d075bf30565cc429e7242a47683272975068df1d327ec1d8ff609c
- Source class
- Nvd cpe vulnerable target
- Assertions
- 24
- Mapping revision
- cpe23-exact-mapping-v1
- Observed cutoff
- Aug 27, 2026
Retained assertion IDs
2e3b2f1e-431f-4c43-be50-3f1dfa010f7e4129311f-95f7-47a7-a273-82493738732847ca13b5-4621-4902-9ae2-dc43106291e9591406e3-aad2-44a5-bf2c-8a6566b5a5a55a1816d1-0a53-4441-9c52-bb12739b5487626cd30b-19b4-4413-b385-2f1140fe362965dfe2dc-87ba-4e32-8d53-696c124fca176ea492bb-be17-4b23-beb5-e074bd6a49e76f960436-417b-4da0-8801-9d7fe141f6e689d9e94c-124f-4798-8073-6d5fed2400eb965e9ed5-ca64-4030-bab2-775e5cc12feb96d11f53-fc8b-4611-bdee-6967d536a8cca2645528-76c7-403b-8a1b-3367ebfc7051a7c55138-dba0-444a-afa6-9b18d0d23401a8d047e7-3be2-4abb-b154-23c129a738e5b4334514-b5af-4a9f-bd58-c8afdf138a9bb994656d-9d92-4100-bffb-a8d1afa5b982bbc01f6b-6bee-452e-9f62-1cc29266dcb7c5719e21-f2c6-489a-b4b9-6b1843c97aa7c5c9ddd6-24d3-492c-a8e8-4eb4463dd75ac78224b6-6404-4a9e-a9e5-e556bab2408cd1391bef-7dfc-40b0-94ef-198f5157aa43de25cba0-d6e6-4104-99cd-1e6d5236128bffcc3a53-aebf-497f-8145-89a4212053b4Canonical linkage remains open; the cited source assertion is retained below.
Vendor specified only by source · Product specified only by source
- Source class
- Direct cve affected
- Assertions
- 24
- Mapping revision
- cpe23-exact-mapping-v1
- Observed cutoff
- Aug 27, 2026
Retained assertion IDs
0afc0480-b5d6-442b-aa71-2aafc3bda13623748520-2877-4034-b324-6f0958100a8b2cfaefe0-d6c2-4b78-b2d7-78bbb7c1c7a0319f870e-2386-4b61-b498-a9af82150ffa417ac340-86b2-4659-ac25-27c726c4ccb6455aca86-36c5-4f48-a004-ff4a73fbcf364a139d29-5e20-4d3e-a1f1-cad9830acd8753806b4b-7aed-4c25-932e-b6f350fd151b5ea4fbaf-9f09-47a7-8e59-dcf5bad7552762ba2915-ace4-4204-b198-9523b3bb62a76b2e4409-21ba-4632-a64d-88a3589940ae8c90bbfa-a99a-42c3-b211-0ccf6c313a5d8d35b2f7-f440-43d4-9bd1-4ac7548b4d4191799291-6f30-4f1f-a970-194c9056fe9ca0db2b2d-f132-453c-b145-ea88e2c3ac53a37390e6-20e9-4909-a55d-5b45f1f6be68bd5eaaf1-b2c0-4a5b-9da3-0ea78124b25ec0b0ff45-ac7b-471e-89f0-b4ae9f53329ad8d6724c-be35-4424-a8fc-bd64e1939afcdd2f77ad-0d3e-42b8-8c9d-f8ee539cba4edd7ee9c1-1726-46b5-a9a7-a0057036e923e04176c8-a5d6-4708-a98b-5f1a79027140f9438ca0-1f5e-43a1-a50b-3645e16abf58f95e312b-9d21-425c-9329-02e03b5c7885Assessments
CVSS by origin
AV:N/AC:L/Au:N/C:P/I:P/A:PCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:CDirect CVE/CNA normalized decisions
microsoft
CVSS 3.1 · Primary · Original assertion · rank 1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C- Validation
- Valid match
- Recomputed
- 9.1
- Decision reason
- Evidence supported
- Policy
- casca-direct-cvss-eligibility-v1
Assessments are retained side by side under closed precedence. Cascade never averages CVSS.
Evidence boundaries
- KEV membership is authoritative for the catalog, not proof of exposure in any environment.
- EPSS is a dated model probability and not an individual-environment prediction.
- Affected or fixed status applies only to the exact cited product and version scope; remaining scope stays source-scoped.
- NVD-carried upstream facts remain derivative; independent corroboration requires a separately authored source.
- Only NVD metrics validated under the generation-bound calculator are Public Priority eligible; direct CVE record metrics remain display-only.
- NVD CVSS source eligibility is closed: NVD-authored, exact record-source, or registered same-CVE container origin; unmapped sources remain display-only.
- Core replay supports the active and immediately prior generation; retrospective valid-at replay is deferred.
- OSV aggregation and OSV-converted NVD material are not independent corroboration.
- Red Hat facts are vendor assertions for the exact supplied products.
- Nonmembership and not-yet-observed states are not proof of safety.