Evidence used
- CISA confirms exploitation in the wild.
- A structured source references public exploit or proof-of-concept material.
- EPSS is 91.74% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-42321High confidence
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft · Exchange
8.8HighCVSS 3.1
Patch now
Patch availableCISA confirms exploitation in the wild and lists 2021-12-01 as the remediation due date.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:Critical, raised one band.upgradedsince 17 Nov 2021
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Increase monitoring for the attack path and post-exploitation behaviour described in the report.
Verification
- Confirm that the asset runs Microsoft Exchange and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
CISA confirms exploitation in the wild and lists 2021-12-01 as the remediation due date.
Patch available01
What, why and how
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.
What
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.
Why
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete.
How
An attacker operating through a network path may attempt exploitation with low privileges. If successful, the issue may execute code or commands in the affected security context.
What
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.
Why
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete.
How
An attacker operating through a network path may attempt exploitation with low privileges. If successful, the issue may execute code or commands in the affected security context.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation Confirmed in the wild
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-17. Known ransomware campaign use is recorded.
Public PoC / exploit material Reference recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.
- Likely attack path
- a network path → Incomplete List of Disallowed Inputs → execute code or commands in the affected security context
- Attack surface
- Network
- Privileges required
- Low: a basic authenticated account is required
- User interaction
- None
- Attack complexity
- Low: no specialised conditions are recorded
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-184 ↗
CWE-184: Incomplete List of Disallowed Inputs. The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:CCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.- Post-exploitation / living off the land
- After compromise, an attacker may use built-in shells, scripting engines, scheduled tasks and native network utilities for discovery, persistence or movement. This is a plausible LoTL path, not evidence that it has occurred for every attack.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Patch available
- Affected
- Microsoft Exchange Server 2019 Cumulative Update 11: 15.02.0 < 15.02.0986.014
- Microsoft Exchange Server 2016 Cumulative Update 21: 15.01.0 < 15.01.2308.020
- Microsoft Exchange Server 2019 Cumulative Update 10: 15.02.0 < 15.02.0792.019
- Microsoft Exchange Server 2016 Cumulative Update 22: 15.0.0 < 15.01.2375.017
- Action
- Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
- Workaround
- No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04
Evidence and provenance
Published 10 Nov 2021 · Last source change 19 Aug 2026, 16:27 UTC · CWE-184 · Incomplete List of Disallowed Inputs
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-29296
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCISA KEV
NVD statusNVD enriched
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- 15.01.0 < 15.01.2308.020; 15.0.0 < 15.01.2375.017; 15.02.0 < 15.02.0792.019; 15.02.0 < 15.02.0986.014 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
- After
- Microsoft Exchange Server 2016 Cumulative Update 21: 15.01.0 < 15.01.2308.020; Microsoft Exchange Server 2016 Cumulative Update 22: 15.0.0 < 15.01.2375.017; Microsoft Exchange Server 2019 Cumulative Update 10: 15.02.0 < 15.02.0792.019; Microsoft Exchange Server 2019 Cumulative Update 11: 15.02.0 < 15.02.0986.014 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
- Vendor guidanceAuthoritative vendor guidance changed: added patch: msrc.microsoft.com/CVE-2021-42321; added remediation: msrc.microsoft.com/CVE-2021-42321; removed remediation: portal.msrc.microsoft.com/CVE-2021-42321.secure@microsoft.com ↗
- Before
- patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: portal.msrc.microsoft.com/CVE-2021-42321
- After
- patch: msrc.microsoft.com/CVE-2021-42321 · patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: msrc.microsoft.com/CVE-2021-42321
- Vendor guidanceAuthoritative vendor guidance changed: added remediation: portal.msrc.microsoft.com/CVE-2021-42321; removed patch: msrc.microsoft.com/CVE-2021-42321; removed remediation: msrc.microsoft.com/CVE-2021-42321.portal.msrc.microsoft.com ↗
- Before
- patch: msrc.microsoft.com/CVE-2021-42321 · patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: msrc.microsoft.com/CVE-2021-42321
- After
- patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: portal.msrc.microsoft.com/CVE-2021-42321
- Vendor guidanceAuthoritative vendor guidance changed: added patch: msrc.microsoft.com/CVE-2021-42321; added remediation: msrc.microsoft.com/CVE-2021-42321; removed remediation: portal.msrc.microsoft.com/CVE-2021-42321.msrc.microsoft.com ↗
- Before
- patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: portal.msrc.microsoft.com/CVE-2021-42321
- After
- patch: msrc.microsoft.com/CVE-2021-42321 · patch: portal.msrc.microsoft.com/CVE-2021-42321 · remediation: msrc.microsoft.com/CVE-2021-42321
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- 15.01.0 < 15.01.2308.020; 15.02.0 < 15.02.0792.019; 15.0.0 < 15.01.2375.017; 15.02.0 < 15.02.0986.014 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
- After
- 15.01.0 < 15.01.2308.020; 15.0.0 < 15.01.2375.017; 15.02.0 < 15.02.0792.019; 15.02.0 < 15.02.0986.014 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
CVE published
Added to CISA KEV
Fixed release or patch reference recorded
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.