Security readout for executives and security teams
Plain-English summary
This flaw can let an attacker execute code on affected Microsoft Exchange Server installations, potentially compromising email, sensitive data, and server availability. Its 7.8 CVSS score is high, and CISA lists it as known exploited. Successful exploitation requires local access and user interaction according to the supplied CVSS vector.
Executive priority
Treat remediation as urgent for confirmed affected systems. Known exploitation and potentially complete system impact justify accelerated patching, exposure validation, and compromise review. Systems outside the listed versions should still be checked against current Microsoft guidance before being deprioritized.
Technical view
CVE-2021-26858 is a remote code execution vulnerability affecting specified Exchange Server 2013, 2016, and 2019 releases. The supplied CVSS 3.1 vector is AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, indicating low complexity, no privileges, required user interaction, and potentially complete confidentiality, integrity, and availability impact.
Likely exposure
Exposure applies to organizations running the listed Exchange Server 2013 CU21–23, Exchange Server 2016 CU8–19, or Exchange Server 2019 through CU4 releases. The bundle does not establish exposure for other versions or products. Confirm exact installed cumulative updates and security-update status.
Exploitation context
CISA KEV inclusion supports that this vulnerability has been exploited in the wild. The supplied evidence does not describe current campaign activity, attacker groups, exploitation frequency, prerequisites beyond the CVSS vector, or indicators of compromise. Do not infer internet-direct exploitation because the vector specifies local access.
Researcher notes
The title says remote code execution, while CVSS specifies local attack vector and required user interaction. Preserve that distinction during triage. The bundle provides no CWE, root-cause detail, exploit-chain description, indicators, or affected-build boundaries beyond its product list; consult Microsoft’s advisory for authoritative update applicability.
Mitigation direction
- Apply the applicable Microsoft security update identified in the vendor advisory.
- Prioritize affected Exchange servers because CISA records known exploitation.
- Follow current Microsoft guidance for unsupported or out-of-date cumulative updates.
- Use vendor-recommended mitigations if immediate patching is operationally impossible.
Validation and detection
- Inventory Exchange Server editions, versions, and cumulative-update levels.
- Confirm the applicable Microsoft security update is installed successfully.
- Compare every deployed release with the affected-version list in the advisory.
- Review relevant Exchange and security telemetry for unexplained activity around affected systems.
- Use Microsoft-provided detection guidance if available in the current advisory.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
Execution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-26858 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.8 (3.1)
- Known Exploited
- Yes
- Published
Vector: 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
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
SSVC decision data
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C1.85.9microsoftVulnerability scoring details
Base CVSS 3.1 score
7.8HighVector: 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
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- Added to KEVCISA-ADP
CISA Known Exploited Vulnerabilities metadata lists this CVE as known exploited.
- ADP timelineCISA-ADP
CVE-2021-26858 added to CISA KEV
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
- 2021-11-03T00:00:00.000Z: CVE-2021-26858 added to CISA KEV
Source materials
- CVE List V5 sourceCVE List V5
- Microsoft Exchange Server Remote Code Execution VulnerabilityCVE reference, microsoft · vendor-advisory, patch
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26858CVE reference, CVE · x_refsource_MISC, x_transferred
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858CVE reference, CISA-ADP · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.