Security readout for executives and security teams
Plain-English summary
CVE-2021-26855 is a critical Microsoft Exchange Server vulnerability that lets an unauthenticated network attacker compromise affected servers without user interaction. Successful exploitation can expose or alter sensitive information. Because CISA lists it as known exploited, organizations should treat any unpatched, network-reachable affected server as an urgent incident risk.
Executive priority
Immediate. Confirm affected-server inventory, patch status, and network exposure, then investigate potentially exposed systems. KEV status means this is not a theoretical risk. Leadership should require evidence of remediation and security review rather than relying only on planned patching.
Technical view
The issue is classified as CWE-918 and scored CVSS 9.1. Its vector indicates remote exploitation with low complexity, no privileges, and no user interaction, with high confidentiality and integrity impact. The supplied record covers listed Exchange Server 2013, 2016, and 2019 releases. Microsoft’s advisory is identified as providing a patch.
Likely exposure
Exposure is most likely where an organization operates one of the listed Exchange Server 2013, 2016, or 2019 builds and has not applied Microsoft’s security update. Network-reachable servers warrant the fastest review. The bundle does not provide configuration-specific exceptions or complete version-detection guidance.
Exploitation context
Active exploitation is confirmed by this CVE’s inclusion in CISA’s Known Exploited Vulnerabilities catalog. The vulnerability is remotely reachable and requires neither authentication nor user action. The supplied sources do not identify current threat actors, exploitation volume, or evidence that any particular organization was compromised.
Researcher notes
The bundle labels this an Exchange Server remote-code-execution vulnerability and maps it to CWE-918. CVSS indicates no availability impact but high confidentiality and integrity impact. Avoid extending the affected set beyond the listed releases. Detailed exploit-chain prerequisites, indicators, and patch identifiers are not included, so consult the cited vendor and government records.
Mitigation direction
- Apply Microsoft’s security update for CVE-2021-26855 using the vendor advisory.
- Prioritize affected Exchange servers that are reachable from untrusted networks.
- Restrict unnecessary network access while patch status is being resolved.
- Follow current Microsoft and CISA guidance for any additional mitigations or investigation.
Validation and detection
- Inventory Exchange Server versions and cumulative updates against the supplied affected list.
- Confirm Microsoft’s CVE-2021-26855 security update is installed on every affected server.
- Identify how each Exchange server is reachable from untrusted networks.
- Review relevant server and access logs for suspicious activity during the exposure period.
- Escalate unexplained indicators for incident-response investigation.
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.
CWE-918: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution 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-2021-26855 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
- Critical
- CVSS
- 9.1 (3.1)
- Known Exploited
- Yes
- Published
Vector: 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
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:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C3.95.2microsoftVulnerability scoring details
Base CVSS 3.1 score
9.1CriticalVector: 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
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-2021-26855 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-2021-26855 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-2021-26855CVE reference, CVE · x_refsource_MISC, x_transferred
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855CVE 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.
Server-Side Request Forgery (SSRF)
Server-Side Request Forgery (SSRF) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.