Security readout for executives and security teams
Plain-English summary
This flaw can let attacker-controlled code run on an affected Microsoft Exchange Server, threatening email confidentiality, integrity, and availability. The supplied CVSS vector indicates local access and user interaction are required, so the evidence does not establish a standalone internet-based attack path. CISA nevertheless confirms exploitation in the wild.
Executive priority
Treat confirmed affected deployments as urgent. Known exploitation raises the possibility of prior compromise, so pair prompt patching with investigation rather than treating update installation alone as proof that the environment is clean.
Technical view
CVE-2021-26857 is an Exchange Server remote code execution vulnerability associated with unsafe deserialization (CWE-502). It scores CVSS 7.8 with AV:L, PR:N, and UI:R; successful exploitation can severely affect confidentiality, integrity, and availability. Microsoft provides a security update advisory.
Likely exposure
The supplied evidence identifies listed releases of Exchange Server 2010 SP3, Exchange 2013, Exchange 2016 CUs 8–19, and Exchange 2019 through CU2. Organizations should verify actual product, cumulative update, and security update levels rather than assuming exposure from the major version alone.
Exploitation context
CISA's Known Exploited Vulnerabilities catalog confirms real-world exploitation. The supplied sources do not identify threat actors, campaign scope, indicators, exploitation frequency, or whether attackers used this vulnerability alone or as part of a chain.
Researcher notes
Interpret the attack path carefully: Microsoft labels this remote code execution, but the supplied CVSS vector specifies local attack vector, no privileges, and required user interaction. CWE-502 indicates unsafe deserialization. The bundle does not explain prerequisites or support inferring a directly network-reachable standalone exploit.
Mitigation direction
- Inventory Exchange servers and match installed versions and cumulative updates against Microsoft's affected-product entries.
- Apply the security update referenced by Microsoft's official CVE advisory after appropriate change validation.
- If immediate patching is impossible, consult current Microsoft guidance; the supplied bundle names no standalone workaround.
- Preserve and review relevant Exchange, endpoint, and authentication telemetry for possible compromise.
- Escalate confirmed affected systems through incident-response procedures because CISA records known exploitation.
Validation and detection
- Record the product version, cumulative update, and installed security update level for every Exchange server.
- Compare each server's state with Microsoft's CVE advisory and affected-product information.
- Confirm the required security update installed successfully using approved asset or patch-management tooling.
- Review telemetry for unexplained code execution or account activity; the supplied bundle provides no specific indicators.
- Document exceptions and revalidate every affected server after remediation.
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-502: Code execution behavior lookup
Code execution and unsafe deserialization weaknesses often justify reviewing execution behavior and process telemetry. 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-2021-26857 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-2021-26857 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-2021-26857 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-2021-26857CVE reference, CVE · x_refsource_MISC, x_transferred
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26857CVE 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.
Deserialization of Untrusted Data
Deserialization of Untrusted Data represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.