Security readout for executives and security teams
Plain-English summary
CVE-2026-26109 is a high-severity Microsoft Excel remote code execution issue. The source describes an out-of-bounds read that could let an unauthorized attacker run code locally. It affects multiple Excel, Office, Microsoft 365 Apps, Office LTSC, Mac LTSC, and Office Online Server products.
Executive priority
Treat this as a high-priority patching item, especially across managed Office fleets and Office Online Server. There is no provided evidence of active exploitation, but potential code execution with high impact justifies prompt remediation.
Technical view
Microsoft identifies this as CWE-125, out-of-bounds read, with CVSS 3.1 score 8.4. The vector is local, low complexity, no privileges, no user interaction, unchanged scope, and high confidentiality, integrity, and availability impact. Microsoft lists an official fix; exploitation maturity is marked unproven in the provided CVSS temporal data.
Likely exposure
Exposure is likely where affected Microsoft Excel, Office, Microsoft 365 Apps for Enterprise, Office LTSC, Mac LTSC, or Office Online Server versions remain unpatched. Prioritize endpoints and Office Online Server assets with the listed versions.
Exploitation context
The provided sources do not show CISA KEV listing or active exploitation. The CVSS vector indicates local attack conditions, no privileges, and no user interaction. Evidence is insufficient to describe a specific exploit path beyond Microsoft’s stated local code execution impact.
Researcher notes
Do not assume remote delivery mechanics from the title alone. The supplied CVSS vector is local and UI:N. Focus validation on version exposure, patch state, and Microsoft advisory details. Current evidence does not support claims of public exploit availability or KEV exploitation.
Mitigation direction
- Apply Microsoft’s official update for CVE-2026-26109.
- Prioritize systems running the affected Microsoft Office and Excel product lines.
- Check Microsoft guidance for any product-specific deployment notes.
- Reduce exposure of unpatched Office Online Server instances until updated.
- Use normal enterprise controls to limit local execution on high-risk systems.
Validation and detection
- Inventory installed Microsoft Office, Excel, Microsoft 365 Apps, and Office Online Server versions.
- Confirm affected assets have received Microsoft’s CVE-2026-26109 fix.
- Review patch compliance reports for systems missing March 2026 Office security updates.
- Verify Office Online Server update status separately from endpoint Office installations.
- Track exceptions where business constraints delay patching.
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-125: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. 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-2026-26109 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
- 8.4 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/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.
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:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C2.55.9microsoftVulnerability scoring details
Base CVSS 3.1 score
8.4HighVector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/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.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- Microsoft Excel Remote Code Execution VulnerabilityCVE reference, microsoft · vendor-advisory, patch
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.
Out-of-bounds Read
Out-of-bounds Read represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.