Security readout for executives and security teams
Plain-English summary
A flaw in Microsoft Excel can let an attacker run code after required user interaction, potentially compromising data, accounts, and device availability. The listed Microsoft 365, Office, Excel, Mac, LTSC, and Office Online Server products require review. The source bundle identifies an official Microsoft update but does not provide fixed build numbers.
Executive priority
Treat this as a high-priority patching issue because successful exploitation can fully affect data and system operations. Accelerate remediation for externally exposed document workflows and high-value users. Emergency incident status is not justified solely by this bundle because it provides no evidence of active exploitation.
Technical view
CVE-2026-55899 is a stack-based buffer overflow caused by improper input validation in Microsoft Office Excel. It is rated CVSS 3.1 7.8 high, with local attack vector, low complexity, no privileges required, and user interaction required. Successful exploitation can compromise confidentiality, integrity, and availability within the affected security scope.
Likely exposure
Exposure is most likely where listed Office or Excel versions remain unpatched and users process untrusted spreadsheet content. Office Online Server is also listed, but the bundle does not explain its specific attack path. Exact vulnerable and fixed build boundaries must be confirmed through Microsoft's advisory and organizational inventory.
Exploitation context
The supplied sources do not establish active exploitation, and this CVE is not identified as CISA KEV in the bundle. User interaction is required, reducing fully automated exploitation risk, but no privileges are required and successful exploitation could execute code locally. Specific triggering files, delivery methods, and exploit availability are not documented here.
Researcher notes
The record maps the issue to CWE-121 and CWE-20. CVSS metrics indicate local access, low complexity, no privileges, required user interaction, unchanged scope, and high impact across all three security objectives. The bundle does not provide root-cause depth, affected component names, fixed builds, proof-of-concept status, or product-specific attack-path details.
Mitigation direction
- Review Microsoft's CVE advisory and apply the designated security updates to every affected installation.
- Prioritize systems where users routinely open spreadsheets received from external or untrusted sources.
- Restrict or isolate untrusted spreadsheet handling until applicable updates are verified.
- Check Microsoft guidance for Office Online Server and Mac-specific remediation details.
- Use existing endpoint protections and attachment controls to reduce exposure during patch rollout.
Validation and detection
- Inventory the listed Microsoft Office, Excel, Mac, LTSC, and Office Online Server products.
- Compare installed versions and update status with Microsoft's current CVE advisory.
- Confirm security updates deployed successfully through endpoint and software-management reporting.
- Review update failures and unsupported installations requiring manual remediation or replacement.
- Monitor security telemetry for suspicious processes launched by Excel or unusual post-document activity.
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-121: 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 lookupCWE-20: 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-2026-55899 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
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/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:R/S:U/C:H/I:H/A:H/E:U/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: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.