CVE-2026-97739

Executive Summary

CVE-2026-97739 is a remote code execution vulnerability in Microsoft Office Excel caused by an integer underflow (wrap or wraparound) condition that, per the NVD description, allows an unauthorized attacker to execute code over a network. The flaw carries a CVSS v3.1 base score of 8.8 (HIGH), driven by network attack vector, low attack complexity, no privileges required, and high confidentiality, integrity, and availability impact — with the sole countervailing factor being required user interaction (UI:R). No patch, workaround, affected-version range, public proof-of-concept, or CISA KEV listing currently exists in the tracked intelligence sources, meaning defenders must treat this as an unmitigated, unconfirmed-in-the-wild but high-severity client-side execution risk.

Technical Analysis

The NVD description states that CVE-2026-97739 is an "integer underflow (wrap or wraparound)" vulnerability in Microsoft Office Excel that permits an unauthorized attacker to execute code over a network. The CWE mapping provided in the intelligence data is CWE-122, "Heap-based Buffer Overflow."

These two data points together define the vulnerability class with reasonable precision even in the absence of a vendor advisory or function-level detail. An integer underflow/wraparound arises when a computed size, offset, or length value is decremented below zero (or below the minimum representable value of the integer type in use), causing the value to wrap to a very large positive number. When that wrapped value is subsequently used to govern a memory allocation or a copy operation, the result is an allocation that is far too small relative to the data later written into it — the textbook precondition for a heap-based buffer overflow (CWE-122). In the Excel context, this class of defect is typically reachable through the parsing of a structured document element whose declared size or count field is attacker-controlled. Because the attack vector is network (AV:N) with required user interaction (UI:R), the realistic delivery surface is a malicious Excel document (or a document-embedded structure) that a victim opens, triggering the vulnerable parsing logic in the Excel client process.

The exploitability profile is significant. AV:N indicates the payload can be delivered across a network (for example, via email attachment, a link to a hosted document, or a shared file location) without the attacker needing local access to the target host. AC:L indicates no special conditions or race windows are required to trigger the flaw. PR:N confirms the attacker needs no authenticated position. The UI:R component is the only barrier: a user must take an action, consistent with opening or interacting with a crafted document. The scope is unchanged (S:U), meaning the impact is contained to the vulnerable Excel process and its security context rather than crossing an authorization boundary — but with C:H/I:H/A:H, successful exploitation yields full compromise of confidentiality, integrity, and availability within that context, which in a typical desktop deployment equates to code execution as the logged-in user.

No confirmed affected build numbers, no exact vulnerable function or component, and no patch or workaround are available from the tracked sources. Any claim about a specific internal parsing routine, file format structure, or trigger offset would be speculation and is therefore not asserted here. What can be stated with confidence is the vulnerability class (integer-underflow-induced heap overflow), the delivery model (network-delivered document with user interaction), and the resulting impact (remote code execution in the Excel client context).

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred strictly from the vulnerability class and the CVSS/NVD data, and deliberately avoids inventing function names, endpoints, or file paths that are not present in the source material.

1. Reconnaissance and target identification. The attacker identifies targets that use Microsoft Office Excel on Windows or macOS endpoints. Because the flaw is reachable via a network-delivered document and requires user interaction, high-value targeting focuses on organizations where Excel documents are routinely exchanged across trust boundaries — finance, legal, HR, engineering, and any team that ingests spreadsheets from external parties. Public indicators used for targeting include MX records, exposed Outlook Web Access or Microsoft 365 login portals, and corporate document-sharing links.

2. Triggering the specific flaw class. The attacker crafts a document containing a malformed structured element whose embedded size or length field is set such that the parsing logic in Excel decrements it below its minimum value. This causes the integer to wrap to a very large value. That wrapped value is then used where a smaller, correct value was expected, leading to an undersized heap allocation (CWE-122). The trigger condition is satisfied at the moment Excel parses the maliciously structured element — no special privileges, authentication, or network position is required by the attacker, consistent with AC:L and PR:N.

3. Payload and execution method. Because the resulting buffer is undersized, the subsequent write of attacker-controlled data overflows the heap. The attacker's objective at this stage is to place controlled data adjacent to the corrupted allocation and influence execution flow within the Excel process. The generic exploitation pattern for a heap-based overflow is to shape the heap, overwrite a function pointer or return address, and redirect execution to a payload. The specific payload staging technique is not documented in the available intelligence and is therefore not asserted.

4. Post-exploitation impact. Given S:U and C:H/I:H/A:H, successful exploitation results in code execution at the privilege level of the user running Excel. In typical enterprise deployments this is a standard user context, from which an attacker would pursue credential access, lateral movement, and persistence. The absence of a public PoC or in-the-wild reporting (no CISA KEV entry) means current exploitation is assessed as theoretical-to-likely rather than confirmed, but the low complexity and high impact make weaponization attractive.

Impact Assessment

The CVSS v3.1 base score is 8.8 (HIGH). Subscore analysis:

• Attack Vector (N — Network): The vulnerability can be reached across a network, aligning with delivery of a malicious document via email, messaging, or a hosted file location. This widens the blast radius beyond physically local attacks.
• Attack Complexity (L — Low): No race condition, no specialized configuration, and no non-default environment is required. Weaponization is comparatively straightforward, which raises risk.
• Privileges Required (N — None): The attacker needs no foothold or credentials on the target system, lowering the barrier to entry.
• User Interaction (R — Required): A user must open or interact with the crafted document. This is the primary limiting factor and is the classic mitigation lever — user awareness and attachment controls can reduce, though not eliminate, exposure.
• Scope (U — Unchanged): The impact is confined to the vulnerable Excel process's security context; the attacker does not automatically gain control of other components outside that authorization boundary.
• Confidentiality / Integrity / Availability (H / H / H): Successful exploitation yields high impact in all three dimensions. When translated to a desktop code-execution scenario, this means an attacker could read, modify, and destroy data accessible to the logged-in user and potentially disrupt the endpoint.

Business impact: an unpatched spreadsheet-parsing RCE in a ubiquitous productivity suite represents a persistent risk surface. Even without a public exploit, the low attack complexity and network vector mean that a determined attacker or a capable researcher could develop a working exploit relatively quickly. For organizations in regulated sectors, an unpatched high-severity RCE in an end-user application may also carry compliance implications. Blast radius spans any workstation or server-side document-processing pipeline that uses affected Excel components; because no affected version list is published, defenders cannot currently scope the affected install base with precision.

Detection Guidance

Sigma Rule

title: Suspected Excel Document Parsing Exploitation via Child Process Spawn
id: 3f9b2c7e-1a4d-4b8e-9c2f-7d6e5a1b0c34
status: experimental
description: Detects Excel spawning suspicious child processes, a common post-exploitation pattern following client-side Office RCE (CVE-2026-97739).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-97739
author: valtersit.com threat intelligence
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\EXCEL.EXE'
  selection_suspicious_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\pwsh.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
      - '\curl.exe'
      - '\certutil.exe'
  condition: selection_parent and selection_suspicious_child
falsepositives:
  - Legitimate Excel macros invoking system utilities in approved workflows
level: high
tags:
  - attack.t1203
  - cve.2026.97739

YARA Rule

rule CVE_2026_97739_Excel_Integer_Underflow_Exploit
{
    meta:
        description = "Detects potential exploit documents targeting the Excel integer underflow heap overflow described in CVE-2026-97739"
        author = "valtersit.com threat intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-97739"
        date = "2026-01-01"
        severity = "high"

    strings:
        $mz = { 4D 5A }
        $pe = { 50 45 00 00 }

    condition:
        $mz at 0 and $pe and filesize < 5MB
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"CVE-2026-97739 Possible Microsoft Excel Exploit Document Transfer"; flow:established,to_client; file_data; content:"PK"; depth:2; content:"xl/"; within:2048; content:"Microsoft"; nocase; within:4096; filestore; sid:1009773901; rev:1; metadata:created_at 2026_01_01, cve CVE-2026-97739, confidence medium; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-97739;)

Elastic Detection

process where event.type == "start" and
  process.parent.name : ("EXCEL.EXE", "excel.exe") and
  process.name : ("cmd.exe", "powershell.exe", "pwsh.exe", "wscript.exe",
                  "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe",
                  "curl.exe", "certutil.exe")

Splunk SPL Query

index=endpoint OR index=sysmon source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
| eval parent=lower(ParentImage), child=lower(Image)
| where like(parent, "%excel.exe") AND (like(child, "%cmd.exe") OR like(child, "%powershell.exe") OR like(child, "%wscript.exe") OR like(child, "%mshta.exe") OR like(child, "%rundll32.exe") OR like(child, "%regsvr32.exe") OR like(child, "%certutil.exe"))
| stats count min(_time) as first_seen max(_time) as last_seen by host, User, ParentImage, Image, CommandLine
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="windows,sysmon,cve-2026-97739,">
  <rule id="100739" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">EXCEL.EXE</field>
    <field name="win.eventdata.image">\.(cmd|powershell|pwsh|wscript|cscript|mshta|rundll32|regsvr32|certutil)\.exe$</field>
    <description>CVE-2026-97739: Possible Microsoft Excel exploitation - suspicious child process spawned by EXCEL.EXE</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_97739,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, no workaround, and no affected-version list are available. The following guidance is therefore mitigation-first.

1. Immediate mitigations

• Block or quarantine inbound Excel documents from untrusted external senders at the email gateway and web proxy; treat unsolicited spreadsheets as executable content.
• Enable Microsoft Defender Attack Surface Reduction (ASR) rules that prevent Office applications from creating child processes and from creating executable content, where applicable to the deployment.
• Enforce Mark-of-the-Web and Protected View so that documents originating from the internet open in a restricted, non-executing context.
• Restrict user privileges so that the Office process runs as a standard user rather than an administrator, limiting the impact of post-exploitation actions.
• Apply application allow-listing to prevent unexpected child processes (script hosts, LOLBins) from spawning from Excel.
• On server-side document-processing pipelines, sandbox or containerize Excel/Office parsing and reject documents that fail structural validation.

2. Patch / upgrade steps

• No patch is currently available from tracked sources. Subscribe to the Microsoft Security Response Center (MSRC) and Microsoft 365/Office update channels, and monitor the NVD entry for CVE-2026-97739 for an updated description, affected-version list, and vendor advisory.
• When a vendor fix is published, prioritize patching endpoints running Excel and any server components that parse Office documents; stage updates through change control and verify through scanning/vulnerability management.

3. Long-term hardening

• Reduce reliance on macro- and document-based workflows; where macros are required, enforce trusted locations and signed-macro policies.
• Maintain inventory of all systems running Microsoft Office/Excel components, including server-side converters and previewers, so that patch coverage can be verified once a fix exists.
• Integrate the Sigma, Suricata, and EDR rules above into detection pipelines to catch post-exploitation behavior even before a signature or patch is available.
• Conduct periodic phishing simulations and user education emphasizing the risk of opening unsolicited spreadsheets.
• Monitor threat intelligence for PoC publication; absence of a public exploit today does not reduce long-term priority given CVSS 8.8 and low attack complexity.

References

• National Vulnerability Database — CVE-2026-97739: https://nvd.nist.gov/vuln/detail/CVE-2026-97739
• MITRE CWE-122 — Heap-based Buffer Overflow: https://cwe.mitre.org/data/definitions/122.html
• MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
• Microsoft Security Response Center (vendor advisory pending — no advisory identified for this CVE at time of writing): https://msrc.microsoft.com/update-guide
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-97739 NOT currently listed — no confirmed in-the-wild exploitation): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor / exploitation status: No public PoC, no Nuclei template, no exploit code, and no CISA KEV entry were identified in the tracked intelligence. No named threat actor campaign is associated with this CVE in the available data. EPSS has not yet been scored by FIRST.org, and no fabricated value is provided.