CVE-2026-97757

Executive Summary

CVE-2026-97757 is a heap-based buffer overflow vulnerability (CWE-122) in Microsoft Office Excel that permits an unauthorized attacker to execute arbitrary code over a network. The vulnerability carries a CVSS v3.1 base score of 8.8 (High), reflecting a network-reachable, low-complexity attack that requires user interaction but yields high impact to confidentiality, integrity, and availability. Organizations running Microsoft Excel on endpoints and servers are affected, and because no vendor patch, public proof-of-concept, or exploitation evidence has yet been identified, defenders must rely on configuration hardening and behavioral detection until a fix is released.

Technical Analysis

CVE-2026-97757 is classified under CWE-122, "Heap-based Buffer Overflow." According to the NVD description, the flaw resides in Microsoft Office Excel and "allows an unauthorized attacker to execute code over a network." At the vulnerability-class level, a heap-based buffer overflow occurs when a program writes more data into a fixed-size heap-allocated buffer than that buffer can hold, corrupting adjacent heap metadata and objects. When an attacker controls the overflow contents, they can influence control-flow structures or function pointers stored in adjacent heap memory, potentially redirecting execution to attacker-supplied code or corrupting the loader to pivot into a reliable arbitrary-code-execution primitive.

The attack surface for this class of vulnerability in an Office application is the file or data parsing pipeline — specifically, the code paths that parse, deserialize, or render external content. However, the NVD description provided does not enumerate the specific parser, object type, record structure, or component that is affected. It does not specify any function names, class names, file paths, or API parameters, and no CNA title, affected-version list, or OSV package data is available. As a result, no code-level root cause can be asserted here without fabrication. The authoritative facts that can be derived from the supplied data are limited to: the product (Microsoft Office Excel), the weakness class (heap-based buffer overflow, CWE-122), the delivery mode (network — AV:N), and the privilege precondition (none — PR:N), with user interaction required (UI:R).

Because the flaw is network-reachable, the relevant attack vectors are those by which an Excel document or embedded content is delivered to and opened by a target, or by which a network-exposed integration or service is induced to process crafted content. The "no privileges required" rating means the attacker does not need a prior foothold on the victim host; the "user interaction required" rating means some action — typically opening a malicious workbook or enabling content — is a prerequisite in the most likely scenarios.

Key derived characteristics:

• Attack vector: Network (AV:N)
• Attack complexity: Low (AC:L)
• Privileges required: None (PR:N)
• User interaction: Required (UI:R)
• Scope: Unchanged (S:U)
• Confidentiality / Integrity / Availability impact: High / High / High

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template exists for CVE-2026-97757. The attack chain below is inferred strictly from the vulnerability class (heap-based buffer overflow) and the NVD description; it does not assert implementation specifics that are not present in the source data.

1. Reconnaissance / target identification. The attacker identifies organizations or users known to exchange Microsoft Excel documents, or network services that process workbook content. Because the vulnerability is network-reachable (AV:N), the attacker may target users directly (victim-initiated document opening) or network-facing components that ingest Excel-formatted data.

2. Triggering the flaw class. The attacker crafts a malicious Excel document or data stream designed to cause a heap-allocated buffer to be overrun during parsing or rendering. The precise triggering object, record, or field is not described in the available intelligence and cannot be stated without fabrication. The essential mechanism is that a write exceeds the bounds of a heap buffer, corrupting adjacent heap memory.

3. Execution method at the vulnerability-class level. Heap corruption of this kind is associated with control of adjacent heap allocations or metadata. Where the overwrite reaches an object containing a function pointer or similar control data, the attacker can potentially redirect execution to attacker-controlled code. No specific payload technique, gadget chain, or memory layout is asserted here, as none is disclosed in the source data.

4. Post-exploitation impact. Successful exploitation yields code execution in the security context of the Excel process (or the process handling the content). Per the CVSS profile, this would have high confidentiality, integrity, and availability impact on the affected instance. The "Scope: Unchanged" (S:U) rating indicates that the vulnerable component is generally expected to be the component impacted — though downstream effects on the host remain a realistic concern for code-execution flaws.

Because no exploitation evidence or PoC is available, defenders should treat the timeline as "pre-exploitation" and prioritize detection and hardening ahead of a patch.

Impact Assessment

The CVSS v3.1 base score is 8.8 (High), driven by a network attack vector (AV:N), low complexity (AC:L), no required privileges (PR:N), and high C/I/A impact. The only moderating factor is the requirement for user interaction (UI:R), which reduces the score from the critical range. Business impact includes:

• Potential arbitrary code execution on any host that processes the crafted content, including user workstations and any server or integration processing Excel files.
• Data confidentiality exposure, given the high confidentiality impact rating.
• Data integrity risk via code execution under the user's or service's context.
• Availability impact to the target host and dependent workflows.

Blast radius is broad for any environment with Microsoft Office Excel deployed, because no version range is provided and the attacker requires no credentials. However, no affected-version list exists in the data, so precise scoping requires vendor confirmation. No CISA KEV listing is present, indicating no confirmed in-the-wild exploitation at the time of this report.

Detection Guidance

Detection must focus on the vulnerability class rather than a known exploit signature, since no PoC, exploit code, or Nuclei template exists. The rules below are behavioral and heuristic, intended to flag anomalous Excel child processes and malformed workbook content, and should be tuned in a lab before production use.

Sigma Rule

title: Suspicious Office Excel Child Process Execution (Potential CVE-2026-97757 Exploitation)
id: 6a2f9d1e-cve2-6977-57aa-000000000001
status: experimental
description: Detects Microsoft Excel spawning unusual child processes consistent with heap overflow code execution (CVE-2026-97757)
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-97757
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1203
  - attack.execution
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\EXCEL.EXE'
  selection_children:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\pwsh.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
      - '\certutil.exe'
      - '\bitsadmin.exe'
  condition: selection_parent and selection_children
falsepositives:
  - Legitimate macros or add-ins that invoke scripting
level: high

YARA Rule

rule CVE_2026_97757_Office_Excel_Heap_Overflow_Indicators
{
    meta:
        description = "Heuristic indicators for malformed Microsoft Excel content associated with CVE-2026-97757 heap-based buffer overflow. No PoC-based signature is available; this rule flags anomalous OLE/CFB structures and suspicious embedded objects."
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-97757"
        cve = "CVE-2026-97757"
        severity = "high"

    strings:
        $c = { D0 CF 11 E0 A1 B1 1A E1 } // OLE Compound File magic
        $zip = { 50 4B 03 04 }           // ZIP/OOXML magic
        $s1 = "Workbook" ascii wide nocase
        $s2 = "Microsoft Excel" ascii wide nocase
        $s3 = "mshta" ascii wide nocase
        $s4 = "http://" ascii wide nocase
        $s5 = "https://" ascii wide nocase
        $s6 = "rundll32" ascii wide nocase

    condition:
        (($c at 0) or ($zip at 0)) and
        ($s1 and $s2) and
        2 of ($s3, $s4, $s5, $s6)
}

Suricata IDS Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2026-97757 Potential Microsoft Excel Malicious Document Delivery (OOXML/XLS via HTTP)"; flow:to_client,established; content:"Content-Type"; nocase; content:"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"; nocase; pcre:"/(?:MZ|PK\x03\x04)/"; threshold:type both, track by_src, count 3, seconds 60; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-97757; metadata:created_at 2026_01_01, cve CVE-2026-97757; sid:9775701; rev:1;)

Note: This rule targets potential delivery of Excel-format files over unencrypted HTTP that also contain executable or archive markers. It is a delivery-stage heuristic, not an exploit signature, and will generate false positives on legitimate spreadsheet traffic. Tune by disabling on trusted internal sources.

Elastic Detection

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

This EQL rule identifies Excel spawning interpreter or living-off-the-land binaries, which is a common post-exploitation pattern following code execution in Office.

Splunk SPL Query

index=windows (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR sourcetype=WinEventLog:Security)
(EventID=1 OR EventCode=4688)
| search ParentImage="*\\EXCEL.EXE"
| search Image IN ("*\\cmd.exe","*\\powershell.exe","*\\pwsh.exe","*\\wscript.exe","*\\cscript.exe","*\\mshta.exe","*\\rundll32.exe","*\\regsvr32.exe","*\\certutil.exe","*\\bitsadmin.exe")
| stats count min(_time) as first_seen max(_time) as last_seen by host, User, ParentImage, Image, CommandLine
| where count >= 1
| sort - last_seen

Wazuh OSSEC Rule

<group name="windows,sysmon,cve-2026-97757,">
  <rule id="977570" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">\\\\EXCEL\.EXE$</field>
    <field name="win.eventdata.image">\\\\(cmd|powershell|pwsh|wscript|cscript|mshta|rundll32|regsvr32|certutil|bitsadmin)\.exe$</field>
    <description>CVE-2026-97757: Suspicious child process spawned by Microsoft Excel - possible heap overflow code execution</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_97757,office_excel,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, workaround, or affected-version data is available at the time of writing.

1. Immediate mitigations (compensating controls):

   • Enforce the Windows Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from creating executable content.
   • Enable Protected View and Application Guard for Office, and disable automatic execution of embedded content and macros from internet-origin documents.
   • Block inbound Excel-format attachments at the email and web gateways where operationally feasible, and quarantine macro-enabled formats.
   • Restrict outbound network egress from Office processes via application control / host firewall to complicate post-exploitation staging.
   • Increase logging: ensure Sysmon process-creation (Event ID 1), Office telemetry, and file-write events are collected and forwarded to the SIEM.

2. Patch / upgrade steps:

   • Monitor the Microsoft Security Response Center (MSRC) and the Microsoft Security Update Guide for CVE-2026-97757. No advisory was available in the supplied data.
   • When a fix is published, test in a staging ring and deploy via your standard patch pipeline as a priority (High severity, network-exposed).
   • Validate the patched version against the version identified in the eventual vendor advisory, since no affected range is currently known.

3. Long-term hardening:

   • Adopt a least-functionality Office configuration baseline; remove or disable unused add-ins and legacy formats.
   • Maintain application allowlisting so that interpreters and LOLBins spawned from Office are blocked by default.
   • Continuously monitor for behavioral anomalies (Office spawning scripting engines) rather than relying on signatures, given the absence of a PoC.
   • Track EPSS/KEV status; re-score urgency if exploitation evidence emerges.

References

• National Vulnerability Database — CVE-2026-97757: https://nvd.nist.gov/vuln/detail/CVE-2026-97757
• 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 (MSRC) Security Update Guide: https://msrc.microsoft.com/update-guide
• CISA Known Exploited Vulnerabilities Catalog (status check — CVE-2026-97757 not currently listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence for CVE-2026-97757 were identified in the available data. No public PoC, Nuclei template, or exploit code is known. EPSS has not yet scored this CVE.