CVE-2026-97777

Executive Summary

CVE-2026-97777 is a heap-based buffer overflow (CWE-122) in Microsoft Office Excel that allows an unauthorized attacker to execute code over a network. Per the NVD description, exploitation requires no privileges (PR:N) but does require user interaction (UI:R) — consistent with a malicious document or workbook that a victim must open. With a CVSS 3.1 base score of 8.8 and High impacts across confidentiality, integrity, and availability, this is a high-severity remote code execution class vulnerability; no public proof-of-concept, exploit code, or patch is currently available, and the CVE is not listed in CISA KEV.

Technical Analysis

The vulnerability class is a heap-based buffer overflow, mapped to CWE-122. Per the NVD description: "Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code over a network."

The heap is a region of dynamically allocated process memory. A heap-based buffer overflow occurs when a program writes data past the boundaries of a heap-allocated buffer, corrupting adjacent heap metadata or neighboring object contents. In productivity application contexts, this class of flaw is typically reached during parsing or processing of attacker-controlled input — for a spreadsheet application, that input is normally an untrusted document or workbook supplied by the attacker. Because the flaw is heap-based rather than stack-based, successful exploitation commonly proceeds by corrupting adjacent heap structures or object pointers to influence control flow, and then redirecting execution toward attacker-supplied data.

Attack surface: the application's document/workbook parsing and processing path. The attack is network-deliverable (AV:N) — the malicious file or data can be delivered remotely — but requires user interaction (UI:R), meaning the victim must take an action that causes Excel to process the crafted input.

Scope is Unchanged (S:U), indicating the impact is confined to the vulnerable Excel process and its security authority rather than crossing into other security scopes.

Insufficient technical detail is available to identify specific vulnerable functions, parsing routines, file format structures, offsets, or memory layouts. The NVD description does not disclose the affected component within Excel, the file format or input channel involved, or the exact trigger condition. Any claim of a specific function, parameter, or code path would be fabrication and is therefore omitted.

Notable intelligence gaps, all of which materially raise uncertainty:

• No vendor advisory, CVE record detail beyond the description, or researcher write-up is available.
• No affected version list has been published (Affected Versions: []).
• No patch evidence exists from any tracked source (Patch confidence 0/4).
• No EPSS score has been assigned by FIRST.org; exploit likelihood is therefore unquantified.
• No public PoC, exploit code, or Nuclei template exists.

Exploitation Scenarios

Note: No public PoC or exploit code is available. The following chain is inferred strictly from the vulnerability class (CWE-122) and the NVD description, at the vulnerability-class level. No specific function names, file paths, or parameters are asserted because none are provided in the source data.

1. Reconnaissance / target identification. Because the flaw is triggered by Excel processing attacker-influenced input, the attacker's reconnaissance centers on identifying targets that use Microsoft Office Excel and whose users can be induced to open an untrusted workbook. Delivery is network-based (AV:N); the attacker needs a channel to get a crafted document to the victim — for example phishing email attachments, links to hosted files, or shared/cloud-hosted documents. Public reporting and Shodan-style exposure of Office web surfaces may inform targeting, though the description does not specify a server-side component.

2. Triggering the flaw. The attacker crafts a malicious spreadsheet/workbook such that, when Excel parses or processes it, a write to a heap-allocated buffer exceeds the buffer's bounds, corrupting adjacent heap memory. The exact malformed structure that reaches the vulnerable code path is not disclosed in the available data; it is characterized here only at the class level (heap buffer overflow during processing of untrusted input). User interaction (UI:R) is required — the victim must open or otherwise cause Excel to process the crafted file.

3. Payload / execution method. In heap-overflow exploitation generally, the attacker seeks to shape the heap so that the out-of-bounds write corrupts an adjacent object, pointer, or metadata structure, and then influences program control flow to redirect execution toward attacker-controlled data, ultimately executing arbitrary code in the context of the Excel process. The specific primitive (e.g., which adjacent structure is targeted) is not derivable from the provided description and is not asserted here.

4. Post-exploitation impact. Successful exploitation yields code execution with the privileges of the user running Excel. Per the CVSS vector, this maps to High Confidentiality, Integrity, and Availability impact (C:H/I:H/A:H), meaning potential theft of data accessible to that user, modification of data, and disruption of the affected process/system. Because scope is Unchanged (S:U), the immediate impact is bounded to the vulnerable application's context, but code execution at the user level can still enable credential theft, lateral movement, and persistence depending on the environment.

Prerequisites and constraints: the attacker needs a delivery vector for the malicious file and must persuade the victim to interact with it (UI:R). No authentication or privileges are required on the attacker's side (PR:N).

Impact Assessment

• Base score: 8.8 (High) under CVSS 3.1.
• Exploitability: AV:N / AC:L / PR:N / UI:R — network-reachable, low attack complexity, no privileges required, but user interaction required. The UI:R requirement is the principal obstacle to mass exploitation and explains why file-delivery social engineering (e.g., phishing) is the expected vector.
• Impact subscores: C:H / I:H / A:H — high impact to confidentiality, integrity, and availability.
• Scope: Unchanged — impact remains within the vulnerable component's security authority.

Business impact: any environment with Microsoft Office Excel in use is in scope, which in practice is nearly every enterprise endpoint population. Exploitation leading to arbitrary code execution at user privilege can result in malware delivery, data exfiltration, credential harvesting, and ransomware staging. Blast radius is a function of how widely the crafted file can be delivered and how many users can be induced to open it; the low-complexity, no-privilege profile makes weaponization attractive once a reliable trigger is developed.

Risk amplifier: the absence of a patch (0/4 confidence, unpatched) means defenders cannot currently remediate by version upgrade. Risk must therefore be managed through delivery-vector controls and endpoint hardening. Note that no KEV listing, no EPSS score, and no observed exploitation are documented, so this is a vulnerability with high potential impact but unconfirmed real-world exploitation as of the available data.

Detection Guidance

Because the vulnerable file structure, affected component, and exploit technique are not disclosed, detections below target the general exploitation pattern for an Office file-triggered heap overflow: suspicious Excel child-process behavior, malformed-document delivery, and network retrieval of document payloads. These rules are heuristic and require environment-specific tuning; they are not derived from a known PoC.

Sigma Rule

title: Potential Microsoft Excel Heap Overflow Exploitation - Suspicious Child Process
id: 3f9a1c72-8b4e-4d21-9c6f-7a2e5d10b9c4
status: experimental
description: Detects Excel spawning unusual child processes, consistent with code execution following exploitation of a client-side vulnerability such as CVE-2026-97777 (heap-based buffer overflow).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-97777
author: Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1203
  - attack.execution
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'
      - '\bitsadmin.exe'
  condition: selection_parent and selection_suspicious_child
falsepositives:
  - Legitimate Excel macros or add-ins invoking signed automation (validate publisher and signed macro policy)
level: high

YARA Rule

rule EXPL_Office_HeapOverflow_Indicator_CVE_2026_97777
{
    meta:
        description = "Heuristic detection of anomalous Office document characteristics consistent with exploitation of a heap-based buffer overflow (CVE-2026-97777). High false-positive risk; tune before deployment."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-97777"
        cwe = "CWE-122"
        date = "2026-01-01"
        severity = "high"

    strings:
        // Office container magic (OOXML / OLE) for scoping to document files
        $zip = { 50 4B 03 04 }
        $ole = { D0 CF 11 E0 A1 B1 1A E1 }

        // Suspicious embedded executable / script markers commonly abused in doc-borne attacks
        $pe   = { 4D 5A }
        $ps1  = "powershell" nocase ascii wide
        $mshta = "mshta" nocase ascii wide
        $wscript = "wscript" nocase ascii wide

    condition:
        ($zip at 0 or $ole at 0) and
        (uint16(0) == 0x4B50 or uint16(0) == 0xCFD0) and
        ($pe or $ps1 or $mshta or $wscript)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-97777 Possible Excel Heap Overflow Payload Exfiltration or Callback"; flow:established,to_server; content:"POST"; http_method; content:"|4D 5A|"; http_client_body; depth:2; threshold:type limit, track by_src, count 1, seconds 300; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-97777; metadata:affected_product Microsoft_Office_Excel, cve CVE-2026-97777; sid:202697777; rev:1;)

alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2026-97777 Possible Malicious Excel Document Download (OOXML/OLE)"; flow:established,to_client; content:"|50 4B 03 04|"; http_server_body; depth:4; threshold:type limit, track by_dst, count 1, seconds 60; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-97777; metadata:affected_product Microsoft_Office_Excel, cve CVE-2026-97777; sid:202697778; rev:1;)

Note: content matches target generic Office/PE signatures; refine once the concrete delivery pattern is known. Tune http_client_body/http_server_body buffers and thresholds to your environment.

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", "bitsadmin.exe")

Companion query for document-borne delivery in email/web contexts:

file where event.action == "creation" and
  file.extension : ("xls", "xlsx", "xlsm", "xlsb", "xlam") and
  file.path : ("*\\Downloads\\*", "*\\AppData\\Local\\Temp\\*", "*\\Outlook\\*")

Splunk SPL Query

index=windows (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR source="WinEventLog:Security")
(EventCode=1 OR EventID=4688)
| eval parent=coalesce(ParentImage, Process_Parent)
| eval child=coalesce(Image, New_Process_Name)
| where (lower(parent) LIKE "%excel.exe" OR lower(parent) LIKE "%excelcnv.exe")
  AND (lower(child) LIKE "%cmd.exe" OR lower(child) LIKE "%powershell.exe"
       OR lower(child) LIKE "%wscript.exe" OR lower(child) LIKE "%cscript.exe"
       OR lower(child) LIKE "%mshta.exe" OR lower(child) LIKE "%rundll32.exe"
       OR lower(child) LIKE "%regsvr32.exe" OR lower(child) LIKE "%bitsadmin.exe")
| stats count values(child) as children min(_time) as first_seen max(_time) as last_seen by host parent
| where count > 0
| sort - count

Optional correlation on network retrieval of spreadsheet payloads:

index=proxy OR index=web
| search (url="*.xls*" OR url="*.xlsx*" OR url="*.xlsm*")
| stats count values(url) as urls by src_ip
| where count > 0

Wazuh OSSEC Rule

<group name="windows,sysmon,exploit,">
  <rule id="2026977" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">EXCEL.EXE$</field>
    <field name="win.eventdata.image" type="pcre2">(?i)\\(cmd|powershell|pwsh|wscript|cscript|mshta|rundll32|regsvr32|bitsadmin)\.exe$</field>
    <description>CVE-2026-97777: Excel spawned a suspicious child process (possible heap overflow code execution)</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>process_creation,attack_execution,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, advisory, or workaround has been identified. The following measures reduce exposure to the exploitation pattern but do not remediate the underlying memory-safety defect.

1. Immediate mitigations

• Enforce strict email and web delivery controls. Because the expected vector is a malicious document requiring user interaction (UI:R), block or sandbox high-risk attachments (.xls, .xlsx, .xlsm, .xlsb, .xlam, and legacy formats such as .xls) at the gateway and in cloud mail filters.
• Disable or tightly restrict macros in Excel via Group Policy / Trust Center, and block macros from the internet (Mark-of-the-Web enforcement). Do not permit macros from untrusted locations.
• Configure Excel Protected View for files originating from the internet and other potentially unsafe locations; disable automatic DDE and OLE object activation.
• Application control / allowlisting on endpoints to prevent child-process execution from Office applications (block Excel spawning command shells, script hosts, and LOLBins).
• Deploy and tune endpoint protection with attack-surface-reduction rules that block Office child-process creation, executable content from Office apps, and process injection techniques.
• Isolate or virtualize browsing/email handling where feasible; treat opening untrusted spreadsheets as a high-risk activity in user awareness training.

2. Patch / upgrade steps

• No patch exists as of this report. Monitor the Microsoft Security Update Guide and Microsoft Security Response Center for an Excel security update addressing this CVE.
• When a fix is released, inventory all Excel deployments and apply the update via your patch-management pipeline on an expedited timeline consistent with a High (8.8) severity rating.
• Track the NVD record for an affected-version list and CPE detail, which are currently absent, to scope which builds are vulnerable and to validate update applicability.

3. Long-term hardening

• Reduce default reliance on client-side document macros and untrusted file types in business workflows; migrate to safer formats and signed, trusted automation only.
• Implement least-privilege user endpoints, credential protection (e.g., LSA protections, credential guard where available), and rapid containment capability for endpoint incidents.
• Maintain robust egress filtering and network monitoring to detect post-exploitation callbacks and exfiltration.
• Adopt a vulnerability-intelligence workflow that flags "unpatched / no-PoC / no-EPSS" CVEs for compensating-control enforcement until a vendor fix materializes.

References

• NVD — CVE-2026-97777: https://nvd.nist.gov/vuln/detail/CVE-2026-97777
• 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 Update Guide (monitor for a future Excel fix): https://msrc.microsoft.com/update-guide
• CISA Known Exploited Vulnerabilities Catalog (CVE not currently listed; monitor): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor and exploit-in-the-wild evidence: none available. No public PoC, no exploit code, no Nuclei template, no CISA KEV listing, and no EPSS score were identified in any tracked source. Exploit likelihood is currently unquantified, and no known campaigns are associated with this CVE.