CVE-2026-88163

Executive Summary

CVE-2026-88163 is a remote code execution vulnerability in Microsoft Office SharePoint, caused by an integer overflow or wraparound condition (CWE-122). An authorized attacker with valid credentials and network access to a SharePoint instance can exploit this flaw to execute code in the context of the SharePoint application. With a CVSS base score of 8.8 (High), successful exploitation yields full compromise of confidentiality, integrity, and availability of the affected service.

Technical Analysis

The NVD description classifies this vulnerability as an "integer overflow or wraparound" (CWE-122 is the heap-based buffer overflow weakness, and the CNA text explicitly names integer overflow/wraparound as the root mechanism). In this class of defect, an arithmetic operation on a size, offset, or length value produces a result that exceeds the maximum value of the integer type used to store it. The wrapped value is then used as an allocation size, a buffer bound, or an index limit. When the wrapped value is smaller than the true intended size, a subsequent write can exceed the allocated buffer, producing a heap (or adjacent memory) corruption primitive that an attacker can steer toward controlled code execution.

The affected component is Microsoft Office SharePoint, an ASP.NET/.NET server application. The attack surface is the network-facing SharePoint web application, reachable over HTTP/HTTPS. The CVSS vector (AV:N) confirms the vulnerable logic is reachable across a network boundary — no local access is required. The vector records PR:L and UI:N, meaning the attacker must hold a valid, low-privileged authenticated session on the SharePoint instance, but no action by a legitimate victim user is needed to trigger the flaw. The scope is unchanged (S:U), so the exploit executes within the security authority of the SharePoint application itself.

Because no CVE-specific technical detail (affected code path, parameter, or function) is present in the supplied intelligence, the precise location of the integer overflow within SharePoint cannot be stated. The vulnerability class, reachability, and required privilege level are known; the specific trigger is not. No public proof-of-concept, Nuclei template, or exploit code is available.

Exploitation Scenarios

The attack chain below is inferred from the vulnerability class and the CVSS vector. No public PoC exists; specific implementation details are not available.

1. Reconnaissance / target identification. The attacker identifies internet- or intranet-exposed SharePoint instances (commonly discovered via search-engine dorks such as SharePoint sign-in pages and product fingerprints). Any SharePoint deployment with a network-reachable endpoint that processes integer-derived size or length values from request data is a candidate.

2. Precondition — authenticated access. Per PR:L, the attacker must first obtain valid low-privileged credentials. This could be via phishing, credential stuffing, password reuse, purchase of leaked credentials, or a separate initial-access vector. Anonymous access is not sufficient; a session established against the SharePoint application is required.

3. Triggering the integer overflow. Using the authenticated session, the attacker submits crafted input over the network to the vulnerable SharePoint code path. Because the root cause is integer overflow/wraparound, the malicious input is designed to drive an arithmetic size or length computation past its integer limit so the computed value wraps to a small number. That wrapped value is then used where the true (larger) size was intended, so a following memory write exceeds the allocated region.

4. Payload / execution (class level). Heap corruption of this sort is converted into code execution by overwriting adjacent heap metadata or a controlled object's data with attacker-influenced values, then hijacking a function pointer or similar control-flow target during normal processing. The exact technique cannot be specified without knowing the affected code. Execution occurs in the security context of the SharePoint application (typically the application pool identity), which may be highly privileged on the host.

5. Post-exploitation impact. Successful exploitation gives the attacker code execution on the SharePoint server with the permissions of the SharePoint application pool account, potentially enabling lateral movement, persistence, and access to content stored in or reachable from SharePoint.

Impact Assessment

• Confidentiality (C:H): Full read access to data reachable by the SharePoint application process, including site collections, documents, and configuration.
• Integrity (I:H): The attacker can modify application data and, via code execution, the server environment.
• Availability (A:H): Code execution and heap corruption can crash or destabilize the service, and the attacker can disrupt availability.
• CVSS 8.8 (High), AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Low attack complexity and no user interaction make this attractive to a motivated, authenticated attacker; only the PR:L requirement limits the unauthenticated blast radius.
• Blast radius: every affected SharePoint server accessible to the attacker. A compromised application pool identity on SharePoint is a high-value foothold in Windows/Active Directory environments, so the effective blast radius can extend well beyond the SharePoint farm.

Detection Guidance

Sigma Rule

title: Potential Integer Overflow Exploitation Attempt Against SharePoint (CVE-2026-88163)
id: 8f3a1c2e-6b47-4e91-9d2a-1c0e5b7a9f31
status: experimental
description: Detects anomalous SharePoint worker process behavior consistent with heap corruption / integer overflow exploitation (CVE-2026-88163).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-88163
author: valtersit.com
date: 2026-01-01
tags:
  - attack.t1190
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith: '\w3wp.exe'
  selection_suspicious_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wmic.exe'
      - '\net.exe'
  condition: selection_parent and selection_suspicious_child
falsepositives:
  - Legitimate administration scripts executed from the SharePoint application pool
level: high

YARA Rule

rule CVE_2026_88163_SharePoint_Integer_Overflow
{
    meta:
        description = "Heuristic detection of suspicious .NET/SharePoint artifacts consistent with CVE-2026-88163 exploitation attempts"
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-88163"
        severity = "high"

    strings:
        $sharepoint = "Microsoft.SharePoint" ascii wide nocase
        $w3wp = "w3wp.exe" ascii wide nocase
        $overflow = "Integer overflow" ascii wide nocase
        $alloc = "Marshal.AllocHGlobal" ascii wide
        $copy = "Buffer.BlockCopy" ascii wide

    condition:
        uint16(0) == 0x5A4D and
        $sharepoint and
        (2 of ($overflow, $alloc, $copy))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-88163 Potential SharePoint Integer Overflow Exploitation Attempt"; flow:established,to_server; http.method; content:"POST"; http.header; content:"SharePoint"; http.request_body; content:"|00 00 00 00|"; threshold:type both, track by_src, count 15, seconds 60; classtype:attempted-admin; reference:cve,2026-88163; sid:202688163; rev:1;)

Note: this Suricata rule uses generic HTTP request-body heuristics and is not CVE-specific, because no CVE-specific request structure is published. Tune thresholds before production deployment.

Elastic Detection

sequence by host.name with maxspan=30s
  [ process where event.type == "start" and process.parent.name == "w3wp.exe" ]
  [ process where event.type == "start" and
      process.name in ("cmd.exe", "powershell.exe", "wmic.exe", "net.exe")]

Splunk SPL Query

index=windows (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR source="WinEventLog:Security")
(EventCode=1 OR EventID=4688)
| where match(ParentImage, "(?i)\\\\w3wp\.exe$") AND match(Image, "(?i)\\\\(cmd|powershell|wmic|net)\.exe$")
| stats count min(_time) as first_seen max(_time) as last_seen by ComputerName, User, ParentImage, Image, CommandLine
| where count > 0
| convert ctime(first_seen) ctime(last_seen)

Wazuh OSSEC Rule

<group name="windows,sharepoint,cve-2026-88163,">
  <rule id="100881" level="12">
    <if_sid>61600</if_sid>
    <field name="win.eventdata.parentImage">w3wp\.exe$</field>
    <field name="win.eventdata.image">\\(cmd|powershell|wmic|net)\.exe$</field>
    <description>CVE-2026-88163: SharePoint worker process (w3wp.exe) spawned a suspicious child process - possible integer overflow exploitation</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_88163,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix or workaround is published for CVE-2026-88163 at this time; the guidance below is defensive and class-based.

1. Immediate mitigations

   • Reduce exposure: do not expose SharePoint directly to the internet; place it behind a reverse proxy, VPN, or zero-trust access broker so only authenticated, trusted networks can reach it.
   • Enforce least privilege on SharePoint accounts: since exploitation requires only PR:L, tighten account provisioning, enforce MFA, and remove stale or unused accounts to shrink the population of usable credentials.
   • Detect/block credential abuse (phishing, stuffing, reuse) that could grant the required authenticated foothold.
   • Monitor the SharePoint application pool identity; run it with the minimum permissions required, and never as a highly privileged domain account.
   • Deploy WAF rules and the detection content above to flag anomalous authenticated request patterns and suspicious w3wp.exe child processes.

2. Patch / upgrade steps

   • Monitor Microsoft's security update channels (MSRC, Microsoft 365 / SharePoint security advisories) for a fix and the exact affected version matrix.
   • No affected-version list or fixed version is available from tracked sources; when a patch is released, inventory all SharePoint servers/appliances, prioritize internet-facing and high-value farms, and apply during a controlled maintenance window.

3. Long-term hardening

   • Establish a SharePoint/Office patch and inventory baseline (versions, exposed endpoints, credential inventory).
   • Adopt network segmentation between SharePoint, Active Directory, and internal data stores to limit lateral movement after a compromise.
   • Enable comprehensive logging (HTTP access logs, Sysmon process creation, application pool events) and forward to the SIEM, with the detections above tuned and validated.
   • Run periodic authenticated-access reviews and privilege audits; remove standing administrative rights from service accounts.
   • Track this CVE for patch and EPSS updates; re-evaluate exploitation likelihood when public PoC or KEV status changes.

References

• NVD — CVE-2026-88163: https://nvd.nist.gov/vuln/detail/CVE-2026-88163
• CWE-122 (Heap-based Buffer Overflow): https://cwe.mitre.org/data/definitions/122.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• Microsoft Security Response Center (MSRC) security update guide: https://msrc.microsoft.com/update-guide
• CISA Known Exploited Vulnerabilities (KEV) catalog (status check for CVE-2026-88163): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No vendor advisory, public PoC, exploit code, or confirmed in-the-wild campaign is currently reported for CVE-2026-88163. The vulnerability is not listed in CISA KEV. No FIRST.org EPSS score is available. Detection content above is heuristic and class-based because no CVE-specific technical detail is published.