CVE-2026-88155: Microsoft SharePoint Server Buffer Overflow

CVE-2026-88155 Overview

CVE-2026-88155 is a heap-based buffer overflow vulnerability in Microsoft SharePoint Server. The flaw allows an authenticated attacker to execute arbitrary code over the network against a vulnerable SharePoint instance. Microsoft published the advisory on October 13, 2026, and the issue is tracked under CWE-122: Heap-based Buffer Overflow.

Exploitation requires low-level privileges but no user interaction. A successful attack results in full compromise of confidentiality, integrity, and availability on the affected SharePoint server. The vulnerability affects the SharePoint Server Subscription Edition.

Critical Impact
Authenticated remote attackers can trigger a heap overflow in SharePoint Server to execute arbitrary code with the privileges of the SharePoint service account.

Affected Products

Microsoft SharePoint Server Subscription Edition
SharePoint deployments exposed to authenticated users over the network
On-premises SharePoint environments using the vulnerable component

Discovery Timeline

2026-10-13 - CVE-2026-88155 published to NVD
2026-10-13 - Microsoft releases security update guidance
2026-10-14 - Last updated in NVD database

Technical Details for CVE-2026-88155

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] within Microsoft SharePoint Server. Heap overflows occur when a program writes data beyond the bounds of a dynamically allocated memory region. In SharePoint, such corruption can overwrite adjacent heap metadata or object pointers used by the worker process.

Attackers who control the overflowed bytes can manipulate program flow and execute code in the context of the SharePoint application pool identity. Because SharePoint commonly runs with elevated service privileges and holds sensitive site content, successful exploitation gives attackers broad access to data and lateral-movement opportunities.

Root Cause

The root cause is improper validation of input length or bounds when SharePoint processes attacker-supplied data into a heap buffer. The affected code path does not enforce size limits before copying content into the allocated region. Microsoft has not published detailed component-level analysis in the public advisory.

Attack Vector

The attack vector is network-based and requires valid authenticated access to the SharePoint server. Attack complexity is low, and no user interaction is required. An attacker sends a crafted request to a vulnerable SharePoint endpoint, triggering the overflow during server-side processing. Successful exploitation yields remote code execution within the SharePoint process context.

No public proof-of-concept exploit, CISA KEV listing, or in-the-wild exploitation has been reported at the time of publication. The EPSS probability is 0.819% (56th percentile) as of October 14, 2026.

Detection Methods for CVE-2026-88155

Indicators of Compromise

Unexpected crashes or faulting threads in the SharePoint worker process (w3wp.exe) tied to the SharePoint application pool
Anomalous child processes spawned by w3wp.exe such as cmd.exe, powershell.exe, or script interpreters
Outbound network connections initiated by the SharePoint service to unknown or newly observed hosts
Creation of new files in SharePoint LAYOUTS, _vti_bin, or wwwroot directories by the application pool identity

Detection Strategies

Monitor IIS logs for abnormally large request bodies, malformed SOAP or REST payloads, and repeated requests to the same SharePoint handler
Alert on process-tree anomalies where w3wp.exe spawns command shells, scripting engines, or encoded PowerShell
Correlate SharePoint ULS logs with Windows crash dumps to identify repeated heap corruption events
Hunt for authenticated sessions originating from unusual geographies or service accounts accessing SharePoint APIs

Monitoring Recommendations

Enable verbose IIS and SharePoint ULS logging and forward to a centralized analytics platform
Capture process creation and command-line auditing (Windows Event ID 4688, Sysmon Event ID 1) on SharePoint hosts
Baseline normal SharePoint API traffic to make anomalous payloads visible
Alert on web shell indicators under SharePoint content directories

How to Mitigate CVE-2026-88155

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guidance for CVE-2026-88155 as soon as change windows allow
Inventory all SharePoint Server Subscription Edition instances and prioritize internet-exposed systems
Audit SharePoint account privileges and revoke unnecessary standing access to reduce the pool of potential attackers
Review authentication logs for suspicious logins preceding the patch window

Patch Information

Microsoft published the official fix through the Microsoft Security Response Center. Administrators should apply the SharePoint Server Subscription Edition cumulative update that remediates CVE-2026-88155. Full details, download links, and version information are available in the Microsoft Security Update Guidance.

Workarounds

Restrict network access to SharePoint endpoints using firewall rules, VPN, or Zero Trust Network Access controls until patching completes
Enforce multi-factor authentication on all SharePoint user and service accounts to limit abuse of authenticated access
Place a web application firewall in front of SharePoint and tune rules against oversized or malformed request bodies
Isolate SharePoint servers from direct internet exposure where business requirements permit

bash

# Example: restrict SharePoint HTTPS access to trusted subnets via Windows Firewall
New-NetFirewallRule -DisplayName "Restrict SharePoint HTTPS" `
  -Direction Inbound `
  -Protocol TCP `
  -LocalPort 443 `
  -RemoteAddress 10.0.0.0/8,192.168.0.0/16 `
  -Action Allow