CVE-2026-88155

Executive Summary

CVE-2026-88155 is a heap-based buffer overflow vulnerability (CWE-122) in Microsoft Office SharePoint that allows an authenticated, authorized attacker to achieve remote code execution over a network. With a CVSS base score of 8.8 (High), the flaw requires low privileges (PR:L) and no user interaction (UI:N) but grants full confidentiality, integrity, and availability impact within the vulnerable component's security scope. Organizations running on-premises SharePoint deployments that expose authenticated endpoints to network-adjacent or internet-reachable users should treat this as a high-priority exposure pending vendor remediation.

Technical Analysis

CVE-2026-88155 is classified as a heap-based buffer overflow (CWE-122). This weakness class occurs when a program writes more data into a fixed-size memory buffer allocated on the heap than the buffer can hold, corrupting adjacent heap metadata and objects. In the context of a network-facing application such as SharePoint, a heap overflow typically arises when attacker-supplied input (for example the body or a structured field of an HTTP request) is copied or serialized into a heap-allocated structure without adequate bounds checking or length validation.

The CVSS vector (AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) indicates the following about the attack surface:

• AV:N (Network): The vulnerable component is reachable over a network, consistent with a web application front end such as SharePoint.
• AC:L (Low complexity): No specialized conditions or significant attacker effort are required to trigger the flaw.
• PR:L (Low privileges): The attacker must possess some form of valid authorization — an authenticated account or otherwise authorized session — before the vulnerable code path can be reached.
• UI:N (No user interaction): Exploitation does not depend on a victim user opening a file or clicking a link.
• S:U (Unchanged): The impact is confined to the vulnerable component's security authority.
• C:H / I:H / A:H: Successful exploitation yields high impact to confidentiality, integrity, and availability.

A heap overflow in a server-side component can be leveraged to corrupt heap structures and, in favorable conditions, redirect execution flow to attacker-controlled code. Because the impacted component is a server reached over the network, this maps to remote code execution under the security context of the SharePoint process.

Important caveat on technical specificity: The provided intelligence contains no CNA title, no affected version list, no file paths, no function or method names, no HTTP endpoints, and no parameter names. Per reporting policy, this report does not invent any such details. The precise vulnerable code path, the request structure that reaches it, and the exact heap object involved are not derivable from the available data and are marked as follows: insufficient technical detail available to specify the vulnerable function, request parameter, or exploitation primitive beyond the buffer-overflow class described above.

Exploitation Scenarios

No public proof-of-concept is known (POC_STATUS: None), and there is no evidence of exploit code or a Nuclei template at the time of writing. The attack chain below is therefore described strictly at the vulnerability-class level and is inferred from the CWE-122 classification, the CVSS metrics, and the fact that the impacted product is a network-accessible collaboration platform.

1. Reconnaissance / Target Identification. An adversary identifies internet-facing or network-reachable SharePoint deployments. This is typically accomplished via search-engine and internet-scanning services using product fingerprints (e.g., HTTP response signatures and TLS certificate subject fields associated with SharePoint), and by querying for authentication portals that confirm the service is exposed.

2. Obtaining Authorized Access. Because the CVSS vector specifies PR:L, the attacker must first hold a low-privileged, authorized account or session. This could be a legitimate but low-privileged user, a compromised account, a guest or externally shared identity, or credentials obtained through prior phishing or credential-stuffing activity. The overflow is not reachable pre-authentication based on the stated metrics.

3. Triggering the Heap Overflow. With an authorized position, the attacker submits crafted input to the vulnerable server-side processing path. At the vulnerability-class level, this means supplying input whose length or structure exceeds the bounds allocated to a heap buffer, causing an out-of-bounds write and heap corruption. The exact request element, encoding, and delivery mechanism are not specified in the available intelligence and are therefore not asserted here.

4. Achieving Code Execution. Heap corruption of this nature, when reliably controlled, can enable the attacker to influence control flow, potentially landing on attacker-controlled data and executing arbitrary code in the context of the SharePoint server process. This aligns with the CVSS assessment of High integrity and confidentiality impact and the RCE framing in the NVD description.

5. Post-Exploitation Impact. Successful code execution within a SharePoint web front end can yield access to application configuration, credentials, and connected data stores, and can serve as a foothold for lateral movement or persistence within the hosting environment. Given "Authorized attacker" and "over a network" wording, the realistic initial objective is server-side compromise of the SharePoint application tier rather than client-side compromise of an end-user workstation.

Note: No public PoC available; attack chain is inferred from vulnerability class. No specific endpoints, parameters, or code primitives are fabricated in this section.

Impact Assessment

The CVSS v3.1 base score is 8.8 (High). Subscore contributions:

• Confidentiality: High. Executed code within the SharePoint process can read data reachable by that process, including documents, configuration, and potentially cached secrets.
• Integrity: High. An attacker able to run code or alter server-side state can modify content, configuration, or logic.
• Availability: High. Heap corruption and code execution can crash or destabilize the service, and deliberate actions could render content unavailable.
• Exploitability: AV:N / AC:L / PR:L / UI:N. The combination of low privileges, no user interaction, and network reachability makes this realistically exploitable by any actor who already holds a valid low-privileged account. The absence of a UI requirement means no victim-side trigger is needed.
• Scope: Unchanged. Impact is contained to the vulnerable component's authority, though in practice a SharePoint server compromise can have significant downstream consequences for connected systems.

Business impact. SharePoint commonly serves as the central document-management and collaboration platform for an organization, frequently hosting sensitive business, legal, HR, and strategic content. A remote code execution flaw in this tier threatens the confidentiality and integrity of that corpus, can undermine trust in the platform, and may provide a pivot point into adjacent enterprise systems. Affected systems are Microsoft Office SharePoint deployments (on-premises and/or hosted configurations consistent with the NVD wording); however, no specific affected version range is provided in the available intelligence, so scoping must be performed directly against vendor advisories and asset inventories. Blast radius is a function of how widely the platform is deployed and how many principals hold authorized (even low-privileged) accounts.

Detection Guidance

Sigma Rule

title: Potential SharePoint Heap Buffer Overflow Exploitation Attempt (CVE-2026-88155)
id: 3b9f6c1a-8a4d-4f72-9c3e-cve202688155
status: experimental
description: Detects anomalous server-side process behavior consistent with heap-based buffer overflow exploitation of Microsoft SharePoint (CWE-122). Generic correlation logic; tune to environment.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-88155
author: valtersit.com
date: 2026/01/01
logsource:
  product: windows
  service: application
detection:
  selection_process:
    Provider_Name: 'ASP.NET'
    Image|endswith: '\w3wp.exe'
  selection_anomaly:
    EventID:
      - 1000   # Application Error / crash
      - 1026   # .NET Runtime unhandled exception
    Data|contains:
      - 'System.AccessViolationException'
      - 'BufferOverrun'
      - 'STATUS_HEAP_CORRUPTION'
  condition: selection_process or selection_anomaly
falsepositives:
  - Legitimate application crashes unrelated to exploitation
  - Routine w3wp.exe recycling under load
level: medium
tags:
  - attack.t1190
  - cve.2026.88155

YARA Rule

rule CVE_2026_88155_SharePoint_Heap_Overflow_Generic
{
    meta:
        description = "Generic heuristic for heap-corruption exploitation artifacts associated with CVE-2026-88155 (SharePoint, CWE-122)"
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-88155"
        cve = "CVE-2026-88155"
    strings:
        $a1 = "STATUS_HEAP_CORRUPTION" ascii wide
        $a2 = "HEAP_CORRUPTION_DETECTED" ascii wide
        $a3 = "0xC0000374" ascii wide
        $a4 = "AccessViolationException" ascii wide
    condition:
        any of them
}

Note: This is a behavioral/heuristic rule to flag corruption-related artifacts; it is not a signature for a specific known exploit, since no PoC or exploit code is available.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-88155 Possible SharePoint Heap Buffer Overflow Attempt (Oversized Request)"; flow:established,to_server; http.method; content:"POST"; http.header_names; content:"|0d 0a|Content-Length|0d 0a|"; dsize:>100000; metadata:service http; reference:cve,2026-88155; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-88155; classtype:attempted-admin; sid:202688155; rev:1;)

Note: Generic oversize-request heuristic for the overflow class; no specific byte signature is known because no PoC exists. Tune dsize and thresholds to baseline traffic.

Elastic Detection

sequence by host.id with maxspan=1m
  [process where process.name == "w3wp.exe" and event.category == "process"]
  [file where event.action == "creation" and process.name in ("cmd.exe","powershell.exe","w3wp.exe")]

Note: Correlates SharePoint worker process activity with suspicious child-process or file creation, a common post-RCE indicator. Adapt indices and field names to your Elastic deployment.

Splunk SPL Query

index=windows (source="XmlWinEventLog:Application" OR EventCode=1000 OR EventCode=1026)
(Provider_Name="ASP.NET" OR Image="*\\w3wp.exe" OR Message="*HEAP_CORRUPTION*" OR Message="*0xC0000374*")
| stats count min(_time) as first_seen max(_time) as last_seen by host, EventCode, Message
| where count > 0
| sort - count

Note: Detects application-error and crash signatures consistent with heap corruption in the SharePoint worker process. Expand to endpoint logs for richer correlation.

Wazuh OSSEC Rule

<group name="windows,cve-2026-88155,">
  <rule id="100881" level="8">
    <if_sid>60000</if_sid>
    <field name="Provider_Name">ASP.NET</field>
    <field name="Data" type="pcre2">HEAP_CORRUPTION|0xC0000374|BufferOverrun|AccessViolationException</field>
    <description>CVE-2026-88155: Possible SharePoint heap buffer overflow exploitation artifact detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Note: Wazuh rule ID is environment-local and must be within your custom range. Tune field mappings to the decoded Windows Application event channel.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, workaround, or affected-version list is available in the intelligence provided, and CVE-2026-88155 is not listed in CISA KEV. Until a vendor patch is published, prioritize compensating controls and continuous monitoring.

1) Immediate mitigations

• Restrict network exposure of SharePoint: place internet-facing or network-reachable deployments behind authenticated reverse proxies, VPNs, or zero-trust access gateways so that only trusted, authorized principals can reach the application. This directly reduces the exploitable surface, since exploitation requires low privileges (PR:L).
• Apply the principle of least privilege to SharePoint accounts and minimize the number of principals with access to potentially vulnerable content-handling functionality.
• Enforce strong authentication (MFA) and monitor for credential abuse, since a valid low-privileged session is the prerequisite for exploitation.
• Deploy the detection rules above and enable verbose application/crash logging on SharePoint web front ends (including .NET Runtime and Application Error events) to surface heap-corruption indicators.
• Increase scrutiny of unusually large or malformed requests to SharePoint endpoints and baseline normal request sizes to tune the Suricata and Sigma thresholds.

2) Patch / upgrade steps

• No patch is currently available per tracked sources. Monitor Microsoft's official security update channels in your licensing tier and subscribe to vendor advisories for SharePoint.
• When a fix is released, inventory all SharePoint deployments (on-premises and hybrid) and stage, test, and deploy the update in line with change management. Validate the fix by confirming the affected build and re-testing the previously exposed path once vendor guidance identifies it.
• Re-verify affected-version scope against the vendor advisory when published, as no version range is specified in the available intelligence.

3) Long-term hardening

• Segment SharePoint server tiers from general corporate networks to limit lateral movement following any compromise.
• Re-baseline the platform security posture: apply the principle of least privilege to service accounts, harden the underlying OS and IIS, and disable unnecessary features and content types.
• Establish continuous vulnerability and asset management so that new SharePoint advisories (including affected-version data) can be mapped quickly to deployed instances.
• Adopt a compensating-control-first policy for network-facing collaboration platforms: assume exposure and build monitoring, segmentation, and access control that reduce impact independently of patch availability.

References

• NVD — CVE-2026-88155: https://nvd.nist.gov/vuln/detail/CVE-2026-88155
• CWE-122 — Heap-based Buffer Overflow: https://cwe.mitre.org/data/definitions/122.html
• FIRST.org EPSS (CVE-2026-88155 not yet scored): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-88155 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Microsoft Security Response Center / Security Update Guide: https://msrc.microsoft.com/update-guide

Threat actor / exploitation evidence: None identified. There is no public PoC, no exploit code, no Nuclei template, and no CISA KEV entry for CVE-2026-88155. Any claim of active in-the-wild exploitation would be unsupported by the available intelligence and is explicitly not asserted here.