CVE-2026-85880: Microsoft Windows Heap-Based Buffer Overflow Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Microsoft Windows. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-09-08, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-09-22; treat that as the outer limit, not the target.
• EPSS 3.6%, percentile 89: FIRST’s model estimates a 3.6% chance of exploitation activity in the next 30 days, higher than 89% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.
• Editorial verdict: Patch this week. Windows Zero-Days: Update Stack and ALPC EoP (Sep 2026) — Make sure every Windows machine has the September 2026 or a later cumulative update; both bugs are exploited, but an attacker needs code running on the box first.

What it is

Heap-based buffer overflow in Windows ALPC allows an authorized attacker to elevate privileges locally.

CISA’s summary

Microsoft Windows Advanced Local Procedure Call contains a heap-based buffer overflow vulnerability that allows an attacker to elevate privileges locally.

Weakness type: CWE-122, CWE-908.

Product: Microsoft Windows · Assigned by: microsoft · Published: 2026-09-08

Affected and fixed versions

Severity scores

• Attack vector: local access needed
• Attack complexity: low
• Privileges required: low (any account)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-09-08; federal due date 2026-09-22. Known ransomware use: Unknown.
• CISA required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
• CISA SSVC assessment: exploitation active, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 3.62% probability of exploitation activity in the next 30 days; percentile 89.

Timeline

• 2026-09-08 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-09-08 — CVE record published by microsoft
• 2026-09-08 — erroneo.us brief: Windows Zero-Days: Update Stack and ALPC EoP (Sep 2026)
• 2026-09-22 — CISA remediation deadline for US federal agencies
• 2026-10-14 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Windows Advanced Local Procedure Call (ALPC) Elevation of Privilege Vulnerability msrc.microsoft.com
2. Listed in CISA KEV notes (msrc.microsoft.com) msrc.microsoft.com

Vulnerability records

1. CVE.org record CVE-2026-85880 cve.org
2. NVD entry CVE-2026-85880 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
2. Listed in CISA KEV notes (cisa.gov) cisa.gov
3. Listed in CISA KEV notes (cisa.gov) cisa.gov

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