Microsoft's September 2026 Patch Tuesday fixes 999 CVEs, a record, with two zero-day privilege escalation flaws already exploited in the wild.
Microsoft published 974 own-product vulnerabilities plus 25 non-Microsoft CVEs, totaling 999 — the most CVEs Microsoft has ever released in a single day. Two flaws are exploited in the wild: CVE-2026-85880, an out-of-bounds write in Windows ALPC granting SYSTEM privileges, and CVE-2026-81963, an improper link resolution flaw in the Windows Update Stack also leading to SYSTEM. Chrome's V8 zero-day CVE-2026-85046 was patched in Edge on September 2, but Microsoft had not published a corresponding advisory, leaving uncertainty about other Chromium fixes in Edge. October 14 lifecycle changes end servicing for Windows 11 24H2 Home/Pro, Office 2021, and Exchange Server 2016/2019.
Heap-Based Buffer Overflow in Windows ALPC Enables Local Privilege Escalation
CVE-2026-85880 is a heap-based buffer overflow in Windows Advanced Local Procedure Call (ALPC), the Windows mechanism for local inter-process communication. An authorized local attacker can trigger the overflow by submitting crafted input over ALPC, corrupting heap memory in the component that handles the request. Successful exploitation allows the attacker to execute code with elevated privileges, typically gaining SYSTEM-level control of the local host, which is especially valuable as a post-exploitation or sandbox-escape step. Affected products include Windows 10 (1607, 1809, 21H2, 22H2) and Windows Server 2012, 2016, 2019, and 2022, meaning most on-premises Windows estates are in scope. The flaw was fixed in Microsoft's record 974-CVE September 2026 Patch Tuesday and was added to CISA's KEV on 2026-09-08, confirming exploitation in the wild; press reports describe Windows zero-days being chained with a Chrome zero-day in 'BlueMoon' kit attacks, though the data does not explicitly confirm this CVE is the Windows flaw in that chain.
· Microsoft Windows 10 1607, 1809, 21H2, 22H2 · Microsoft Windows Server 2012, 2016, 2019, 2022 KEVmass
Local Privilege Escalation via Link Following in Windows Update Stack
CVE-2026-81963 is a link-following flaw (CWE-59, improper link resolution before file access) in the Microsoft Windows Update Stack, in which the component fails to correctly resolve file links before opening them. A local attacker with low privileges can plant or manipulate a link (symlink/junction) that the privileged update stack follows during operation, redirecting its file access to an attacker-controlled target. The result is local privilege escalation — CVSS 3.1 rates this 7.8 (high) with high confidentiality, integrity, and availability impact — allowing an authorized local user or malware already on the machine to gain elevated rights. Affected products are Windows 11 23H2, 24H2, 25H2, and 26H1 and Windows Server 2025; any unpatched system on those versions is exposed to any local account holder. The flaw was fixed in Microsoft's record September 2026 Patch Tuesday (974 CVEs), was added to CISA's KEV on 2026-09-08 as one of two Windows zero-days reported as exploited in the wild, and has no known public PoC or confirmed ransomware use.
· Microsoft Windows 11 23H2, 24H2, 25H2, 26H1 · Microsoft Windows Server 2025 KEVmass
Missing Authorization in Microsoft Entra ID Enables Privilege Escalation
CVE-2026-83941 is a missing-authorization flaw (CWE-862) in Microsoft Entra ID, the cloud identity service behind Microsoft 365 and Azure. An already-authenticated, low-privileged user can send a network request to an Entra ID endpoint that fails to enforce proper authorization checks, requiring no user interaction. Exploitation lets the attacker elevate their privileges within the directory, with high confidentiality and integrity impact (CVSS 9.9, scope changed). Any organization that uses Microsoft Entra ID is in the affected population. The flaw was patched in Microsoft's September 2026 Patch Tuesday release; it is not in CISA KEV, has no known public proof-of-concept, and carries a low EPSS of roughly 0.7%.
Permissive Cross-Domain Policy Privilege Elevation in Microsoft Azure Arc
Microsoft Azure Arc (CVE-2026-62895) contains a privilege elevation flaw caused by a permissive cross-domain policy that trusts untrusted domains, mapped by Microsoft to CWE-942 (permissive cross-domain policy with untrusted domains), CWE-1390 (weak authentication) and CWE-89 (SQL injection). An unauthenticated attacker can exploit it over a network, with CVSS scoring indicating some user interaction is required (AV:N/AC:L/PR:N/UI:R), by abusing the overly permissive cross-domain trust to elevate privileges, with high impact on confidentiality, integrity and availability (CVSS 3.1: 8.8 High). Any organization using Azure Arc to manage hybrid or multi-cloud resources is potentially affected. There is no evidence of exploitation in the wild, no known public proof-of-concept, the flaw is not in CISA's KEV catalog, and EPSS estimates only a 0.7% probability of exploitation in the next 30 days (52nd percentile). Microsoft addressed the flaw as part of its September 2026 Patch Tuesday release, which fixed 966 flaws including 2 zero-days.
Authorization Bypass Through User-Controlled Key in Microsoft Azure Cosmos DB
CVE-2026-69857 is an authorization bypass through a user-controlled key (CWE-639, an IDOR-style flaw) in Microsoft Azure Cosmos DB, Microsoft's managed NoSQL database service: an authenticated principal can supply or alter a resource identifier (key) that the service fails to validate against the caller's permissions. Triggered over a network by a low-privileged, authorized user who manipulates such a key (CVSS 3.1 vector AV:N/AC:L/PR:L/UI:N), the flaw lets the attacker bypass access checks and perform spoofing, i.e., impersonate another identity or act outside their authorized scope. Per the CVSS vector, successful exploitation carries high impact on the confidentiality, integrity, and availability of data in the affected database account, since the attacker can read or modify data as a different authorized identity. Any organization using Azure Cosmos DB is potentially affected; because it is a fully managed Azure service there are no customer-managed installations, and no affected version ranges are specified in the advisory data. The flaw was addressed in Microsoft's September 2026 Patch Tuesday cycle (966 flaws fixed, including 2 zero-days); no public proof-of-concept is known, it is not in CISA's KEV, and EPSS estimates only a ~0.4% chance of exploitation within 30 days.
Improper Authentication in Microsoft Authenticator Enables Local Privilege Escalation
CVE-2026-80097 is an improper authentication flaw (CWE-287) in Microsoft Authenticator that allows an unauthorized attacker to elevate privileges locally, rated 8.6 (high) with no privileges required, required user interaction, and a changed scope. Exploitation requires local access to a device running the app plus user interaction, and because the scope is changed, a successful attack crosses a security boundary beyond the Authenticator component itself. A successful exploit yields local privilege elevation with high impact to confidentiality, integrity, and availability. Anyone running affected versions of Microsoft Authenticator is affected; the app is Microsoft's standard multi-factor authentication app for Entra ID/Microsoft 365 and is widely deployed across enterprise and personal mobile devices. As of the September 2026 disclosure it is not known to be exploited - no public PoC, not in CISA KEV, EPSS 0.4% (33rd percentile) - and it was patched amid the record 974-flaw Patch Tuesday, though the two actively exploited zero-days in that release are Windows flaws, not this one.
Local Privilege Escalation in Microsoft Xbox Gaming Services (CVE-2026-58611)
CVE-2026-58611 is an improper authorization flaw (CWE-285) in Microsoft's Xbox Gaming Services on Windows in which the service fails to properly validate authorization for privileged operations. It is triggered locally: an attacker who already has a low-privileged foothold on a machine where the service is installed abuses the service without any user interaction. Successful exploitation elevates the attacker's privileges on the local machine, with high impact on confidentiality, integrity, and availability (CVSS 7.8 High). Potentially affected are Windows 10/11 PCs with the Gaming Service installed, which is typically bundled with the Xbox app, PC Game Pass, and many Microsoft Store games. As of the September 2026 Patch Tuesday there is no known exploitation, no public proof-of-concept, the flaw is not in CISA KEV, and EPSS assigns only a 0.2% probability of exploitation within 30 days.
Insufficiently Protected Credentials in Microsoft Azure CycleCloud
CVE-2026-77909 is a credential-protection flaw (CWE-522) in Microsoft Azure CycleCloud, Microsoft's tool for creating and managing HPC clusters in Azure, in which credentials are stored or transmitted with insufficient protection. A remote attacker who already holds a low-privileged authorized account can trigger the flaw over the network, with no user interaction required. Because the CVSS scope is changed with high confidentiality impact, the exposed credentials can likely be used in another security scope — for example, to retrieve or reuse secrets that grant access to additional resources beyond the immediate component — resulting in information disclosure, though integrity and availability are unaffected. Organizations running Azure CycleCloud to orchestrate HPC workloads in their Azure subscriptions are affected. There is no known public proof-of-concept, it is not in the CISA KEV catalog, EPSS is a modest 0.6% over 30 days, and no in-the-wild exploitation has been reported; a fix was distributed as part of Microsoft's September 2026 Patch Tuesday, which addressed 966 flaws.
Capture-replay authentication bypass in Microsoft MSAL for Node.js
CVE-2026-84003 is an authentication bypass by capture-replay (CWE-294) in the Microsoft Authentication Library (MSAL) for Node.js, the library Node.js applications use to authenticate users and services against Microsoft's identity platform. An attacker positioned on the network who can capture authentication material in transit can replay it to authenticate as a legitimate user or client, with no privileges or user interaction required; the high attack complexity reflects the difficulty of intercepting and replaying the exchange while it remains valid. Successful exploitation enables spoofing with high impact on confidentiality and integrity (the attacker can act as the victim), though there is no availability impact. Any organization running Node.js server applications, APIs, daemons, or CLIs that depend on MSAL for Node.js is affected, with end users of those applications exposed through them. As of this writing there is no known public proof-of-concept, the flaw is not in CISA KEV, and EPSS puts the 30-day exploitation probability at 0.4% (37th percentile); the fix shipped in Microsoft's September 2026 Patch Tuesday.
· Microsoft Authentication Library (MSAL) for Node.js (@azure/msal-node)mass
OS Command Injection Privilege Elevation in Microsoft Azure HDInsights
CVE-2026-81349 is an operating-system command injection flaw (CWE-78) in Microsoft's Azure HDInsights managed big-data service. It is triggered when the service improperly neutralizes special elements passed into an OS command, and per the CVSS vector an attacker must already hold high-level authorized access, exploiting it over the network with no user interaction required. Successful exploitation allows the attacker to elevate privileges, with high impact on the confidentiality, integrity, and availability of the affected HDInsight environment. Only organizations running Azure HDInsight clusters are affected; the flaw was fixed as part of Microsoft's September 2026 Patch Tuesday, which addressed 966 flaws. No public proof-of-concept is known, it is not listed in CISA's KEV catalog, and EPSS estimates roughly a 0.7% chance of exploitation within 30 days.
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