CVE-2021-31955: Microsoft Windows Kernel Information Disclosure Vulnerability

CVE-2021-31955 is an information-disclosure vulnerability in the Microsoft Windows kernel. It lets a local attacker running code in user mode read contents of kernel memory. That kind of leak can expose sensitive system state and help an attacker plan further privilege escalation or persistence. Public detail on exact mechanics is limited; treat the Microsoft advisory as the authoritative source.

CISA notes that successful exploitation allows attackers to read kernel memory from a user-mode process. Ransomware use is not documented for this CVE. The required action is to apply updates per vendor instructions.

How it works

The weakness is classed as CWE-497: exposure of sensitive system information to an unauthorized control sphere. In practical terms, the Windows kernel fails to properly restrict what a user-mode process can observe. An attacker who can already execute code on the host (for example via a malicious binary, script, or compromised account) abuses that gap to pull data out of kernel address space that should remain inaccessible.

Kernel memory can contain pointers, object layouts, credentials material, or other internal state. Even without a full remote code-execution chain, the disclosed information reduces the work needed for follow-on attacks. Exact trigger conditions, IOCTLs, or structures involved are not specified in the public summary; confirm those details against the vendor advisory rather than assuming a particular exploit path.

Am I affected? How to find it in your systems

The vulnerability affects Microsoft Windows. It is relevant anywhere the Windows kernel is running: workstations, servers, virtual machines, and cloud instances based on Windows. Inventory every Windows host in your environment, including domain controllers, jump boxes, VDI pools, and build agents.

• Use your asset-management or endpoint-management console (SCCM/ConfigMgr, Intune, WSUS reporting, vulnerability scanners) to list installed Windows editions and cumulative update levels.
• Compare each host’s patch level against the security updates Microsoft lists for CVE-2021-31955 in its advisory; do not rely on version guesses.
• Confirm whether local code execution is possible for untrusted users or processes—shared workstations and servers that allow interactive logon or third-party software installation raise exposure.
• Review endpoint telemetry and Windows security/system logs for anomalous user-mode processes that issue unusual kernel queries or that crash shortly after starting; information-disclosure attempts often leave only subtle traces, so treat absence of clear IOCs as inconclusive.

If your scanner or Microsoft’s own tools flag the CVE as applicable, assume the host needs the update until you verify otherwise.

How to remediate

Patch first. Apply the security updates Microsoft released for this vulnerability, following the vendor’s instructions exactly. Use your standard deployment ring (test, then pilot, then broad) but prioritize hosts that allow local code execution by multiple users or that hold high-value data.

• Deploy the relevant cumulative or security-only update through WSUS, Intune, SCCM, or your patch platform; reboot as required so the kernel changes take effect.
• After patching, verify the update is installed (Get-HotFix, Windows Update history, or your CMDB) and that the CVE no longer appears as open in vulnerability scans.
• Harden residual risk for this class of flaw: enforce least privilege so ordinary users cannot run arbitrary binaries, enable application control (AppLocker, WDAC) where feasible, and keep credential-guard and similar kernel-isolation features enabled on supported editions.
• Maintain prompt patch cadence for the Windows kernel going forward; information-disclosure bugs are frequently chained with other local elevation issues.

If you can't patch immediately

When immediate patching is blocked by change windows or compatibility testing, reduce the attack surface until the update can be applied.

• Segment and restrict local logon: limit who can interactively log on to servers; remove unnecessary local admin rights; prefer privileged-access workstations for administration.
• Application control and ASR rules: block untrusted executables, scripts, and office macros that could give an attacker the initial user-mode foothold needed to abuse the disclosure.
• Increase monitoring: alert on new processes running as STANDARD users that load unusual drivers or that repeatedly touch kernel interfaces; forward Sysmon or equivalent logs to your SIEM and hunt for post-exploitation patterns that commonly follow kernel leaks.
• Virtual patching is limited for pure local kernel information disclosure; host-based controls and rapid isolation of suspicious endpoints matter more than network WAF rules.
• Schedule the official Microsoft update as soon as practicable—compensating controls only buy time.

If your data may have been exposed

Actively exploited kernel vulnerabilities can be a stepping stone into broader compromise even when ransomware use is not documented for the specific CVE. If you have evidence of exploitation or of suspicious local code execution on unpatched hosts, follow your incident-response process: isolate affected systems, preserve memory and disk evidence, rotate credentials that may have been present in memory, and check for lateral movement.

As a quick additional check, you can run a free exposure scan of your email addresses against known breach data to see whether your accounts already appear in public breach corpora, then tighten passwords and MFA accordingly.