CVE-2025-55696: Windows Kernel TOCTOU Token Race Privilege Escalation

A newly recorded Windows kernel vulnerability, tracked as CVE-2025-55696, is a time‑of‑check/time‑of‑use (TOCTOU) race in the NtQueryInformationToken implementation (ntifs.h) that can allow a local attacker to elevate privileges to SYSTEM when exploited; Microsoft has published the entry in its Security Update Guide and independent vulnerability databases show a High (7.8) CVSS v3.1 base score for the issue.

Background​

Windows has long exposed token‑related APIs to allow usermode and kernel components to query and manipulate security tokens representing process or thread identities. One such routine — the query path behind NtQueryInformationToken (declared in ntifs.h) — is intended to return token metadata and limited internals to callers with appropriate privileges or contexts. A TOCTOU race in this path can briefly expose kernel data or allow inconsistent validation that attackers can abuse to modify or forge token state, leading to privilege escalation. Public trackers and vendor metadata classify the root cause of CVE‑2025‑55696 as a TOCTOU race condition and map it to the kernel token query path.
The practical concern is that token objects are the linchpin of Windows privilege model: once an attacker can manipulate or impersonate a token that maps to SYSTEM or another privileged principal, they can perform actions normally restricted to administrators. While the vulnerability is local (an attacker must be able to run unprivileged code on the host), the attacker may not need preexisting administrator rights to trigger the bug.

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What we know — verified, high‑confidence facts​

• The vulnerability is recorded as CVE‑2025‑55696 and described as a TOCTOU race in the NtQueryInformationToken implementation (ntifs.h). This is the vendor‑registered identifier and description.
• Public CVSS v3.1 reporting (vendor‑supplied) lists a base score of 7.8 (High) with a vector indicating a local attack vector and high confidentiality/integrity/availability impact. This score reflects the operational severity if the bug is weaponized.
• The attack vector is local: successful exploitation requires running code or otherwise interacting with the vulnerable API on the target host. It is not documented as remotely exploitable over a network-only channel.
• Microsoft has published the advisory entry in the Security Update Guide (the canonical KB mapping location) and has released security updates to address related kernel token or information‑disclosure/TOCTOU classes; administrators are instructed to consult the MSRC update guide for exact KB numbers for each OS build.

These points are corroborated across vendor metadata and widely referenced vulnerability mirrors; they form the high‑confidence baseline for defenders and engineers.

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Technical summary — how the vulnerability is believed to work​

TOCTOU races and token queries: a short primer​

A TOCTOU race arises when code checks a condition (time‑of‑check) and then later uses the previously validated data (time‑of‑use) with the assumption that nothing changed in the interim. In kernel contexts this can happen when:

• The kernel validates a pointer, object, or buffer and later uses that same pointer while trust boundaries (other threads, device interrupts, or usermode actors) can modify the referenced data; or
• The kernel temporarily writes sensitive data into a user‑supplied buffer and later sanitizes that buffer, creating a narrow window where a racing usermode thread can sample the sensitive contents.

When this pattern appears in token query or token management code, the consequences can be severe: a race can reveal kernel addresses (defeating KASLR), leak token fragments, or allow inconsistent validation that attackers manipulate to create or impersonate privileged tokens. Public technical write‑ups on related kernel token/IOCTL races show practical exploit sequences that pair this class of information leakage with driver write primitives to escalate to SYSTEM.

What public analysis and community write‑ups suggest about CVE‑2025‑55696​

Independent trackers and community analysis (mirroring patterns from earlier 2025 kernel info‑leak and token races) indicate the vulnerability likely manifests as a narrow TOCTOU window within NtQueryInformationToken where the kernel either:

• briefly exposes kernel pointers or token internals to a user buffer, or
• validates a token or pointer and then uses it after an intervening operation that could change the underlying object.

That transient inconsistency can be exploited by racing reads/writes from usermode to capture kernel pointers or to induce inconsistent state that makes token swapping or manipulation possible. Those inferences are consistent with multiple recent Windows kernel advisories where NtQueryInformationToken or similar APIs were used in published proofs‑of‑concept. However, the vendor advisory itself is intentionally terse and does not enumerate the exact IOCTLs, offsets, or internal structure fields; until a patch diff or independent technical postmortem is published, the precise low‑level mechanics should be treated as informed inference, not confirmed fact.

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Impact: why a local TOCTOU on token code matters more than the “local” label suggests​

On paper the attack vector is local (AV:L), which sometimes leads non‑specialists to deprioritize fixes. That is a dangerous misreading for three reasons:

• A local exploit can be chained: attackers commonly obtain a low‑privilege foothold through phishing, a malicious installer, or another vulnerability; an EoP to SYSTEM on a single host allows total compromise of that machine and often rapid lateral movement in an enterprise environment.
• Token primitives are powerful: token theft, duplication, or manipulation bypasses many user‑level defenses because Windows authorization checks rely on tokens to decide privileges. An attacker who can impersonate a SYSTEM token effectively gains administrative control of the host.
• Information leaks accelerate exploitation economics: exposing kernel pointers or token fragments defeats KASLR and can convert once‑fragile kernel exploits into reliable payloads by giving exploit developers deterministic offsets. That transformation often turns a medium‑scored CVE into high‑urgency operational risk.

Because of these cascading effects, organizations must treat CVE‑2025‑55696 as a high operational priority for hosts that accept local code execution (VDI, terminal servers, developer build machines, multi‑user desktops, and kiosks).

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Exploitability and public PoC status​

As of the latest vendor entry and public trackers: there is no widely‑published, reliable proof‑of‑concept (PoC) exploit tied specifically to CVE‑2025‑55696 indexed by major repositories at the moment of disclosure. That absence reduces immediate mass‑exploit risk but is not a guarantee of safety; historically similar token or kernel TOCTOU issues were weaponized rapidly once researchers published exploit primitives. Treat claims about in‑the‑wild exploitation as provisional until confirmed by vendor incident reports or national CERT advisories.

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Who should prioritize patching​

High priority (patch first):

• Domain controllers, jump/privileged access workstations (PAWs), and admin consoles.
• VDI/RDS servers and multi‑user terminal servers.
• Build servers, CI agents, and developer workstations that execute untrusted third‑party code.
• Cloud/virtualization hosts and guest images that allow guest‑initiated operations touching host token/query surfaces.

Medium priority:

• Standard employee desktops where local code execution is restricted but possible.
• Servers that do not accept user logons or run untrusted workloads.

Low priority (but not ignored): air‑gapped systems used for dedicated single‑purpose appliances where no local code execution is possible.
These operational priorities reflect the exploitation economics: hosts where multiple untrusted actors can run code are the most attractive targets for leveraging a local EoP.

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Practical mitigation and remediation playbook​

Patch is primary: consult Microsoft’s Security Update Guide entry for CVE‑2025‑55696 and extract the exact KB(s) for each OS SKU before deploying. The vendor update is the authoritative remediation and must be applied per standard change‑control processes.
If immediate patching is impossible, implement these compensating controls and detection measures:

• Enable Memory Integrity (HVCI) where hardware supports it to increase the cost of kernel tampering.
• Enforce the Microsoft Vulnerable Driver Blocklist and block known unsigned or vulnerable drivers from loading.
• Implement application allow‑listing (WDAC or AppLocker) to prevent untrusted binaries from running in user contexts.
• Remove unnecessary local admin privileges and reduce the number of interactive logons on high‑value hosts.
• Temporarily isolate or harden VDI/RDP hosts and restrict local file or device mounting on such endpoints.

Detection and hunting recommendations:

• Add EDR rules to flag unusual, repeated calls to token query APIs (e.g., high‑frequency NtQueryInformationToken usage) from non‑privileged processes.
• Alert on processes that perform rapid usermode memory sampling loops immediately after token or device queries; this is a common pattern in TOCTOU PoCs.
• Monitor for sudden token impersonation events, unexpected privilege grants, or scheduled task/service creation by low‑privilege accounts — these are common post‑exploit artifacts.
• If compromise is suspected, capture full memory images and preserve EDR telemetry for forensic analysis before attempting remediation. Forensic artifacts are crucial to reconstruct exploitation timelines and to validate whether a chain was used.

Patch validation checklist (recommended sequence):

• Query Microsoft Security Update Guide for CVE‑2025‑55696 and capture the KB number(s) for each OS SKU.
• Deploy the update to a pilot ring representative of your environment (include app compatibility and driver compatibility checks).
• Monitor pilot devices for 48–72 hours for functional regressions, driver incompatibilities, or new crash patterns.
• Roll updates broadly via WSUS/ConfigMgr/Intune with compliance reporting and a rollback plan.

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Detection recipes and indicators of attempted exploitation​

Add EDR/SIEM rules for:

• Repeated calls to NtQueryInformationToken from low‑privilege processes or sandboxes.
• High‑rate usermode reads of buffers immediately after kernel queries (spin loops that sample buffers are a telltale sign).
• Sudden appearance of SYSTEM‑level scheduled tasks or services created by unprivileged users.
• Unexpected loads of unsigned drivers or sudden driver updates around the time of suspicious token API use.

When hunting, correlate kernel‑level telemetry (driver loads, crash dumps) with user‑space behavior (process call sequences) to spot the subtle race patterns that typical log sources miss. Preserve memory captures and full EDR traces in suspected cases for later patch diff analysis and incident response.

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Risks, unknowns, and things to watch for​

• Vendor discretion on details: Microsoft’s MSRC advisory entries for kernel info‑leaks and TOCTOU races are often intentionally terse to avoid lowering the exploitation bar; the advisory for CVE‑2025‑55696 follows this practice. That means many of the low‑level mechanics (exact offsets, IOCTL IDs, or struct field names) are unpublicized until a patch diff or researcher write‑up appears. Treat public technical inferences as plausible defensive assumptions, not confirmed detail.
• PoC publication risk: the absence of a public PoC on disclosure does not mean the vulnerability is safe. Historically, once PoC or partial details appear, weaponization and scanning accelerate quickly. Watch reputable technical blogs and major vulnerability repositories for post‑disclosure write‑ups.
• CVE aggregation confusion: third‑party trackers sometimes fragment related CDP/NTFS/token issues across multiple CVE IDs; always rely on the Microsoft Security Update Guide to map CVE → KB for your exact OS build. Mis‑mapped KBs are a common operational error during rapid patch cycles.

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Strengths and limitations of public reporting​

Strengths:

• The CVE is registered and mapped in vendor channels, which gives IT teams an authoritative starting point for remediation.
• Community write‑ups and playbooks already provide pragmatic detection and mitigations for kernel TOCTOUs that can be adapted to this CVE.

Limitations and risks:

• The vendor advisory intentionally omits exploit‑level details; many community technical claims about precise IOCTLs, exact buffer offsets, or driver names are inferred, not vendor‑confirmed. Those claims should be flagged as unverified until a patch diff or independent postmortem confirms them.
• Third‑party CVE mirrors may list slightly different metadata or lag in KB mapping. Rely on Microsoft’s Security Update Guide as the single source of truth for KB numbers.

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Longer‑term recommendations for hardening against token‑level races​

• Enforce least privilege across the estate: fewer local admin users reduces the attack surface for local EoP exploits.
• Expand application allow‑listing and isolate build/CI hosts and developer workstations from production resources. These hosts frequently run untrusted binaries that make TOCTOU exploitation practical.
• Increase kernel telemetry coverage: ensure EDR captures driver loads, kernel crashes, and suspicious IOCTL sequences; correlate them with user‑mode behavior to detect reconnaissance.
• Maintain an up‑to‑date inventory of installed kernel drivers and enforce driver signing policies; many kernel exploit chains rely on loading an unsigned or vulnerable driver as a write primitive.

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Conclusion​

CVE‑2025‑55696 is a kernel‑level TOCTOU race in the NtQueryInformationToken implementation (ntifs.h) with a high operational impact if weaponized. Although the vector is local, the power of token manipulation combined with modern exploit chaining means this is a remediation that should be prioritized for multi‑user hosts, admin workstations, and any system that accepts local code execution. Apply Microsoft’s patch(es) from the Security Update Guide as your primary remediation, harden hosts with Memory Integrity and application allow‑listing where possible, and tune detection to hunt for the characteristic high‑frequency token queries and userbuffer‑sampling loops used in TOCTOU exploit primitives. Remediation should follow a staged pilot→monitor→deploy model to catch compatibility issues while prioritizing high‑risk hosts first.
Flagged caution: until Microsoft or reputable researchers publish patch diffs or a technical postmortem, any claims about the exact IOCTLs, driver names, or literal memory offsets involved in CVE‑2025‑55696 must be treated as unverified inference, and operational decisions should rely on vendor KB mappings and the defensive playbook above.

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Source: MSRC Security Update Guide - Microsoft Security Response Center