CVE-2026-20929 Elevation of Privilege in Windows HTTP.sys: Patch Now

Microsoft’s Security Update Guide has recorded CVE-2026-20929 as an elevated-risk elevation-of-privilege vulnerability in the Windows HTTP.sys component, and the vendor’s public entry confirms the issue exists while providing only limited technical detail at the time of publication.

Background / Overview​

HTTP.sys is the kernel-mode HTTP protocol stack used by Windows to terminate HTTP requests for IIS, HTTP Server APIs, and many other inbox services. Because HTTP.sys runs in kernel mode, flaws in this component can produce high-impact outcomes: denial-of-service, information disclosure, local privilege escalation, or — in some cases historically — remote code execution when paired with other weaknesses. High-profile past vulnerabilities in HTTP.sys show the range of possible outcomes, from high-availability impacts to full system compromise. Microsoft’s listing for CVE-2026-20929 in the Security Update Guide establishes three immediate facts administrators must treat as authoritative: (1) the vulnerability identifier exists and is mapped in MSRC’s advisory system, (2) the affected component is HTTP.sys, and (3) the impact is an Elevation of Privilege condition in Windows. The vendor’s public advisory text for this CVE is intentionally brief, which is a common disclosure posture for inbox kernel or privileged components while patches are staged. Treat the MSRC entry as confirmation of existence and the canonical source for KB ↔ CVE mapping once the advisory page renders the full remediation data.

Important note: The publicly visible MSRC entry may show minimal exploit mechanics by design. Where low-level exploit details are absent from the vendor post, that absence should not be read as evidence of low risk — rather, it typically reflects a conservative disclosure stance until fixes are widely available.

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What the public record says now​

• Microsoft’s Security Update Guide lists CVE-2026-20929 against the HTTP.sys component and classifies the impact as Elevation of Privilege. This confirms the vulnerability exists (high vendor confidence in existence).
• The initial public advisory provides limited technical detail and does not (at the time of entry) publish a comprehensive exploit chain or public proof-of-concept. That pattern is consistent with how Microsoft often handles high-impact kernel/inbox-component CVEs.
• Because HTTP.sys executes privileged kernel-mode logic, an exploit that converts a flaw into arbitrary memory modifications or control-flow redirection can yield SYSTEM-level privileges. Historical HTTP.sys CVEs illustrate a range of impact vectors and exploitation models administrators should consider.

If your operations team needs a single operational takeaway: assume the MSRC record is authoritative and prepare to patch immediately when Microsoft publishes the KB mappings and update packages for your OS builds. The remainder of this article explains why, what to watch for, and how to mitigate while you stage updates.

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Technical analysis — plausible fault classes and exploitation models​

Microsoft’s initial advisory text is deliberately concise. Where low-level exploit mechanics are not published, it is appropriate for defenders to reason from the component’s role and historical vulnerability classes. The following are evidence-based, plausible defect classes for an HTTP.sys elevation-of-privilege issue. Each is presented as a plausible model — not a confirmed exploit recipe — and any claim about the exact exploit chain should be treated as unverified until corroborated by vendor or independent technical analysis.

1. Kernel memory-safety issues (use-after-free / buffer overflow)​

Kernel-mode drivers process structured network inputs and often perform complex parsing. A memory-safety bug such as a use-after-free, heap overflow, or out-of-bounds write in a request-parsing path can be escalated into privilege escalation by:

• leaking kernel pointers (information disclosure) that defeat kernel address layout randomization;
• converting an arbitrary write into a token swap or overwriting credentials / security descriptors; or
• hijacking function pointers or callback structures to run attacker-controlled code in kernel context.

Kernel-mode vulnerabilities historically produce reliable post‑compromise primitives because kernel processes run at the highest privilege level.

2. Improper validation of request-derived pointers / handles​

If a component accepts user-provided handles, or constructs kernel pointers from parsed request data without proper sanitization, a maliciously crafted request could cause the component to dereference or overwrite kernel data structures in privileged context. This pattern is particularly dangerous in protocol stacks that must marshal user-supplied metadata into kernel structures.

3. Race conditions and TOCTOU (time-of-check/time-of-use)​

Race windows in the path between authorization and action may allow a low-privileged request to trick privileged logic into performing sensitive actions against attacker-controlled resources. TOCTOU and concurrency defects can be weaponized into privilege escalation even when parsing logic appears correct at a high level.

4. Authorization bypasses in admin paths​

If HTTP.sys hosts administrative or management endpoints (or influences how privileged services accept metadata), insufficient authorization or misrouted privilege checks can lead to higher-level service logic performing privileged operations on behalf of an untrusted caller.
Caveat: these are plausible classes given HTTP.sys’s role; the exact root cause for CVE-2026-20929 remains vendor-controlled until MSRC or independent researchers publish a technical writeup or patch diffs. Flag any descriptions of the exact exploit chain as unverified unless corroborated by Microsoft or at least two independent technical analyses.

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Why HTTP.sys vulnerabilities are high-risk​

• Kernel mode execution: HTTP.sys runs with kernel privileges. Memory or control-flow vulnerabilities in kernel code often allow direct elevation to SYSTEM or equivalent trust.
• Wide attack surface: Many Windows servers expose HTTP endpoints (IIS, WCF services, custom apps using HTTP Server API). Even local-only exploit vectors can be weaponized in combination with other footholds.
• Diverse downstream impact: Successful exploitation can lead to service compromise, persistence, credential theft, or the ability to subvert update or management workflows.
• Historical precedence: Past HTTP.sys CVEs have ranged from denial-of-service to critical remote code execution and wormable scenarios — demonstrating that small implementation mistakes in the HTTP stack can have outsized effects.

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Practical risk assessment (what defenders should assume now)​

• Existence: Confirmed by Microsoft (high confidence). Administrators should treat the CVE ID and affected component mapping as authoritative for triage.
• Exploit details: Not publicly detailed in MSRC at publication (limited vendor disclosure). That means attackers have limited public guidance today, but the lack of public PoC does not mean there is no private exploit. Historical precedent shows exploit code often appears shortly after patches or patch diffs are published.
• Urgency: High for hosts that expose HTTP services or run IIS/HTTP-dependent inbox services; patch scheduling should be prioritized for externally reachable servers, management hosts, domain controllers that terminate HTTP, and crucial internal services that rely on HTTP.sys.
• Likely attacker model: Local escalation for an already compromised account or process is a plausible starting point; depending on the precise bug class, remote weaponization cannot be ruled out without vendor confirmation. Treat both local and network-exposed services conservatively.

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Immediate mitigation and detection recommendations​

While waiting for Microsoft to publish KB mappings and update packages, apply a defense-in-depth posture that reduces exposure and gives detection time to catch attempted exploitation.

Short-term mitigations (apply immediately if patching is not yet possible)​

• Block and segment:
• Block inbound HTTP/S access to non-essential servers at the network perimeter.
• Use firewall rules to limit access to admin and management ports to trusted management subnets.
• Disable or restrict unnecessary HTTP.sys features:
• If specific HTTP.sys features (e.g., HTTP/2, trailer support, or certain kernel-mode acceleration features) are not required, disable them through documented registry or feature toggles until a vendor fix is installed. Historical advisories have offered similar temporary mitigations for specific HTTP.sys features; apply such mitigations only when documented by Microsoft for this specific CVE. (Do not assume a specific registry key applies to CVE-2026-20929 without vendor confirmation.
• Reduce attack surface:
• Remove or stop the HTTP Server API bindings for services that do not require external HTTP access.
• For IIS hosts that are not internet-facing, ensure external routing does not reach the server.

Detection and hunting (EDR/SIEM rules)​

• Add hunts for:
• Unexplained kernel crashes or hangs correlating with HTTP.sys activity.
• Creation of SYSTEM-level child processes spawned from HTTP-related processes or from unusual service contexts.
• Sudden changes to service binaries or unexpected writes in system directories that coincide with HTTP traffic.
• Log sources and telemetry:
• Collect Windows Event logs and IIS logs; look for anomalous HTTP requests at odd endpoints or unusual request sizes/formats.
• Monitor netstat and firewall logs for unexpected inbound connections to ports typically used by HTTP.sys-backed services.
• Crash and memory forensic signals:
• Kernel memory leaks, repeated page faults in http.sys, or consistent exploit-like crashes in the HTTP stack should be treated as high-priority incidents.

Operational controls​

• Enforce least privilege:
• Reduce the number of accounts that can run code on servers that expose HTTP endpoints.
• Enforce application allow-listing/whitelisting on critical hosts.
• Harden update and deployment pipelines:
• Confirm the KB-to-SKU mapping in Microsoft’s Update Guide before mass deployment. Microsoft’s advisory pages are the canonical source for exact KB numbers for each Windows build.

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Patch management guidance — how to act when Microsoft publishes updates​

• Validate the MSRC entry and KB mapping for your Windows builds.
• Microsoft’s Security Update Guide is the canonical mapping; the advisory entry will show which KB(s) and which OS builds are affected. Use that mapping in your SCCM/WSUS/Intune automation to target the correct packages.
• Stage updates:
• Pilot the update on a small, representative set of systems (including a test IIS host and an admin workstation) and verify functionality and compatibility before broad rollout.
• Prioritize rollout:
• Order of precedence: externally-exposed HTTP servers → internal high-value HTTP servers (management, automation) → endpoints and workstations.
• Post-patch validation:
• Confirm the patch installed successfully and that previously observed exploitation indicators have ceased.
• Re-run the detection hunts and ensure no residual anomalous behavior remains.

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Controls and compensations for constrained environments​

For environments that cannot apply vendor patches immediately due to business constraints, adopt compensating controls until remediation is possible:

• Network-level blocking of ports and listeners for affected hosts.
• Temporarily deprovision or restrict the HTTP Server API usage on high-value hosts.
• Move critical workloads away from affected hosts or turn off unnecessary HTTP-dependent roles.
• Apply strict process and file integrity controls (EDR policies) to detect post-exploit behavior such as persistence mechanisms or unapproved child process creation.

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Cross-referencing and verification — what was checked​

• The Microsoft Security Update Guide lists CVE-2026-20929 against the Windows HTTP.sys component and labels the impact as Elevation of Privilege; this MSRC entry is the authoritative vendor record. Because the MSRC advisory page is rendered client-side and often intentionally concise until updates ship, its presence confirms the issue even where exploit mechanisms are not published.
• Historical HTTP.sys vulnerabilities and their impacts were referenced to illustrate plausible exploit models and risk: prior high-impact CVEs demonstrate that small protocol- or parser-level defects in HTTP.sys have led to denial-of-service and, in some circumstances, full system compromise — these examples underscore why CVE-2026-20929 demands priority remediation.
• Microsoft’s internal “confidence” / disclosure posture was discussed based on common MSRC practice and mirrored in community analyses of adjacent Windows CVEs; community guidance stresses mapping CVEs to KBs in the Update Guide and treating vendor acknowledgements as operationally binding even when technical details are minimal.

If Microsoft releases an expanded advisory, a patch diff, or independent writeups, those documents will be the definitive sources for the precise exploit primitives and for any recommended feature-specific mitigations (for example, registry keys to disable particular behaviors).

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What defenders should not do​

• Do not wait for public proof-of-concept code or proof-of-exploit telemetry before planning remediation. Vendor confirmation of a vulnerability in a kernel or privileged component is sufficient operational basis to prioritize patching.
• Do not assume a single KB applies to all SKUs — Microsoft typically publishes different package numbers and servicing notes across servicing channels and Windows builds. Always confirm the KB ↔ SKU mapping in Microsoft’s Update Guide or Update Catalog before mass deploy.
• Avoid applying unvetted community "fixes" or third-party mitigations that have not been validated; these can break services and complicate patch rollouts.

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Recommended 10-point action checklist (immediate to 72 hours)​

• Confirm whether any systems in your estate use HTTP.sys directly (IIS hosts, API hosts, services using HTTP Server API).
• Query Microsoft’s Update Guide for CVE-2026-20929 → get the KB mapping for each Windows build in your inventory.
• Stage the vendor patches in a pilot ring that includes representative server roles.
• Block external HTTP access to non-essential servers and management endpoints while you patch.
• Add EDR hunts for kernel crashes tied to http.sys and abnormal SYSTEM child process creation.
• Monitor IIS and Windows Event logs for unusual POSTs or malformed request patterns.
• Enforce firewall rules to segment HTTP/S traffic to trusted subnets only.
• Harden admin hosts and jump boxes; isolate management-plane hosts from general-purpose networks.
• Prepare rollback / recovery plans should the patch introduce regressions in critical workloads.
• Document the KBs applied, timestamp of remediation, and follow-up detection results for audit and post-mortem.

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Final appraisal and concluding guidance​

CVE-2026-20929’s registration in Microsoft’s Security Update Guide is the critical operational signal: the vulnerability exists in HTTP.sys, and Microsoft’s entry is the authoritative starting point for remediation. The public advisory’s concise style means low-level exploit mechanics are intentionally withheld until fixes are staged — a vendor posture that reduces short-term weaponization risk but increases the operational need to patch promptly when updates are issued. Administrators should assume high priority for hosts that terminate HTTP traffic or host management services, and they should treat the MSRC entry as a direct call to action: map CVE → KB for each Windows build, stage patches in a controlled pilot, and roll out to exposed hosts with urgency. At the same time, apply compensating controls (network segmentation, firewalling, targeted feature toggles where Microsoft documents them) and tune detection to catch both exploit attempts and the typical post‑exploit behaviors of privilege escalation.
Flag any operational claim about how exactly this CVE is exploited as unverified until Microsoft releases patch diffs or independent technical writeups appear; historical HTTP.sys vulnerabilities show that small parsing or kernel-mode logic errors can yield disproportionately large effects, and that is the prudent threat model to adopt now.
In short: presume the MSRC record is authoritative, prioritize patching and segmentation for HTTP-exposed hosts, instrument detection for kernel-level anomalies tied to HTTP.sys, and validate KB mappings before mass deployment.

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Source: MSRC Security Update Guide - Microsoft Security Response Center