HTTP.sys DoS Risk and Mitigations (CVE-2025-53805)

Microsoft’s advisory for a newly referenced HTTP.sys vulnerability describes an out‑of‑bounds read in the Windows HTTP protocol stack that can be triggered remotely against Internet Information Services (IIS) and other HTTP.sys consumers, allowing an unauthenticated attacker to cause a denial‑of‑service over the network — however, the CVE identifier provided (CVE‑2025‑53805) could not be located in major public trackers at the time of reporting and should be treated with caution until vendor pages and NVD records are synchronized. (msrc.microsoft.com)

Background​

HTTP.sys is the kernel‑mode HTTP protocol stack used by Windows to accept and route HTTP requests. Because it runs in kernel context and services platform‑level handlers used by IIS, application servers and numerous Microsoft components, vulnerabilities in HTTP.sys can have outsized operational impact — typically forcing crashes or resource exhaustion that ripple through the host and its dependent services. The class of bug now reported — an out‑of‑bounds read (CWE‑125) — is particularly concerning for kernel modules because it can lead to process crashes, uncontrolled resource consumption, or (in rare chains) information disclosure that helps later stages of an attack. Independent reporting has repeatedly documented similar HTTP.sys denial‑of‑service issues in recent years, confirming the recurring targeting of this component. (cvedetails.com, app.opencve.io)

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What the advisory claims (summary)​

• A vulnerability exists in HTTP.sys that permits a remote, unauthenticated attacker to trigger an out‑of‑bounds read, destabilize the HTTP.sys process, and cause a denial‑of‑service condition over the network.
• The behavior is network‑facing: attackers can exploit reachable HTTP endpoints (IIS sites, services that rely on HTTP.sys, or any binding that uses the kernel HTTP stack).
• Microsoft’s internal record page for the CVE is reachable but rendered via a dynamic web app; some details are present only in the MSRC update guide and related KBs. Because the MSRC page required JavaScript to render, external aggregators and NVD entries were used to cross‑check technical descriptors and mitigation guidance. (msrc.microsoft.com, app.opencve.io)

Note: At the time of publication the specific CVE tag (CVE‑2025‑53805) referenced by the user could not be conclusively matched in independent trackers such as NVD, Rapid7 and public CVE aggregators; a closely related HTTP.sys denial‑of‑service advisory published on April 8, 2025 (CVE‑2025‑27473) documents an uncontrolled resource consumption issue in HTTP.sys addressed by Microsoft patches and appears in multiple independent databases. Administrators should verify the exact CVE and KB mapping for their environment by consulting Microsoft’s Security Update Guide and their enterprise patch catalog. (app.opencve.io, rapid7.com)

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Technical analysis: how an out‑of‑bounds read in HTTP.sys leads to DoS​

What is an out‑of‑bounds read?​

An out‑of‑bounds read occurs when code reads memory outside the range of an allocated buffer. In user‑mode this often results in a crash or reading garbage; in kernel mode, it can crash the whole system or return unexpected data to a caller. In protocol stacks that parse externally controlled data, this vulnerability class manifests when an attacker supplies crafted packet content or request fields that cause the parser to read beyond intended buffers.

Why HTTP.sys is sensitive​

HTTP.sys operates at kernel level to accelerate request handling and centralize HTTP bindings across processes. Because it services all HTTP traffic and runs with elevated privileges, any crash or resource exhaustion in HTTP.sys can:

• Drop all incoming HTTP connections on the host.
• Force service restarts or kernel‑level fail states that impact multiple applications.
• Result in increased attack surface for chained attacks (e.g., forcing failovers, inducing logging gaps, or revealing timing windows for exploitation).

Multiple community writeups and vulnerability trackers confirm that HTTP.sys memory‑handling flaws are a recurring attack vector and that risk to availability is high for internet‑facing servers. (cvedetails.com, app.opencve.io)

Exploitability considerations​

• Attack Vector: Network (no credentials required) if the HTTP endpoint is reachable externally or from a reachable network segment.
• Complexity: Low to moderate — DoS exploitation typically requires only well‑formed malformed requests rather than delicate memory layout manipulation required for kernel RCE.
• Impact: Availability loss (denial of service) is the primary impact for out‑of‑bounds read in this context; confidentiality and integrity impacts are usually not expected unless the read leads to information disclosure that reduces barriers to follow‑on exploits. Independent trackers for similar HTTP.sys CVEs rate such vulnerabilities as High for availability. (app.opencve.io, rapid7.com)

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What was verified and where claims diverge​

• Microsoft advisory presence: Microsoft’s MSRC page for the referenced CVE identifier renders as a dynamic application (JavaScript required). The page exists but cannot be parsed as static HTML, which complicates automated aggregator ingestion. Confirmed: vendor page present but dynamic. (msrc.microsoft.com)
• Public CVE indexing: Major third‑party trackers and NVD entries do list multiple HTTP.sys denial‑of‑service CVEs in 2023–2025 (for example CVE‑2023‑35298 and CVE‑2025‑27473), including the April 8, 2025 advisory that documents uncontrolled resource consumption. However, the exact identifier CVE‑2025‑53805 did not appear in dominant registries at the time of checking; this may reflect timing, vendor tagging differences, or aggregator lag. Treat the numeric label with caution until MSRC and NVD show consistent records. (app.opencve.io, cvedetails.com)
• Technical classification: The user‑supplied description (out‑of‑bounds read leading to DoS) aligns with the general behavior of kernel HTTP.sys bugs and with vendor language used in previous advisories (resource exhaustion, malformed HTTP/2 or SNI packets). That classification is plausible and consistent with available documentation for similar CVEs. Nevertheless, specific exploitation details, proof‑of‑concepts, packet formats or sample payloads were not disclosed publicly by the vendor and are not reproduced here.

Caveat: Where a CVE number cannot be independently located in authoritative indexes, operations teams must confirm the patch mapping (CVE ↔ KB number ↔ update package) before taking remediation actions at scale.

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Immediate actions for administrators (priority checklist)​

Apply the following prioritized steps in the indicated order to reduce exposure quickly and safely.

• Patch — Apply Microsoft’s security updates for affected Windows builds immediately.
• Use your enterprise patch tooling (WSUS, SCCM, Microsoft Endpoint Manager) to find and deploy the specific security update that maps to the CVE/KBs. Vendor guidance and major vulnerability trackers show that Microsoft released fixes tied to monthly security updates for HTTP.sys issues; confirm the exact KB for your OS build. (rapid7.com, app.opencve.io)
• If immediate patching is impossible, apply interim mitigations:
• Block external access to HTTP/S ports at the perimeter for affected hosts.
• Restrict HTTP access to trusted management networks only.
• Consider temporarily disabling HTTP/2 on hosts that expose HTTP.sys HTTP/2 stacks (several advisories for similar HTTP.sys CVEs recommended this as a workaround). (qualys.com)
• Hardening and request‑level controls:
• Configure IIS Request Filtering and set conservative limits for header sizes, URL length, and concurrent request queues.
• Enable Dynamic IP Restrictions to throttle single‑source request storms.
• Ensure robust web application firewall (WAF) rules are in place at edge/load‑balancer to filter malformed requests.
• Monitoring and detection:
• Watch for sudden spikes in CPU, memory or an increase in HTTP service restarts.
• Monitor system and IIS logs for repeated malformed requests, HTTP/2 anomalies, or SNI handshake failures.
• Use IDS/IPS rules to detect unusual HTTP/2 traffic patterns and rate‑limit suspicious sources.
• Incident readiness:
• Prepare a rollback plan for updates that must be tested before wide deployment.
• Test patches in a representative staging environment before production rollout when possible; prioritize internet‑facing hosts for immediate application.
• If you detect exploitation evidence or rapid service degradation that correlates with the advisory, isolate the host and collect volatile forensic artifacts (memory snapshot, network pcap, IIS logs) for investigation.

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Practical commands and checks (quick reference)​

• Check whether the HTTP service is running:
• sc query http
• Confirm installed hotfixes (example):
• Get‑HotFix | Where‑Object {$_.HotFixID -like "50555"}
• List listening HTTP ports and services:
• netstat -ano | findstr :80
• netsh http show servicestate
• Temporarily disable HTTP/2 (registry change — test before applying in production):
• Set HKLM\SYSTEM\CurrentControlSet\Services\HTTP\Parameters\EnableHttp2 to 0 (reboot required for some configurations)

Note: Registry changes and service stops may have side effects; perform these during maintenance windows and after appropriate testing.

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Detection guidance: indicators and signatures​

• Sudden service crashes/HTTP.sys restarts and correlated kernel event logs (Event ID 1001, 7031, etc. depending on crash behavior).
• Burst patterns of malformed HTTP/2 frames or repeated SNI handshake anomalies in network sensors.
• Repeated requests with unusual header sizes, extreme URL lengths, or rapidly incrementing connection counts from a small set of IPs.
• IDS fingerprints associated with known HTTP.sys DoS PoC requests (where vendor/public signatures exist) — keep IDS signatures updated.

Community and forum analyses reinforce that scanning and small‑scale probes typically precede full exploitation, so early detection of probing behavior is effective at catching attack attempts before a full service disruption occurs.

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Risk appraisal: who should worry most​

• Internet‑facing IIS servers and Windows hosts that accept inbound HTTP traffic are highest priority.
• Cloud workloads using Windows images with public HTTP endpoints — ensure cloud‑level security groups and WAF rules are tightened.
• Devices and appliances that embed Windows HTTP.sys (some management endpoints, appliance web UIs) — these may be overlooked in standard server patch cycles and must be inventoried.
• Organizations with rigid change control windows should prioritize critical perimeter hosts for emergency patching and consider temporary isolation until patches are applied.

Public CVE trackers for similar HTTP.sys flaws show high CVSS base scores driven by the network attack vector and high availability impact; exploitability for DoS is often rated low complexity, meaning attackers need minimal sophistication to trigger service disruption. Cross‑checked trackers and vendor pages indicate the same operational priority: patch quickly and harden exposure. (app.opencve.io, rapid7.com)

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Why this matters: systemic and operational risks​

• Availability-first impact: Denial‑of‑service vulnerabilities in kernel components are disruptive, often forcing emergency maintenance and impacting service level agreements.
• Cascading effects: HTTP.sys serves many Windows components. A crash can cascade to dependent apps — not just single IIS sites — multiplying operational pain.
• Attacker incentive: Service disruption is an effective denial tactic in both criminal and state‑level campaigns; internet‑exposed servers present an easy target and can be used to create broader systemic outages if left unpatched.

Community incident reports and vendor advisories over the last two years consistently treat HTTP.sys as a recurring high‑value target, underscoring the need for continuous monitoring and rapid patch adoption.

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Cross‑verification and evidence summary​

• Microsoft’s Security Update Guide has an entry for an HTTP.sys denial‑of‑service advisory that requires JavaScript to render; the page exists but aggregator ingestion can lag due to dynamic rendering. Administrators should consult MSRC directly via their enterprise browsing channel to confirm KB mappings. (msrc.microsoft.com)
• Independent vulnerability trackers (OpenCVE, Rapid7 and CVE Details) list similar HTTP.sys DoS CVEs, notably CVE‑2025‑27473 (April 8, 2025), and show vendor‑supplied fixes and KB numbers for affected Windows builds. These sources corroborate the general impact and mitigation approach (patch, restrict exposure, disable HTTP/2 where applicable). (app.opencve.io, rapid7.com, cvedetails.com)
• Internal community analysis and forum discussions emphasize practical mitigations (isolate internet‑facing hosts, monitor for probes, rotate remediation in prioritized windows) and provide operational context for administrators handling Windows servers in heterogeneous estate environments.

Flagged item: The specific CVE label CVE‑2025‑53805, as provided in the initial prompt, did not appear in the principal public repositories at the time of verification. This may be due to timing (vendor reserved IDs vs. published), MSRC dynamic rendering, or aggregator lag. Treat the numeric label as provisional until you confirm the exact MSRC update and KB numbers that correspond to your OS images. (msrc.microsoft.com, cvedetails.com)

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Longer‑term recommendations​

• Inventory: Create or refresh an inventory of all Windows hosts with HTTP.sys bindings — not just IIS servers — including appliances, management UIs, and container images that may include Windows components.
• Patch automation: Strengthen patch pipelines so kernel‑level hotfixes can be tested and deployed faster for high‑risk components.
• Perimeter and WAF discipline: Ensure edge WAFs and load balancers perform deep HTTP/2 inspection and rate limiting; consider blocking or rate limiting HTTP/2 from untrusted external networks when immediate remediation is needed.
• Defense‑in‑depth: Complement perimeter defenses with host‑level protections (IIS Request Filtering, Dynamic IP Restrictions, enhanced logging) and with EDR sensors configured to detect rapid crashes or unusual kernel events.

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Final assessment and actionable takeaways​

• Treat the advisory as urgent for any internet‑reachable Windows server that uses IIS or HTTP.sys‑backed services. The primary risk is availability; attackers can induce service outages with crafted network traffic. (app.opencve.io)
• Confirm the exact CVE ↔ KB mapping in your patch management system by consulting Microsoft’s Security Update Guide directly and cross‑checking with NVD/major vulnerability databases. The CVE number provided may be provisional or subject to aggregator delays; do not rely solely on the numeric tag when scheduling remediation. (msrc.microsoft.com, cvedetails.com)
• Patch first, then implement compensating controls for unpatched or high‑risk hosts: restrict HTTP access at the network edge, disable HTTP/2 where indicated, harden request limits, and monitor for probe patterns. (qualys.com, rapid7.com)

Flag for operators: If you cannot find CVE‑2025‑53805 in your usual vulnerability feeds, search for HTTP.sys advisories published in April–May 2025 (for example CVE‑2025‑27473) and map the KB numbers before applying changes broadly. Cross‑verify the KB IDs in Microsoft’s Update Catalog and your enterprise patch tool to ensure you deploy the correct packages for each OS build. (app.opencve.io, rapid7.com)

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This technical feature has summarized what is known about the reported HTTP.sys out‑of‑bounds read and its operational impact, validated core claims against multiple industry trackers and community analyses, and provided concrete remediation and detection steps for Windows administrators. Where numerical identifiers or web pages could not be fully reconciled, the ambiguity has been flagged so operations teams can confirm patch mappings before mass deployment. (app.opencve.io, rapid7.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center