CVE-2015-1635: Microsoft HTTP.sys Remote Code Execution Vulnerability

CVE-2015-1635 is a remote code execution vulnerability in Microsoft’s HTTP protocol stack component, HTTP.sys. An attacker who can reach an affected system over the network may be able to run code in the context of the HTTP stack, which sits at a privileged layer on Windows hosts that accept HTTP traffic. For IT and security teams this matters because HTTP.sys is foundational to IIS and other Windows HTTP services; successful abuse can give an attacker a foothold without needing valid credentials.

Public detail on exact mechanics is limited to the vendor and CISA descriptions; treat the guidance below as class-level defender advice and confirm every version, patch, and configuration detail against the Microsoft advisory for this CVE.

How it works

The weakness is classified as CWE-94 (code injection). In broad terms for this class, the HTTP.sys component improperly handles certain crafted input while processing HTTP requests. An unauthenticated remote attacker can send specially formed traffic to a listening HTTP endpoint; if the stack fails to validate or constrain that input correctly, the attacker’s data can influence execution flow and lead to arbitrary code running with the privileges of the HTTP.sys driver or associated service.

No exploit code or packet-level details are provided in the given facts. Defenders should assume that any network-reachable Windows host running the vulnerable HTTP.sys implementation is in scope until the vendor update is applied and verified. Confirm the precise trigger conditions and affected request types only from the official Microsoft advisory.

Am I affected? How to find it in your systems

HTTP.sys is the kernel-mode HTTP protocol stack used by Internet Information Services (IIS), HTTP Server APIs, and other Windows components that accept HTTP/HTTPS connections. It commonly appears on Windows Server and client systems that host web applications, reverse proxies, or management interfaces.

• Inventory Windows hosts that listen on ports 80, 443, or other HTTP ports; check for the presence of IIS, HTTP.sys-dependent roles, or applications that call the HTTP Server API.
• Query installed updates and the HTTP.sys file version on each host; compare against the fixed versions listed in the Microsoft security update for CVE-2015-1635. Do not rely on guessed build numbers—use the vendor’s published matrix.
• Review configuration: systems that expose HTTP.sys directly to untrusted networks are highest priority. Internal-only listeners still require patching but can be deprioritized after external exposure is addressed.
• Telemetry and log signs of exploitation attempts are not detailed in the supplied facts. In general, look for anomalous or malformed HTTP requests, unexpected crashes or restarts of HTTP.sys/IIS worker processes, and post-exploitation indicators such as new services, scheduled tasks, or outbound connections from web-facing hosts. Correlate with EDR and network logs; treat any confirmed exploitation as an incident.

How to remediate

Patch first. Apply the Microsoft security update that addresses CVE-2015-1635 exactly as directed in the vendor advisory and the CISA required action (“Apply updates per vendor instructions”). After installation, reboot if required and verify the HTTP.sys version and update status on every affected host.

• Use centralized patch management (WSUS, ConfigMgr, Intune, or equivalent) to drive deployment and reporting.
• Prioritize internet-facing and DMZ systems, then tier-1 application servers, then remaining internal hosts.
• After patching, re-inventory to confirm no residual vulnerable instances remain, including golden images, offline VMs, and disaster-recovery replicas.
• For this vulnerability class, also harden the HTTP stack: disable unused HTTP features and bindings, enforce least-privilege application pool identities, and keep the broader Windows baseline current so related components do not reintroduce risk.

If you can't patch immediately

Compensating controls reduce but do not eliminate risk. Implement them only as a bridge until the vendor update is installed.

• Network segmentation and filtering: restrict access to HTTP/HTTPS ports on affected hosts to known management and application source ranges; block unnecessary exposure at the edge and between tiers.
• Virtual patching / WAF: if a web application firewall or reverse proxy sits in front of the service, enable rules that inspect and drop malformed or suspicious HTTP requests characteristic of this class of flaw. Tune carefully to avoid breaking legitimate traffic and confirm rule coverage with the WAF vendor.
• Disable or unload the affected feature where operationally feasible—for example, stop IIS or remove the HTTP Server role on hosts that do not require it.
• Heightened monitoring: increase logging verbosity for HTTP.sys and IIS, forward logs to a SIEM, and alert on process crashes, unusual request patterns, and privilege-escalation or lateral-movement behaviors on web-facing servers. Retain packet captures of anomalous traffic for later analysis.
• Ensure EDR is deployed and healthy on these hosts so post-exploitation activity can be detected even if the initial request succeeds.

If your data may have been exposed

Actively exploited remote code execution vulnerabilities can lead to full host compromise and subsequent data theft or ransomware, although known ransomware use is not documented for this CVE in the supplied facts. If you have evidence of exploitation or cannot rule it out, follow your incident-response process: isolate affected systems, preserve forensic images, rotate credentials, and assess what data the host could access. As a quick additional check, you can run a free exposure scan of your email addresses against known breach datasets to see whether associated accounts appear in prior public breaches while you complete containment and recovery.