Windows HTTP.sys EUVD-2026-56365

The attacker must hold a low-privileged, authenticated local account on the target system, as described by 'authorized attacker' in the CVE and confirmed by CVSS PR:L. HTTP.sys must be active on the target, which is the default state on any Windows system running IIS, WinRM, or HTTP API-dependent services. High attack complexity (CVSS AC:H) means reliable exploitation depends on meeting specific kernel heap state conditions - likely requiring heap grooming or precise timing that is not consistently reproducible. Factors limiting exploitation: the attacker must first achieve local code execution at low privilege before attempting escalation; AC:H makes reliable exploitation technically demanding and not suitable for mass automated exploitation; fully patched systems (per fixed build versions above) are not vulnerable.

The CVSS 7.0 score is appropriately calibrated: the AV:L and AC:H metrics substantially constrain real-world risk relative to the H/H/H impact. Exploitation requires an attacker who already holds a low-privileged local session (PR:L) and must meet high-complexity conditions (AC:H), most likely involving precise kernel heap layout manipulation or race conditions inherent to the overflow trigger. This vulnerability is not listed in CISA KEV and no POC code has been identified, meaning there is no confirmed evidence of active or opportunistic exploitation. EPSS data was not provided, so probabilistic exploitation likelihood cannot be quantified. The primary real-world risk scenario is post-compromise privilege escalation: an attacker with an initial foothold at low privilege uses this flaw to achieve full system control. The breadth of affected Windows versions (Server 2012 through Windows 11 26H1) amplifies enterprise exposure significantly. CVSS vector and description are internally consistent - no conflicting signals were identified.

An attacker who has obtained a low-privileged shell on a Windows server - through phishing, a web application exploit, or credential theft - crafts a sequence of operations that triggers the heap buffer overflow in HTTP.sys under specific kernel heap layout conditions. If the high-complexity prerequisites are met, the overflow corrupts a security-relevant kernel structure, redirecting execution to attacker-controlled code running as SYSTEM, yielding full host compromise. No public proof-of-concept code has been identified at time of analysis, indicating this scenario currently requires significant attacker expertise to execute reliably.