EUVD-2026-43803
GHSA-wrm6-j53r-328q
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Network-reachable, low-complexity, unauthenticated HTTP/2 DoS with no user interaction; impact is availability-only, so C:N/I:N/A:H and unchanged scope.
Primary rating from Vendor (microsoft).
CVSS VectorVendor: microsoft
Lifecycle Timeline
2DescriptionCVE.org
Allocation of resources without limits or throttling in HTTP/2 allows an unauthorized attacker to deny service over a network.
AnalysisAI
Denial of service in the Windows HTTP/2 network stack allows a remote, unauthenticated attacker to exhaust server resources and render affected services unavailable across a broad range of Windows client and server releases (Windows 10/11 and Windows Server 2016 through 2025). Reported by Microsoft with a vendor patch available; there is no public exploit identified at time of analysis and it is not listed in CISA KEV. The 7.5 CVSS reflects high availability impact with no confidentiality or integrity loss.
Technical ContextAI
The flaw lies in Windows' implementation of the HTTP/2 protocol, most likely surfaced through the kernel-mode HTTP.sys stack that underpins IIS, HTTP.sys-hosted services, and other HTTP/2 endpoints on Windows. The root cause is CWE-770 (Allocation of Resources Without Limits or Throttling): the HTTP/2 handler accepts protocol-level constructs without enforcing adequate bounds, allowing an attacker to force disproportionate memory, CPU, or connection-state allocation. HTTP/2's multiplexing, stream, and header/frame handling are a recurring class of DoS amplification primitives (e.g., prior 'Rapid Reset'-style stream abuse), where cheap client requests translate into expensive server-side allocation. The CPE data confirms the affected surface is the OS-level HTTP/2 component shipped across Windows 10 (1607, 1809, 21H2, 22H2), Windows 11 (24H2, 25H2, 26H1), and Windows Server 2016/2019/2022/2025.
RemediationAI
Apply the Microsoft security update for CVE-2026-49788 (Patch available per vendor advisory); consult the per-edition fixed builds in the Microsoft Security Update Guide at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-49788 and deploy the cumulative update matching each affected Windows 10/11 or Windows Server edition. Where immediate patching is not possible on internet-facing hosts, compensating controls include disabling HTTP/2 on the affected endpoint so connections fall back to HTTP/1.1 (on HTTP.sys/IIS this is done via the EnableHttp2 registry settings under HTTP\Parameters, with the trade-off of losing HTTP/2 multiplexing performance benefits), placing a reverse proxy or WAF/load balancer in front to terminate and rate-limit HTTP/2 connections and cap concurrent streams, and restricting exposure of the service to trusted networks. Each mitigation reduces performance or reachability rather than fixing the underlying allocation flaw, so treat them as temporary until the update is installed.
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External POC / Exploit Code
Leaving vuln.today