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CVE-2026-49788 is a high‑severity denial‑of‑service (DoS) vulnerability affecting the HTTP/2 protocol stack in supported Microsoft Windows clients and servers. The flaw stems from an uncontrolled resource‑allocation weakness, classified as CWE‑770, where the software allocates reusable resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.
The HTTP/2 protocol introduces multiplexing, stream prioritisation, and HPACK header compression to improve performance. However, these features also create new stateful objects – streams, header tables, flow‑control windows, and priority trees – that must be tracked per connection. In a vulnerable implementation, an attacker can open a single TCP connection and then initiate a large number of HTTP/2 streams or send specially crafted frames that force the server to allocate memory, CPU, and connection‑tracking structures without enforcing per‑connection or global limits.
Because the vulnerability is reachable over the network with no authentication and low complexity, an unauthenticated remote attacker can send a sequence of HTTP/2 frames that trigger excessive resource consumption. For example, by rapidly opening new streams and never closing them, or by sending CONTINUATION frames that fragment headers into many small pieces, the server’s HTTP.sys kernel‑mode driver is forced to maintain an ever‑growing set of state entries. As these allocations accumulate, memory and CPU usage spike, eventually causing the system to become unresponsive or crash, thereby denying service to legitimate clients.
Microsoft’s CVSS v3.1 vector for this issue is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, reflecting a network‑reachable attack with low complexity, no privileges required, and a high impact on availability. The base score is 7.5 (High). The weakness is rooted in the HTTP.sys component, which handles all HTTP/2 traffic on Windows systems. The vulnerability affects a wide range of Windows versions, including Windows 10 1607, 1809, 21H2, 22H2, Windows 11 24H2, 25H2, 26H1, and Windows Server 2016, 2019, 2022, and 2025.
Microsoft released security updates on July 14, 2026 to address the flaw. The fixes introduce proper throttling and per‑connection limits for HTTP/2 resources, ensuring that no single connection can exhaust system resources. Administrators are urged to apply the updates immediately, as the vulnerability is automatable and could be weaponised in widespread DoS attacks.
DailyCVE Form:
Platform: ……. Windows HTTP.sys
Version: …….. 10.0.14393–10.0.28000
Vulnerability :…… Uncontrolled resource allocation
Severity: ……. 7.5 (High)
date: ………. 2026-07-14
Prediction: ….. 2026-07-14 (already patched)
What Undercode Say:
- Attack Vector: Remote, unauthenticated HTTP/2 frames.
- Resource Exhaustion: Memory + CPU + connection state.
- No User Interaction: Required for successful exploitation.
- Automatable: Yes – scriptable DoS tool possible.
- Observed Exploitation: None reported as of July 2026.
- Microsoft Assessment: Exploitation less likely.
- CISA SSVC: Technical Impact – Partial.
- Patch Availability: Included in July 2026 updates.
- KB References: KB5101650, KB5099539, KB5099535.
- CWE Mapping: CWE‑770 – Allocation without limits.
Analytics under heading What Undercode Say:
Check current Windows build number systeminfo | findstr /B /C:"OS Name" /C:"OS Version" Verify if the system is vulnerable (build below fixed threshold) Example for Windows 10 22H2: fixed build is 19045.7548 wmic os get buildnumber Query installed updates to confirm patch presence wmic qfe list brief /format:texttable | findstr "KB5101650 KB5099539 KB5099535" Enable HTTP/2 logging for forensic analysis (IIS) %windir%\system32\inetsrv\appcmd.exe set config /section:httpLogging /dontLog:False Monitor HTTP.sys resource usage via Performance Monitor typeperf "\HTTP Service\Current Connections" "\HTTP Service\Current Non-Active Requests"
Exploit:
An attacker can exploit CVE‑2026‑49788 by establishing a single TCP connection to a vulnerable Windows server and sending a crafted sequence of HTTP/2 frames that cause uncontrolled resource allocation. Common techniques include:
– Stream Flooding: Opening hundreds or thousands of concurrent streams without sending END_STREAM, forcing the server to maintain state for each stream.
– Header Fragment Attack: Sending CONTINUATION frames that fragment HPACK-encoded headers into many small pieces, causing the server to repeatedly allocate and reallocate memory for header reconstruction.
– Priority Tree Manipulation: Sending PRIORITY frames that create deep or cyclic dependency trees, exhausting CPU cycles during tree traversal.
– Zero‑Window Exploitation: Advertising a zero‑byte flow‑control window and then sending large data frames, forcing the server to buffer data indefinitely.
These attacks require no authentication and can be launched from any network‑reachable host. Public proof‑of‑concept code is expected to emerge shortly, as the vulnerability is automatable.
Protection:
- Apply Official Patches: Install the July 2026 security updates (KB5101650 for Windows 11, KB5099539 for Windows 10, KB5099535 for Windows Server 2016, and corresponding updates for other affected versions).
- Disable HTTP/2 (Temporary Mitigation): If patching is not immediately possible, disable HTTP/2 on IIS servers via the registry:
reg add "HKLM\System\CurrentControlSet\Services\HTTP\Parameters" /v EnableHttp2Tls /t REG_DWORD /d 0 /f reg add "HKLM\System\CurrentControlSet\Services\HTTP\Parameters" /v EnableHttp2Cleartext /t REG_DWORD /d 0 /f
(Reboot required.)
- Network Rate Limiting: Deploy a Web Application Firewall (WAF) or load balancer that enforces per‑client connection and stream limits.
- Monitor Resource Usage: Set up alerts for abnormal CPU, memory, or HTTP.sys connection counts using Performance Monitor or SIEM tools.
- Restrict Access: Where possible, limit HTTP/2 exposure to trusted networks or implement IP whitelisting.
Impact:
- Availability Loss: Successful exploitation causes the HTTP.sys driver to consume all available memory and CPU, leading to a complete denial of service for all HTTP/HTTPS services on the affected system.
- Service Disruption: Web applications, APIs, authentication flows, and management portals become unresponsive, potentially affecting business operations.
- No Data Breach: The vulnerability does not allow data exfiltration or privilege escalation; its impact is strictly limited to availability.
- Wide Attack Surface: The flaw affects all supported Windows versions from Server 2016 to Windows 11 26H1, making it a critical concern for enterprises.
- Potential for Automated Attacks: Because the vulnerability is automatable and requires no user interaction, it could be incorporated into botnets or self‑propagating worms.
- Business Continuity Risk: Repeated attacks can force administrators to restart servers, leading to extended downtime and recovery efforts.
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Sources:
Reported By: nvd.nist.gov
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