CVE-2026-49788 Overview
CVE-2026-49788 is a denial-of-service vulnerability affecting the HTTP/2 implementation in Microsoft Windows and Windows Server. The flaw stems from allocation of resources without limits or throttling [CWE-770] in the HTTP/2 protocol handler. An unauthenticated remote attacker can exhaust server resources by sending crafted HTTP/2 traffic over the network. Successful exploitation renders the target unable to service legitimate requests. The vulnerability requires no user interaction and no prior authentication, making internet-facing Windows servers particularly exposed.
Critical Impact
An unauthorized remote attacker can trigger service unavailability on affected Windows and Windows Server systems through crafted HTTP/2 requests, disrupting web services and any workload dependent on the HTTP.sys stack.
Affected Products
- Microsoft Windows 10 (1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (24H2, 25H2, 26H1)
- Microsoft Windows Server 2016, 2019, 2022, and 2025
Discovery Timeline
- 2026-07-14 - CVE-2026-49788 published to NVD
- 2026-07-20 - Last updated in NVD database
Technical Details for CVE-2026-49788
Vulnerability Analysis
The vulnerability resides in the HTTP/2 protocol handling within the Windows networking stack. HTTP/2 multiplexes multiple concurrent streams over a single TCP connection, and each stream consumes server memory and processing state. When the implementation fails to enforce upper bounds on stream creation, frame processing, or associated buffer allocations, an attacker can weaponize legitimate protocol behavior to consume resources indefinitely.
Attackers exploit this class of flaw by opening many streams, sending oversized headers, or issuing rapid stream resets that force the server to allocate state faster than it can reclaim it. The result is CPU saturation, memory exhaustion, or connection table depletion on the target host. Because the HTTP/2 stack is embedded in HTTP.sys and used by Internet Information Services (IIS) and other Windows-hosted services, exposure spans a broad range of production workloads.
Root Cause
The root cause is missing throttling logic on server-side resource allocation for HTTP/2 streams and frames, classified under [CWE-770] Allocation of Resources Without Limits or Throttling. The server accepts and processes attacker-controlled protocol elements without applying rate or volume ceilings appropriate to the connection.
Attack Vector
The attack vector is network-based and requires no authentication or user interaction. An adversary needs only network reachability to an HTTP/2 endpoint on an affected Windows or Windows Server system. Refer to the Microsoft Vulnerability Advisory CVE-2026-49788 for vendor-specific technical detail.
No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
Detection Methods for CVE-2026-49788
Indicators of Compromise
- Abnormal spikes in HTTP.sys memory or non-paged pool consumption on IIS and Windows Server hosts.
- Large volumes of HTTP/2 stream creation, rapid RST_STREAM frames, or oversized HEADERS frames from a single source.
- Sudden loss of responsiveness on HTTP/2 endpoints without corresponding legitimate traffic increase.
- Application event log entries indicating exhausted worker threads or dropped connections in IIS.
Detection Strategies
- Monitor Windows performance counters for HTTP Service Request Queues and worker process memory to identify anomalous growth.
- Enable HTTP/2 telemetry at network appliances or web application firewalls to flag connections with unusually high stream counts or reset rates.
- Correlate network flow data with Windows event logs to identify single-source connections generating disproportionate protocol activity.
Monitoring Recommendations
- Baseline normal HTTP/2 concurrency per client and alert on deviations that exceed the baseline over short windows.
- Track availability metrics for exposed web services and configure alerting on sustained latency or error-rate increases.
- Review IIS logs and Windows Event Viewer entries under System and Application for repeated connection failures tied to HTTP/2 endpoints.
How to Mitigate CVE-2026-49788
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Vulnerability Advisory CVE-2026-49788 to all affected Windows and Windows Server systems.
- Prioritize patching for internet-facing IIS servers and any host exposing HTTP/2 endpoints to untrusted networks.
- Place rate-limiting controls or a reverse proxy in front of exposed HTTP/2 services until patching completes.
Patch Information
Microsoft has published patch guidance through the Microsoft Security Response Center. Administrators should consult the Microsoft Vulnerability Advisory CVE-2026-49788 for the specific KB article, build numbers, and deployment instructions applicable to each Windows edition and architecture (x86, x64, ARM64).
Workarounds
- Disable HTTP/2 on affected IIS endpoints where the protocol is not required, forcing clients to negotiate HTTP/1.1.
- Deploy an upstream load balancer or WAF that enforces per-connection stream limits, concurrent request caps, and connection-rate throttling.
- Restrict inbound HTTP/2 access to trusted networks via firewall rules where feasible.
# Configuration example: disable HTTP/2 on Windows Server via 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
# Restart the HTTP service to apply
net stop http /y && net start http
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
