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A newly disclosed denial-of-service flaw in multiple HTTP/2 server implementations allows remote, unauthenticated attackers to exhaust memory and crash services by abusing normal flow-control behavior. By stalling outbound data transmission—such as by manipulating SETTINGS_INITIAL_WINDOW_SIZE or withholding WINDOW_UPDATE frames—an attacker can cause vulnerable servers to keep generating and buffering full responses, leading to out-of-memory conditions, swap thrashing, worker-thread exhaustion, service freezes, or kernel OOM kills. CERT/CC is tracking the issue after it was reported by the Okta Red Team, and the coordinated disclosures include CVE-2026-44909, CVE-2026-59173, and CVE-2026-59762.
Affected vendors named in advisories include Apache Traffic Server, Citrix, F5 Networks, Meta, Red Hat, SUSE, and Yahoo. Apache Traffic Server users were advised to upgrade to versions 9.1.14 or 10.1.3, while F5 published guidance for BIG-IP and other vendors issued product-specific fixes or mitigations. Recommended defenses include stricter memory limits, limiting concurrent HTTP/2 streams, terminating stalled connections, and improving timeout and backpressure handling. No public exploitation has been confirmed, but the low complexity of the attack has prompted calls for rapid patching and configuration hardening.
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4 events from the most recent confirmed update back to the earliest known activity.
The HTTP/2 denial-of-service issue was reported by the Okta Red Team. The flaw involves abusing normal flow-control behavior so servers buffer unsent response data until memory is exhausted.
Affected vendors including F5 began releasing fixes, advisories, and mitigations for the HTTP/2 memory-exhaustion flaw. Recommended defenses included stricter memory limits, limiting concurrent HTTP/2 streams, terminating stalled connections, and improving timeout and backpressure handling.
Apache Traffic Server published fixes or advisories for its affected HTTP/2 implementation. Users were advised to upgrade to versions 9.1.14 or 10.1.3.
CERT/CC began tracking the coordinated disclosure covering multiple CVEs affecting HTTP/2 implementations, including CVE-2026-44909, CVE-2026-59173, and CVE-2026-59762. The issue affects multiple vendors and can lead to freezes, crashes, or out-of-memory conditions.
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CERT/CC disclosed **VU#885548**, a denial-of-service condition in some HTTP/2 server implementations that can be triggered remotely without authentication by abusing stalled flow-control behavior. An attacker can advertise settings such as `SETTINGS_INITIAL_WINDOW_SIZE = 0` or stop sending `WINDOW_UPDATE` frames, forcing servers to buffer large response bodies in memory across many concurrent streams. The resulting impact can include memory exhaustion, swap thrashing, worker or connection exhaustion, service instability, and in severe cases a full crash. Okta Red Team was credited with reporting the issue, and the advisory linked the problem to **`CVE-2026-59762`**, **`CVE-2026-59173`**, and **`CVE-2026-44909`**. F5 said **`CVE-2026-59762`** affects **BIG-IP** and related **BIG-IP Next** products when an HTTP/2 profile is configured on a virtual server, allowing crafted requests to drive up memory use in the **TMM** process until performance degrades or TMM restarts; the company rated the flaw **CVSS 7.5** under v3.1 and **8.7** under v4.0 and urged customers to install patched releases. Apache also confirmed **`CVE-2026-59173`** in **Apache Traffic Server** versions **9.0.0 through 9.2.13** and **10.0.0 through 10.1.2**, recommending upgrades to **9.2.14+** or **10.1.3+**. CERT noted that several vendors have already issued fixes, while some products including GNU wget, Go, HAProxy, ISC BIND, lighttpd, and nghttp2 were listed as not affected.
Jul 17, 2026
A newly disclosed denial-of-service vulnerability dubbed **HTTP/2 Bomb** (`CVE-2026-49975`) can let unauthenticated attackers crash vulnerable servers by abusing HTTP/2 header compression and flow-control behavior to amplify small requests into large in-memory allocations. Researchers said the flaw affects widely deployed infrastructure including **nginx**, **Apache httpd**, **Microsoft IIS**, **Envoy**, and **Cloudflare Pingora**, with early Internet scans identifying more than **880,000** potentially exposed websites. Security firms reported reconnaissance activity in the wild, and sectors with large numbers of Internet-facing systems—especially **telecommunications, IT, and healthcare**—were flagged as particularly exposed. A public proof-of-concept is now available for **Apache HTTP Server**, where affected versions **2.4.17 through 2.4.67** can be forced into sustained memory exhaustion through crafted HTTP/2 cookie headers and a zero-window flow-control technique that prevents resources from being released. Apache fixed the issue in **2.4.68**, while other vendors have rolled out patches or mitigations on uneven timelines, including earlier fixes from nginx and Apache, a next-day patch from Envoy, a later mitigation from Microsoft, and no patch yet reported for Cloudflare at the time of coverage. Defenders are being urged to **upgrade immediately**, consider temporarily disabling HTTP/2 where feasible, and watch for abnormal memory growth and other signs of attempted DoS activity.
Jun 29, 2026
Researchers publicly disclosed **HTTP/2 Bomb**, a remote denial-of-service technique that abuses standard HTTP/2 behavior to rapidly exhaust memory on major web servers and proxies, including **nginx, Apache HTTP Server, Microsoft IIS, Envoy, and Cloudflare Pingora**. The attack combines **HPACK header-compression amplification** with a **Slowloris-style flow-control stall**, letting a low-bandwidth client trigger disproportionate server-side allocations and keep that memory pinned by advertising a zero-byte window. Reports said a single machine on a 100 Mbps connection could make vulnerable services inaccessible within seconds, with testing showing **Apache httpd** and **Envoy** consuming roughly **32 GB of RAM** in about **10 to 20 seconds** under default HTTP/2 configurations. Public disclosures and proof-of-concept code indicated the amplification largely comes from per-header bookkeeping rather than oversized decoded header values, allowing common decoded-size limits to be bypassed. **Apache** tracked its exposure as **`CVE-2026-49975`** and released a fix in **`mod_http2 2.0.41`**, while **nginx** addressed the issue in **`1.29.8`**; advisories initially said **IIS, Envoy, and Pingora** lacked patches, though later disclosures showed **Envoy** assigned **`CVE-2026-47774`** and published an advisory. Researchers estimated more than **880,000** internet-exposed HTTP/2 systems could be affected and recommended upgrading where fixes exist, or otherwise disabling HTTP/2, enforcing strict header-count limits, and monitoring for abnormal memory growth.
Jul 20, 2026