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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.
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Apache Traffic Server disclosed that versions 9.0.0 through 9.2.13 and 10.0.0 through 10.1.2 are affected by a stalled HTTP/2 flow-control denial-of-service vulnerability tracked as CVE-2026-59173. The Apache Software Foundation advised upgrading to versions 9.2.14+ or 10.1.3+, and credited Okta Red Team with reporting the issue.
CERT/CC published Vulnerability Note VU#885548 describing a denial-of-service issue in some HTTP/2 server implementations caused by stalled flow-control conditions. The note associated CVE-2026-59762, CVE-2026-59173, and CVE-2026-44909 with the issue, credited Okta Red Team for reporting it, and said several vendors had already issued fixes.
A CVE record for CVE-2026-59173 was reserved by a CVE Numbering Authority, with no public vulnerability details provided at that time.
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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.
Jun 29, 2026
Apache HTTP Server disclosed **CVE-2026-23918**, a high-severity double-free flaw in `mod_http2` affecting version `2.4.66`, with researchers showing it can be triggered remotely over a single TCP connection by sending crafted HTTP/2 frames on the same stream. The bug stems from the same `h2_stream` object being queued twice for cleanup, causing a second `apr_pool_destroy` on freed memory; on multi-threaded MPM deployments such as **event** and **worker**, this can reliably crash worker processes without authentication, while **prefork** is not affected. Public reporting said denial-of-service is the most practical near-term outcome, but under specific conditions the memory corruption can be developed into pre-authenticated remote code execution. Apache fixed the issue in **version 2.4.67** by preventing duplicate cleanup entries and urged administrators to upgrade immediately. Guidance published alongside the disclosure recommended temporarily disabling **HTTP/2** where patching is delayed, using rate limiting or a WAF to reduce opportunistic abuse, and reviewing exposure from related modules after the same release also patched flaws in `mod_rewrite`, `mod_proxy_ajp`, `mod_md`, and `mod_dav_lock`. At the time of publication, sources said there was **no confirmed in-the-wild exploitation**, but warned that Apache HTTP Server's broad deployment and common `mod_http2` enablement make internet-facing systems attractive targets.
Jun 29, 2026