CVE-2026-49975: HTTP/2 Bomb CVE-2026-49975

CVE-2026-49975Memory Allocation with Excessive Size Value vulnerability in Apache HTTP Server's modhttp leads to denial of service via malicious HTTP requests. This issue affects Apache HTTP Server: from 2.4.17 through 2.4.67.

— FortiGuard

We’re publishing HTTP/2 Bomb, a remote denial-of-service exploit against most major web servers, including:

nginx

Apache httpd

Microsoft IIS

Envoy

Cloudflare Pingora

The vulnerable behavior exists in each server's default HTTP/2 configuration.

The attack was discovered by Codex, which chained two techniques known to humans for a decade: a compression bomb and a Slowloris-style hold. The bomb targets HPACK, HTTP/2's header compression scheme: one byte on the wire becomes one full header allocation on the server, repeated thousands of times per request. The hold is a zero-byte flow-control window that keeps the server from ever freeing any of it.

A curious search on Shodan revealed 880,000+ websites supporting HTTP/2 and running one of these servers, though many sit behind a CDN, which is much harder to bring down.

A home computer on a 100Mbps connection can render a vulnerable server inaccessible within seconds. Against Apache httpd and Envoy, a single client can consume and hold 32GB of server memory in roughly 20 seconds.

— Red Hat