CVE-2021-31618: Apache HTTP Server

High severity, CVSS 7.5. EPSS: 51.5% chance of exploitation in the next 30 days.

Apache HTTP Server protocol handler for the HTTP/2 protocol checks received request headers against the size limitations as configured for the server and used for the HTTP/1 protocol as well. On violation of these restrictions and HTTP response is sent to the client with a status code indicating why the request was rejected. This rejection response was not fully initialised in the HTTP/2 protocol handler if the offending header was the very first one received or appeared in a a footer. This led to a NULL pointer dereference on initialised memory, crashing reliably the child process. Since such a triggering HTTP/2 request is easy to craft and submit, this can be exploited to DoS the server. This issue affected mod_http2 1.15.17 and Apache HTTP Server version 2.4.47 only. Apache HTTP Server 2.4.47 was never released.

Affected products

• Apache HTTP Server: version 1.15.17 only; version 2.4.47 only
• Debian Debian Linux: version 9.0 only; version 10.0 only
• Fedoraproject Fedora: version 33 only; version 34 only
• Oracle Enterprise Manager Ops Center: version 12.4.0.0 only
• Oracle Instantis Enterprisetrack: version 17.1 only; version 17.2 only; version 17.3 only
• Oracle ZFS Storage Appliance Kit: version 8.8 only

Published 2021-06-15. Last modified 2026-06-17.