Security Advisory - Apache Software Foundation
Apache HTTPD WebServer / httpd.apache.org
Server memory can be exhausted and service denied when HTTP/2 is used
The Apache HTTPD web server (from 2.4.17-2.4.23) did not apply limitations
on request headers correctly when experimental module for the HTTP/2
protocol is used to access a resource.
The net result is that a the server allocates too much memory instead of denying
the request. This can lead to memory exhaustion of the server by a properly
Apache has limits on the number and length of request header fields. which
limits the amount of memory a client can allocate on the server for a request.
Version 2.4.17 of the Apache HTTP Server introduced an experimental feature:
mod_http2 for the HTTP/2 protocol (RFC7540, previous versions were known as
This module is NOT compiled in by default -and- is not enabled by default,
although some distribution may have chosen to do so.
It is generally needs to be enabled in the 'Protocols' line in httpd by
adding 'h2' and/or 'h2c' to the 'http/1.1' only default.
The default distributions of the Apache Software Foundation do not include
this experimental feature.
- From version 2.4.17, upto and including version 2.4.23 the server failed
to take the limitations on request memory use into account when providing
access to a resource over HTTP/2. This issue has been fixed
in version 2.4.23 (r1772576).
As a result - with a request using the HTTP/2 protocol a specially crafted
request can allocate memory on the server until it reaches its limit. This can
lead to denial of service for all requests against the server.
This can lead to denial of service for all server resources.
All versions from 2.4.17 to 2.4.23.
For a 2.4.23 version a patch is supplied. This will be included in the
Mitigations and work arounds:
As a temporary workaround - HTTP/2 can be disabled by changing
the configuration by removing h2 and h2c from the Protocols
line(s) in the configuration file.
The resulting line should read:
Credits and timeline
The flaw was found and reported by Naveen Tiwari <naveen.tiwari () asu edu>
and CDF/SEFCOM at Arizona State University on 2016-11-22. The issue was
resolved by Stefan Eissing and incorporated in the Apache repository,
ready for inclusion in the next release.
Apache would like to thank all involved for their help with this.
The mod_http2 module in the Apache HTTP Server 2.4.17 through 2.4.23, when
the Protocols configuration includes h2 or h2c, does not restrict
request-header length, which allows remote attackers to cause a denial of
service (memory consumption) via crafted CONTINUATION frames in an HTTP/2
This issue was resolved and addressed in
GLSA 201701-36 at https://security.gentoo.org/glsa/201701-36
by GLSA coordinator Aaron Bauman (b-man).