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Date: Mon, 5 Dec 2016 12:33:40 +0100
From: Eissing Stefan <>
Cc: httpd-security <>
Subject: [FD] CVE-2016-8740,
 Server memory can be exhausted and service denied when HTTP/2 is

         Security Advisory - Apache Software Foundation
               Apache HTTPD WebServer  /

    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
crafted request.

- -----------

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 
Google SPDY).

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.
Versions affected: 
- ------------------
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
next release. 

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:

		Protocols http/1.1

Credits and timeline
- --------------------

The flaw was found and reported by Naveen Tiwari <> 
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.

Patch against 2.4.23 release source:

Download attachment "CVE-2016-8740-2.4.23.diff" of type "application/octet-stream" (1227 bytes)

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