Security Advisory - Apache Software Foundation

              Apache HTTPD WebServer  / httpd.apache.org

   Server memory can be exhausted and service denied when HTTP/2 is used

                CVE-2016-8740

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.

Background:

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

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. 

Details:

- --------

- 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.

Impact:

- -------

This can lead to denial of service for all server resources.

Versions affected: 

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

All versions from  2.4.17 to  2.4.23. 

Resolution:

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

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 <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.

Index: modules/http2/h2_stream.c

===================================================================

--- modules/http2/h2_stream.c	(revision 1771866)

+++ modules/http2/h2_stream.c	(working copy)

@@ -322,18 +322,18 @@

                                            HTTP_REQUEST_HEADER_FIELDS_TOO_LARGE);

             }

         }

-    }

-    

-    if (h2_stream_is_scheduled(stream)) {

-        return h2_request_add_trailer(stream->request, stream->pool,

-                                      name, nlen, value, vlen);

-    }

-    else {

-        if (!input_open(stream)) {

-            return APR_ECONNRESET;

+        

+        if (h2_stream_is_scheduled(stream)) {

+            return h2_request_add_trailer(stream->request, stream->pool,

+                                          name, nlen, value, vlen);

         }

-        return h2_request_add_header(stream->request, stream->pool,

-                                     name, nlen, value, vlen);

+        else {

+            if (!input_open(stream)) {

+                return APR_ECONNRESET;

+            }

+            return h2_request_add_header(stream->request, stream->pool,

+                                         name, nlen, value, vlen);

+        }

     }

 }