The vulnerable system is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or through terminal emulation (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., using social engineering techniques to trick a legitimate user into opening a malicious document).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
Low
PR
The attacker requires privileges that provide basic capabilities that are typically limited to settings and resources owned by a single low-privileged user. Alternatively, an attacker with Low privileges has the ability to access only non-sensitive resources.
User Interaction
None
UI
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
Low
C
There is some impact on confidentiality, but the attacker either does not gain control of any data, or the information obtained does not have a significant impact on the system or its operations.
Integrity
Low
I
Modification of data is possible, but the attacker does not have control over what can be modified, or the extent of what the attacker can affect is limited. The data modified does not have a direct, serious impact on the system.
Availability
None
A
There is no impact on the availability of the system; the attacker does not have the ability to disrupt access to or use of the system.
Apache HTTPD Web Server 2.4.23 Memory Exhaustion 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 <[email protected]>
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.
-------------------------
CVE-2016-8740-2.4.23.diff
-------------------------
Patch against 2.4.23 release source:
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);
+ }
}
}
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